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We explore the potential of fluorine-containing small Mn 2+ chelates as alternatives to perfluorinated nanoparticles, widely used as 19 F MRI probes. In MnL1, the cyclohexanediamine skeleton and two piperidine rings, involving each a metal-coordinating amide group and an appended CF 3 moiety, provide high rigidity to the complex. This allows for good control of the Mn-F distance (r MnF =8.2±0.2 Å determined from 19 F relaxation data), as well as for high kinetic inertness (a dissociation half-life of 1285 h is estimated for physiological conditions). The paramagnetic Mn 2+ leads to a ~150-fold acceleration of the longitudinal 19 F relaxation, with moderate line-broadening effect, resulting in T 2 /T 1 ratios of 0.8 (9.4 T). Owing to its inner sphere water molecule, MnL1 is a good 1 H relaxation agent as well (r 1 =5.36 mM -1  s -1 at 298 K, 20 MHz). MnL1 could be readily visualized in 19 F MRI by using fast acquisition techniques, both in phantom images and living mice following intramuscular injection, with remarkable signal-to-noise ratios and short acquisition times. While applications in targeted imaging or cell therapy monitoring require further optimisation of the molecular structure, these results argue for the potential of such small, monohydrated and fluorinated Mn 2+ complexes for combined 19 F and 1 H MRI detection., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Three water-soluble Mn(III)-porphyrin complexes with cationic pyridyl side groups bearing COOH- or OH-terminated carbon chains in the meta or para positions have been synthesized as probes for both magnetic resonance imaging (MRI) and photodynamic therapy (PDT). The complexes Mn-1 , Mn-2 , and Mn-3 are highly water-soluble, and their relaxivities range between 10 and 15 mM -1 s -1 , at 20-80 MHz and 298 K, 2-3 times higher than that of commercial Gd(III)-based agents. The complexes containing carboxylate ( Mn-2 ) or alcoholic ( Mn-3 ) side chains in the para position are endowed with higher relaxivities and have also shown efficient photoinduced DNA cleavage and singlet oxygen ( 1 O 2 ) generation. Mn-3 with stronger photoinduced DNA cleavage has also revealed stabilizing and binding activities for G4 DNA, at a similar level as the known G4 binder Mn-TMPyP4 . Nevertheless, the G4-binding activity of Mn-3 was nonspecific. Preliminary tests evidenced photocytotoxicity of Mn-3 on HeLa cells without a significant effect in the absence of light. Altogether, these results underline the potential of such water-soluble Mn(III)-porphyrins for the development of multimodal approaches combining MRI and PDT., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Co-published by Nanjing University and American Chemical Society.)

Despite significant progress in cancer imaging and treatment over the years, early diagnosis and metastasis detection remain a challenge. Molecular magnetic resonance imaging (MRI), with its high resolution, can be well adapted to fulfill this need, requiring the design of contrast agents which target specific tumor biomarkers. Netrin-1 is an extracellular protein overexpressed in metastatic breast cancer and implicated in tumor progression and the appearance of metastasis. This study focuses on the design and preclinical evaluation of a novel Netrin-1-specific peptide-based MRI probe, GdDOTA-KKTHDAVR (Gd-K), to visualize metastatic breast cancer. The targeting peptide sequence was identified based on the X-ray structure of the complex between Netrin-1 and its transmembrane receptor DCC. Molecular docking simulations support the probe design. In vitro studies evidenced submicromolar affinity of Gd-K for Netrin-1 ( K D = 0.29 μM) and good MRI efficacy (proton relaxivity, r 1 = 4.75 mM -1 s -1 at 9.4 T, 37 °C). In vivo MRI studies in a murine model of triple-negative metastatic breast cancer revealed successful tumor visualization at earlier stages of tumor development (smaller tumor volume). Excellent signal enhancement, 120% at 2 min and 70% up to 35 min post injection, was achieved (0.2 mmol/kg injected dose), representing a reasonable imaging time window and a superior contrast enhancement in the tumor as compared to Dotarem injection.

Mn 2+ complexes of 2,4-pyridyl-disubstituted bispidine ligands have emerged as more biocompatible alternatives to Gd 3+ -based MRI probes. They display relaxivities comparable to that of commercial contrast agents and high kinetic inertness, unprecedented for Mn 2+ complexes. The chemical structure, in particular the substituents on the two macrocyclic nitrogens N3 and N7, are decisive for the conformation of the Mn 2+ complexes, and this will in turn determine their thermodynamic, kinetic and relaxation properties. We describe the synthesis of four ligands with acetate substituents in positions N3, N7 or both. We evidence that the bispidine conformation is dependent on N3 substitution, with direct impact on the thermodynamic stability, kinetic inertness, hydration state and relaxivity of the Mn 2+ complexes. These results unambiguously show that (i) solely a chair-chair conformation allows for favorable inertness and relaxivity, and (ii) in this family such chair-chair conformation is accessible only for ligands without N3-appended carboxylates., (© 2023 Wiley-VCH GmbH.)

We present the in vitro characterisation of a Gd 3+ -based contrast agent that responds to Zn 2+ upon interaction with Human Serum Albumin. We show that the contradictory in vivo behaviour is related to Gd 3+ -accumulation in Zn-rich tissues. This highlights the importance of the biodistribution of such contrast agents.

Zn 2+ -responsive magnetic resonance imaging (MRI) contrast agents are typically composed of a Gd chelate conjugated to a Zn 2+ -binding moiety via a linker. They allow for Zn 2+ detection in the presence of human serum albumin (HSA). In order to decipher the key parameters that drive their Zn 2+ -dependent MRI response, we designed a pyridine-based ligand, PyAmC2mDPA , and compared the properties of GdPyAmC2mDPA to those of analogue complexes with varying Gd core, Zn-binding moiety, or linker sizes. The stability constants determined by pH potentiometry showed the good selectivity of PyAmC2mDPA for Gd 3+ (log K Gd = 16.27) versus Zn 2+ (log K Zn = 13.58), proving that our modified Zn 2+ -binding DPA moiety prevents the formation of previously observed dimeric species. Paramagnetic relaxation enhancement measurements indicated at least three sites that are available for GdPyAmC2mDPA binding on HSA, as well as a 2-fold affinity increase when Zn 2+ is present ( K D = 170 μM versus K DZn = 60 μM). Fluorescence competition experiments provided evidence of the higher affinity for site II vs site I, as well as the importance of both the Zn-binding part and the Gd core in generating enhanced HSA affinity in the presence of Zn 2+ . Finally, an analysis of nuclear magnetic relaxation dispersion (NMRD) data suggested a significantly increased rigidity for the Zn 2+ -bound system, which is responsible for the Zn 2+ -dependent relaxivity response.

A new fluorinated manganese porphyrin, (Mn-TPP-p-CF 3 ) is reported capable of providing, based on the Mn(III)/Mn(II) equilibrium, dual 1 H relaxivity and 19 F NMR response to redox changes. The physical-chemical characterization of both redox states in DMSO-d 6 /H 2 O evidenced that the 1 H relaxometric and 19 F NMR properties are appropriate for differential redox MRI detection. The Mn(III)-F distance (d Mn-F =9.7-10 Å), as assessed by DFT calculations, is well tailored to allow for adequate paramagnetic effect of Mn(III) on 19 F T 1 and T 2 relaxation times. Mn-TPP-p-CF 3 has a reversible Mn(II)/Mn(III) redox potential of 0.574 V vs. NHE in deoxygenated aqueous HEPES/ THF solution. The reduction of Mn(III)-TPP-p-CF 3 in the presence of ascorbic acid is slowly, but fully reversed in the presence of air oxygen, as monitored by UV-Vis spectrometry and 19 F NMR. The broad 1 H and 19 F NMR signals of Mn(III)-TPP-p-CF 3 disappear in the presence of 1 equivalent ascorbate replaced by a shifted and broadened 19 F NMR signal from Mn(II)-TPP-p-CF 3 . Phantom 19 F MR images in DMSO show a MRI signal intensity decrease upon reduction of Mn(III)-TPP-p-CF 3 , retrieved upon complete reoxidation in air within ~24 h. 1 H NMRD curves of the Mn(III)/(II)-TPP-p-CF 3 chelates in mixed DMSO/water solvent have the typical shape of Mn(II)/Mn(III) porphyrins., (© 2023 Wiley-VCH GmbH.)

A new 15-membered pyridine-based macrocyclic ligand containing one acetate pendant arm ( N -carboxymethyl-3,12,18-triaza-6,9-dioxabicyclo[12.3.1]octadeca-1(18),14,16-triene, L1) was synthesized and its Mn(II) complex MnL1 was investigated in the context of MRI contrast agent development. The X-ray molecular structure of MnL1 confirmed a coordination number of seven with an axially compressed pentagonal bipyramidal geometry and one coordination site available for an inner-sphere water molecule. The protonation constants of L1 and the stability constants of Mn(II), Zn(II), Cu(II) and Ca(II) complexes were determined by potentiometry, and revealed higher thermodynamic stabilities in comparison with complexes of 15-pyN 3 O 2 , the parent macrocycle without an acetate pendant arm. The MnL1 complex is fully formed at physiological pH 7.4, but it shows fast dissociation kinetics, as followed by relaxometry in the presence of an excess of Zn(II). The short dissociation half-life estimated for physiological pH ( ca. 3 minutes) is related to fast spontaneous dissociation of the non-protonated complex. At lower pH values, the proton-assisted dissociation pathway becomes important, while the Zn(II) concentration has no effect on the dissociation rate. 17 O NMR and 1 H NMRD data indicated the presence of one inner-sphere water molecule with a rather slow exchange ( k 298ex = 4.5 × 10 6 s -1 ) and provided information about other microscopic parameters governing relaxation. The relaxivity ( r 1 = 2.45 mM -1 s -1 at 20 MHz, 25 °C) corresponds to typical values for monohydrated Mn(II) chelates. Overall, the acetate pendant arm in L1 has a beneficial effect with respect to 15-pyN 3 O 2 in increasing the thermodynamic stability and kinetic inertness of its Mn(II) complex, but leads to a reduced number of inner-sphere water molecules and thus lower relaxivity.

With the aim of developing more stable Gd(III)-porphyrin complexes, two types of ligands 1 and 2 with carboxylic acid anchors were synthesized. Due to the N-substituted pyridyl cation attached to the porphyrin core, these porphyrin ligands were highly water-soluble and formed the corresponding Gd(III) chelates, Gd-1 and Gd-2 . Gd-1 was sufficiently stable in neutral buffer, presumably due to the preferred conformation of the carboxylate-terminated anchors connected to nitrogen in the meta position of the pyridyl group helping to stabilize Gd(III) complexation by the porphyrin center. 1 H NMRD (nuclear magnetic relaxation dispersion) measurements on Gd-1 revealed high longitudinal water proton relaxivity ( r 1 = 21.2 mM -1 s -1 at 60 MHz and 25 °C), which originates from slow rotational motion resulting from aggregation in aqueous solution. Under visible light irradiation, Gd-1 showed extensive photoinduced DNA cleavage in line with efficient photoinduced singlet oxygen generation. Cell-based assays revealed no significant dark cytotoxicity of Gd-1 , while it showed sufficient photocytotoxicity on cancer cell lines under visible light irradiation. These results indicate the potential of this Gd(III)-porphyrin complex ( Gd-1 ) as a core for the development of bifunctional systems acting as an efficient photodynamic therapy photosensitizer (PDT-PS) with magnetic resonance imaging (MRI) detection capabilities., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Co-published by Nanjing University and American Chemical Society.)

As an essential metal ion and an efficient relaxation agent, Mn 2+ holds a great promise to replace Gd 3+ in magnetic resonance imaging (MRI) contrast agent applications, if its stable and inert complexation can be achieved. Toward this goal, four pyridine and one carboxylate pendants have been introduced in coordinating positions on the bispidine platform to yield ligand L 3 . Thanks to its rigid and preorganized structure and perfect size match for Mn 2+ , L 3 provides remarkably high thermodynamic stability (log K MnL = 19.47), selectivity over the major biological competitor Zn 2+ (log (K MnL / K ZnL ) = 4.4), and kinetic inertness. Solid-state X-ray data show that [MnL 3 (MeOH)](OTf) 2 has an unusual eight-coordinate structure with a coordinated solvent molecule, in contrast to the six-coordinate structure of [ZnL 3 ](OTf), underlining that the coordination cavity is perfectly adapted for Mn 2+ , while it is too large for Zn 2+ . In aqueous solution, 17 O NMR data evidence one inner sphere water and dissociatively activated water exchange ( k ex 298 = 13.5 × 10 7 s -1 ) for MnL 3 . Its water proton relaxivity ( r 1 = 4.44 mM -1 s -1 at 25 °C, 20 MHz) is about 30% higher than values for typical monohydrated Mn 2+ complexes, which is related to its larger molecular size; its relaxation efficiency is similar to that of clinically used Gd 3+ -based agents. In vivo MRI experiments realized in control mice at 0.02 mmol/kg injected dose indicate good signal enhancement in the kidneys and fast renal clearance. Taken together, MnL 3 is the first chelate that combines such excellent stability, selectivity, inertness and relaxation properties, all of primary importance for MRI use.

C -glycopyranosyl derivatives of six-membered heterocycles are scarcely represented in the chemical literature and the title 3-glycopyranosyl-1,2,4-triazines are completely unknown. In this paper, the first synthesis of this compound class is accomplished by the cyclocondensation of C -glycosyl formamidrazones and 1,2-dicarbonyl derivatives. In addition, the synthesis of C -glycopyranosyl 1,2,4-triazin-5(4 H )-ones was also carried out by the transformation of the above formamidrazones with α-keto-carboxylic esters. Inverse electron demand Diels-Alder reactions of 3-glycopyranosyl-1,2,4-triazines with a bicyclononyne derivative yielded the corresponding annulated 2-glycopyranosyl pyridines.

The reactivity of O-peracetylated and O-perbenzoylated 1-COOMe, 1-CONH 2 and 1-CN-substituted glycals was studied against O-, S-, N- and C-nucleophiles in the presence of Lewis acids. Allylic substituted products with exclusive axial stereoselectivity were formed with simple alcohols, N 3 - , and Cl - ions, but with benzyl thiol the Ferrier rearrangement took place and thioglycosides were obtained. The use of a sugar derived thiol resulted in the formation of both the allylic substituted and the rearranged products., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Bispidine (3,7-diazabicyclo[3.3.1]nonane) provides a rigid and preorganized scaffold that is particularly interesting for the stable and inert complexation of metal ions, especially for their application in medical imaging. In this study, we present the synthesis of two bispidine ligands with N -methanephosphonate (H 4 L 1 ) and N -methanecarboxylate (H 3 L 2 ) substituents as well as the physico-chemical properties of the corresponding Mn 2+ and Zn 2+ complexes. The two complexes [Mn( L 1 )] 2- and [Mn( L 2 )] - have relatively moderate thermodynamic stability constants according to potentiometric titration data. However, they both display an exceptional kinetic inertness, as assessed by transmetallation experiments in the presence of 50 equiv excess of Zn 2+ , showing only ∼40 and 20% of dissociation for [Mn( L 1 )] 2 - and [Mn( L 2 )] - , respectively, after 150 days at pH 6 and 37 °C. Proton relaxivities amount to r 1 = 4.31 mM -1 s -1 ([Mn( L 1 )] 2 - ) and 3.64 mM -1 s -1 ([Mn( L 2 )] - ) at 20 MHz, 25 °C, and are remarkable for Mn 2+ complexes with one inner-sphere water molecule ( q = 1); they are comparable to that of the commercial contrast agent [Gd(DOTA)(H 2 O)] - . The presence of one inner-sphere water molecule and an associative water exchange mechanism was confirmed by temperature-dependent transverse 17 O relaxation rate measurements, which yielded k ex 298 = 0.12 × 10 7 and 5.5 × 10 7 s -1 for the water exchange rate of the phosphonate and the carboxylate complex, respectively. In addition, radiolabeling experiments with 52 Mn were also performed with H 2 ( L 1 ) 2- showing excellent radiolabeling properties and quantitative complexation at pH 7 in 15 min at room temperature as well as excellent stability of the complex in various biological media over 24 h.

Porous materials, such as zeolites, have great potential for biomedical applications, thanks to their ability to accommodate positively charged metal-ions and their facile surface functionalization. Although the latter aspect is important to endow the nanoparticles with chemical/colloidal stability and desired biological properties, the possibility for simple ion-exchange enables easy switching between imaging modalities and/or combination with therapy, depending on the envisioned application. In this study, the nanozeolite Linde type L (LTL) with already confirmed magnetic resonance imaging properties, generated by the paramagnetic gadolinium (Gd III ) in the inner cavities, was successfully radiolabeled with a positron emission tomography (PET)-tracer zirconium-89 ( 89 Zr). Thereby, exploiting 89 Zr-chloride resulted in a slightly higher radiolabeling in the inner cavities compared to the commonly used 89 Zr-oxalate, which apparently remained on the surface of LTL. Intravenous injection of PEGylated 89 Zr/Gd III -LTL in healthy mice allowed for PET-computed tomography evaluation, revealing initial lung uptake followed by gradual migration of LTL to the liver and spleen. Ex vivo biodistribution confirmed the in vivo stability and integrity of the proposed multimodal probe by demonstrating the original metal/Si ratio being preserved in the organs. These findings reveal beneficial biological behavior of the nanozeolite LTL and hence open the door for follow-up theranostic studies by exploiting the immense variety of metal-based radioisotopes.

The development of metal-based multimodal imaging probes is a highly challenging field in coordination chemistry. In this context, we have developed a bifunctional hexadentate tripodal ligand (H 3 L2) with three 3,4-HOPO moieties attached to a flexible tetrahedral carbon bearing a functionalizable nitro group. Complexes formed with different metal ions have potential interest for diagnostic applications, namely magnetic resonance imaging (MRI) and positron emission tomography (PET). The capacity of the ligand to coordinate Gd III and Ga III was studied and the thermodynamic stability constants of the respective complexes were determined by potentiometry and spectrophotometry. The ligand forms stable 1 : 1 ML complexes though with considerably higher affinity for Ga III than for Gd III ( p Ga = 26.2 and p Gd = 14.3 at pH 7). The molecular dynamics simulations of the Gd III complex indicate that two water molecules can coordinate the metal ion, thus providing efficient paramagnetic enhancement of water proton relaxation. The relaxation and the water exchange properties of the Gd III chelate, assessed by a combined 17 O NMR and 1 H NMRD study, showed associative activated water exchange with a relatively low rate constant, k 298ex = (0.82 ± 0.11) × 10 7 s -1 , and some aggregation tendency. Biodistribution studies of the 67 Ga-L2 complex suggested good in vivo stability and quick renal clearance. Further anchoring of this ligand with specific biotargeting moieties might open future prospectives for applications of labelled conjugates in both MRI and 68 Ga-PET diagnostic imaging.

The stability constants of lanthanide complexes with the potentially octadentate ligand CHX OCTAPA 4- , which contains a rigid 1,2-diaminocyclohexane scaffold functionalized with two acetate and two picolinate pendant arms, reveal the formation of stable complexes [log K LaL = 17.82(1) and log K YbL = 19.65(1)]. Luminescence studies on the Eu 3+ and Tb 3+ analogues evidenced rather high emission quantum yields of 3.4 and 11%, respectively. The emission lifetimes recorded in H 2 O and D 2 O solutions indicate the presence of a water molecule coordinated to the metal ion. 1 H nuclear magnetic relaxation dispersion profiles and 17 O NMR chemical shift and relaxation measurements point to a rather low water exchange rate of the coordinated water molecule ( k ex 298 = 1.58 × 10 6 s -1 ) and relatively high relaxivities of 5.6 and 4.5 mM -1 s -1 at 20 MHz and 25 and 37 °C, respectively. Density functional theory calculations and analysis of the paramagnetic shifts induced by Yb 3+ indicate that the complexes adopt an unprecedented cis geometry with the two picolinate groups situated on the same side of the coordination sphere. Dissociation kinetics experiments were conducted by investigating the exchange reactions of LuL occurring with Cu 2+ . The results confirmed the beneficial effect of the rigid cyclohexyl group on the inertness of the Lu 3+ complex. Complex dissociation occurs following proton- and metal-assisted pathways. The latter is relatively efficient at neutral pH, thanks to the formation of a heterodinuclear hydroxo complex.

While Mn II complexes meet increasing interest in biomedical applications, ligands are lacking that enable high Mn II complex stability and selectivity vs. Zn II , the most relevant biological competitor. We report here two new bispidine derivatives, which provide rigid and large coordination cavities that perfectly match the size of Mn II , yielding eight-coordinate Mn II complexes with record stabilities. In contrast, the smaller Zn II ion cannot accommodate all ligand donors, resulting in highly strained and less stable six-coordinate complexes. Combined theoretical and experimental data (X-ray crystallography, potentiometry, relaxometry and 1 H NMR spectroscopy) demonstrate unprecedented selectivity for Mn II vs. Zn II (K MnL /K ZnL of 10 8 -10 10 ), in sharp contrast to the usual Irving-Williams behavior, and record Mn II complex stabilities and inertness with logK MnL close to 25., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

A new pyclen-3,9-diacetate derivative ligand (H 2 3,9-OPC2A ) was synthesized possessing an etheric O-atom opposite to the pyridine ring, to improve the dissociation kinetics of its Mn(II) complex (pyclen = 3,6,9,15-tetraazabicyclo(9.3.1)pentadeca-1(15),11,13-triene). The new ligand is less basic than the N-containing analogue (H 2 3,9-PC2A ) due to the non-protonable O-atom. In spite of its lower basicity, the conditional stability of the [Mn( 3,9-OPC2A )] (pMn = -log(Mn(II)), c L = c Mn(II) = 0.01 mM. pH = 7.4) remains unaffected (pMn = 8.69), compared to the [Mn( 3,9-PC2A )] (pMn = 8.64). The [Mn( 3,9-OPC2A )] possesses one water molecule, having a lower exchange rate with bulk solvents ( k ex 298 = 5.3 ± 0.4 × 10 7 s -1 ) than [Mn( 3,9-PC2A )] ( k ex 298 = 1.26 × 10 8 s -1 ). These mild differences are rationalized by density-functional theory (DFT) calculations. The acid assisted dissociation of [Mn( 3,9-OPC2A )] is considerably slower ( k 1 = 2.81 ± 0.07 M -1 s -1 ) than that of the complexes of diacetates or bisamides of various 12-membered macrocycles and the parent H 2 3,9-PC2A . The [Mn( 3,9-OPC2A )] is inert in rat/human serum as confirmed by 52 Mn labeling (nM range), as well as by relaxometry (mM range). However, a 600-fold excess of EDTA (pH = 7.4) or a mixture of essential metal ions, propagated some transchelation/transmetalation in 7 days. The H 2 3,9-OPC2A is labeled efficiently with 52 Mn at elevated temperatures, yet at 37 °C the parent H 2 3,9-PC2A performs slightly better. Ultimately, the H 2 3,9-OPC2A shows advantageous features for further ligand designs for bifunctional chelators.

Drug-loaded liposomes are typical examples of nanomedicines. We show here that doxorubicin, the anti-cancer agent in the liposomal drug Doxil, can sensitize Ytterbium (Yb 3+ ) and generate its near-infrared (NIR) emission. When doxorubicin and amphiphilic Yb 3+ chelates are incorporated into liposomes, the sensitized emission of Yb 3+ is dependent on the integrity of the particles, which can be used to monitor drug release. We also established the first demonstration that the NIR Yb 3+ emission signal is observable in living mice following intratumoral injection of the Yb 3+ -doxorubicin-liposomes, using a commercial macroscopic setup equipped with a NIR camera., (© 2021 Wiley-VCH GmbH.)

Two DO3A-type ligands conjugated to substrates of urokinase (L3) and caspase-3 (L4) via a propyl-amide linker were synthesized and their lanthanide(III) (Ln 3+ ) complexes studied. A model compound without peptide substrate (L2) and an amine derivative ligand mimicking the state after enzymatic cleavage (L1) were also prepared. Proton Nuclear Magnetic Relaxation Dispersion (NMRD) profiles recorded on the gadolinium(III) (Gd 3+ ) complexes, complemented with the assessment of hydration numbers via luminescence lifetime measurements on the Eu 3+ analogues, allowed us to characterize the lanthanide coordination sphere in the chelates. These data suggest that the potential donor groups of the peptide side chains (carboxylate, amine) interfere in metal coordination, leading to non-hydrated LnL3 and LnL4 complexes. Nevertheless, GdL3 and GdL4 retain a relatively high relaxivity due to an important second-sphere contribution generated by the strongly hydrophilic peptide chain. Weak PARACEST effects are detected for the amine-derivative EuL1 and NdL1 chelates. Unfortunately, the GdL3 and GdL4 complexes are not significantly converted by the enzymes. The lack of enzymatic recognition of these complexes can likely be explained by the participation of donor groups from the peptide side chain in metal coordination.

Interactions of paramagnetic metal complexes with their biological environment can modulate their magnetic resonance imaging (MRI) contrast-enhancing properties in different ways, and this has been widely exploited to create responsive probes that can provide biochemical information. We survey progress in two rapidly growing areas: the MRI detection of biologically important metal ions, such as calcium, zinc, and copper, and the use of transition metal complexes as smart MRI agents. In both fields, new imaging technologies, which take advantage of other nuclei ( 19 F) and/or paramagnetic contact shift effects, emerge beyond classical, relaxation-based applications. Most importantly, in vivo imaging is gaining ground, and the promise of molecular MRI is becoming reality, at least for preclinical research., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

We report two macrocyclic ligands based on a 1,7-diaza-12-crown-4 platform functionalized with acetate ( t O2DO2A 2- ) or piperidineacetamide ( t O2DO2AM Pip ) pendant arms and a detailed characterization of the corresponding Mn(II) complexes. The X-ray structure of [Mn( t O2DO2A )(H 2 O)]·2H 2 O shows that the metal ion is coordinated by six donor atoms of the macrocyclic ligand and one water molecule, to result in seven-coordination. The Cu(II) analogue presents a distorted octahedral coordination environment. The protonation constants of the ligands and the stability constants of the complexes formed with Mn(II) and other biologically relevant metal ions (Mg(II), Ca(II), Cu(II) and Zn(II)) were determined using potentiometric titrations ( I = 0.15 M NaCl, T = 25 °C). The conditional stabilities of Mn(II) complexes at pH 7.4 are comparable to those reported for the cyclen-based t DO2A 2- ligand. The dissociation of the Mn(II) chelates were investigated by evaluating the rate constants of metal exchange reactions with Cu(II) under acidic conditions ( I = 0.15 M NaCl, T = 25 °C). Dissociation of the [Mn( t O2DO2A )(H 2 O)] complex occurs through both proton- and metal-assisted pathways, while the [Mn( t O2DO2AM Pip )(H 2 O)] analogue dissociates through spontaneous and proton-assisted mechanisms. The Mn(II) complex of t O2DO2A 2- is remarkably inert with respect to its dissociation, while the amide analogue is significantly more labile. The presence of a water molecule coordinated to Mn(II) imparts relatively high relaxivities to the complexes. The parameters determining this key property were investigated using 17 O NMR (Nuclear Magnetic Resonance) transverse relaxation rates and 1 H nuclear magnetic relaxation dispersion (NMRD) profiles.

Regular monitoring of arsenic concentrations in water sources is essential due to the severe health effects. Our goal was to develop a rapidly responding, sensitive and stable sensing layer for the detection of arsenic. We have designed flagellin-based arsenic binding proteins capable of forming stable filament structures with high surface binding site densities. The D3 domain of Salmonella typhimurium flagellin was replaced with an arsenic-binding peptide motif of different bacterial ArsR transcriptional repressor factors. We have shown that the fusion proteins developed retain their polymerization ability and have thermal stability similar to that of wild-type filament. The strong arsenic binding capacity of the monomeric proteins was confirmed by isothermal titration calorimetry (ITC), and dissociation constants (K d ) of a few hundred nM were obtained for all three variants. As-binding fibers were immobilized on the surface of a gold electrode and used as a working electrode in cyclic voltammetry (CV) experiments to detect inorganic arsenic near the maximum allowable concentration (MAC) level. Based on these results, it can be concluded that the stable arsenic-binding flagellin variant can be used as a rapidly responding, sensitive, but simple sensing layer in a field device for the MAC-level detection of arsenic in natural waters.

Coupling reactions of O-peracylated 2,6-anhydro-aldose tosylhydrazones (C-(β-d-glycopyranosyl)formaldehyde tosylhydrazones) with tetrazoles were studied under metal-free conditions using thermic or microwave activation in the presence of different bases. The reactions proved highly regioselective and gave the corresponding, up-to-now unknown 2-β-d-glycopyranosylmethyl-2H-tetrazoles in 7-67% yields. The method can be applied to get new types of disaccharide mimetics, 5-glycosyl-2-glycopyranosylmethyl-2H-tetrazoles, as well. Galectin binding studies with C-(β-d-galactopyranosyl)formaldehyde tosylhydrazone and 2-(β-d-galactopyranosylmethyl)-5-phenyl-2H-tetrazole revealed no significant inhibition of any of these lectins.

Invited for the cover of this issue are Jean-François Morfin and Éva Tóth at the CNRS in Orléans, and their collaborators from University of Debrecen, University of Coimbra and Université de Toulouse. The image depicts that when an amphiphilic compound is intravenously injected, monomer, pre-micellar and micellar forms can co-exist in the blood and have different affinities for amyloid peptides. Read the full text of the article at 10.1002/chem.202004000., (© 2021 Wiley-VCH GmbH.)

The stability constants of Mn 2+ complexes with ligands containing a trans-1,2-cyclobutanediamine spacer functionalized with picolinate and/or carboxylate functions were determined using potentiometric titrations (25 °C, 0.1 M KCl). The stability constant of the complex with a hexadentate ligand containing four acetate groups (L1 4- , log K MnL = 10.26) is improved upon replacing one (L2 4- , log K MnL = 14.71) or two (L3 4- , log K MnL = 15.81) carboxylate groups with picolinates. The [Mn(L1)] 2- complex contains a water molecule coordinated to the metal ion in aqueous solutions, as evidenced by 1 H NMRD studies and 17 O chemical shifts and transverse relaxation rates. The 1 H relaxivities determined at 60 MHz (3.3 and 2.4 mM -1 s -1 at 25 and 37 °C, respectively) are comparable to those of monohydrated complexes such as [Mn(edta)] 2- . The exchange rate of the inner-sphere water molecule (k = 248 × 10 6 s -1 ) is slightly lower than that of the edta 4- analogue. DFT calculations (M11/def2-TZVP) suggest that the water exchange reaction follows a dissociatively activated mechanism, providing activation parameters in reasonably good agreement with the experimental data. DFT calculations also show that the 17 O hyperfine coupling constant A/ℏ is affected slightly by changes in the Mn-O water distance and the orientation of the water molecule with respect to the Mn-O vector.

We report the Mn(II) complexes with two pyclen-based ligands (pyclen = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene) functionalized with acetate pendant arms at either positions 3,6 ( 3,6-PC2A ) or 3,9 ( 3,9-PC2A ) of the macrocyclic fragment. The 3,6-PC2A ligand was synthesized in five steps from pyclen oxalate by protecting one of the secondary amine groups of pyclen using Alloc protecting chemistry. The complex with 3,9-PC2A is characterized by a higher thermodynamic stability [log K MnL = 17.09(2)] than the 3,6-PC2A analogue [log K MnL = 15.53(1); 0.15 M NaCl]. Both complexes contain a water molecule coordinated to the metal ion, which results in relatively high 1 H relaxivities ( r 1p = 2.72 and 2.91 mM -1 s -1 for the complexes with 3,6-PC2A and 3,9-PC2A , respectively, at 25 °C and 0.49 T). The coordinated water molecule displays fast exchange kinetics with the bulk in both cases; the rates ( k ex 298 ) are 140 × 10 6 and 126 × 10 6 s -1 for [Mn( 3,6-PC2A )(H 2 O)] and [Mn( 3,9-PC2A )(H 2 O)], respectively. The two complexes were found to be remarkably inert with respect to their dissociation, with half-lives of 63 and 21 h, respectively, at pH = 7.4 in the presence of excess Cu(II). The r 1p values recorded in blood serum remain constant at least over a period of 120 h. Cyclic voltammetry experiments show irreversible oxidation features shifted to higher potentials with respect to [Mn(EDTA)(H 2 O)] 2- (H 4 EDTA = ethylenediaminetetraacetic acid) and [Mn(PhDTA)(H 2 O)] 2- (H 4 PhDTA = phenylenediamine- N , N , N ', N '-tetraacetic acid), indicating that the PC2A complexes reported here have a lower tendency to stabilize Mn(III). The superoxide dismutase activity of the Mn(II) complexes was tested using the xanthine/xanthine oxidase/ p -nitro blue tetrazolium chloride assay at pH = 7.8. The Mn(II) complexes of 3,6-PC2A and 3,9-PC2A are capable of assisting decomposition of the superoxide anion radical. The kinetic rate constant of the complex of 3,9-PC2A is smaller by 1 order of magnitude than that of 3,6-PC2A .

Reliable measurement of the binding kinetics of low molecular weight analytes to their targets is still a challenging task. Often, the introduction of labels is simply impossible in such measurements, and the application of label-free methods is the only reliable choice. By measuring the binding kinetics of Ni(II) ions to genetically modified flagellin layers, we demonstrate that: (1) Grating-Coupled Interferometry (GCI) is well suited to resolve the binding of ions, even at very low protein immobilization levels; (2) it supplies high quality kinetic data from which the number and strength of available binding sites can be determined, and (3) the rate constants of the binding events can also be obtained with high accuracy. Experiments were performed using a flagellin variant incorporating the C-terminal domain of the nickel-responsive transcription factor NikR. GCI results were compared to affinity data from titration calorimetry. We found that besides the low-affinity binding sites characterized by a micromolar dissociation constant (K d ), tetrameric FliC-NikR C molecules possess high-affinity binding sites with K d values in the nanomolar range. GCI enabled us to obtain real-time kinetic data for the specific binding of an analyte with molar mass as low as 59 Da, even at signals lower than 1 pg/mm 2 .

The photophysical properties of Eu3+ and Tb3+ complexes of DOTAGA and DO3A-monoamide conjugates of the Pittsburgh compound B (PiB) chromophore, prepared using linkers of different lengths and flexibilities, and which form stable negatively charged (LnL1), and uncharged (LnL2) complexes, respectively, were studied as potential probes for optical detection of amyloid aggregates. The phenylbenzothiazole (PiB) moiety absorbs light at wavelengths longer than 330 nm with a high molar absorption coefficient in both probes, and acts as an antenna in these systems. The presence of the luminescent Ln3+ ion quenches the excited states of PiB through an energy transfer process from the triplet state of PiB to the metal centre, and structured emission is seen from Eu3+ and Tb3+. The luminescence study indicates the presence of a 5D4 → T1 back transfer process in the Tb3+ complexes. It also provides insights on structural properties of the Eu3+ complexes, such as the high symmetry environment of the Eu3+ ion in a single macrocyclic conformation and the presence of one water molecule in its inner coordination sphere. The overall quantum yield of luminescence of EuL1 is higher than for EuL2. However, their low values reflect the low overall sensitization efficiency of the energy transfer process, which is a consequence of the large distances between the metal center and the antenna, especially in the EuL2 complex. DFT calculations confirmed that the most stable conformation of the Eu3+ complexes involves a combination of a square antiprismatic (SAP) geometry of the chelate and an extended conformation of the linker. The large calculated average distances between the metal center and the antenna point to the predominance of the Förster energy transfer mechanism, especially for EuL2. This study provides insights into the behavior of amyloid-targeted Ln3+ complexes as optical probes, and contributes towards their rational design.

Low-density lipoprotein (LDL)-mimetic lipid nanoparticles (LNPs), decorated with MRI contrast agents and fluorescent dyes, were prepared by the covalent attachment of apolipoprotein-mimetic peptide ( P ), Gd(iii)-chelate ( Gd ), and sulforhodamine B ( R ) moieties on the LNP surface. The functionalized LNPs were prepared using the amide-forming potassium acyltrifluoroborate (KAT) ligation reaction. The KAT groups on the surface of LNPs were allowed to react with the corresponding hydroxylamine (HA) derivatives of P and Gd to provide bi-functionalized LNPs ( PGd-LNP ). The reaction proceeded with excellent yields, as observed by ICP-MS (for B and Gd amounts) and MALDI-TOF-MS data, and did not alter the morphology of the LNPs (mean diameter: ca. 50 nm), as shown by DLS and cryoTEM analyses. With the help of the efficient KAT ligation, a high payload of Gd(iii)-chelate on the PGd-LNP surface ( ca. 2800 Gd atoms per LNP) was successfully achieved and provided a high r 1 relaxivity ( r 1 = 22.0 s -1 mM -1 at 1.4 T/60 MHz and 25 °C; r 1 = 8.2 s -1 mM -1 at 9.4 T/400 MHz and 37 °C). This bi-functionalized PGd-LNP was administered to three atherosclerotic apoE -/- mice to reveal the clear enhancement of atherosclerotic plaques in the brachiocephalic artery (BA) by MRI, in good agreement with the high accumulation of Gd in the aortic arch as shown by ICP-MS. The parallel in vivo MRI and ex vivo studies of whole mouse cryo-imaging were performed using triply functionalized LNPs with P , Gd , and R ( PGdR-LNP ). The clear presence of atherosclerotic plaques in BA was observed by ex vivo bright field cryo-imaging, and they were also observed by high emission fluorescent imaging. These directly corresponded to the enhanced tissue in the in vivo MRI of the identical mouse., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

With the aim of developing new molecular theranostic agents, a π-extended Zn(II) porphyrin as photosensitizer for photodynamic therapy (PDT) linked to two GdDOTA-type complexes for magnetic resonance imaging (MRI) detection was synthesized. The relaxivity studies revealed a much higher relaxivity value per Gd ion for this medium sized molecule (19.32 mM -1 s -1 at 20 MHz and 298 K) compared to clinical contrast agents-a value which strongly increases in the presence of bovine serum albumin, reaching 25.22 mM -1 s -1 . Moreover, the photophysical studies showed the strong ability of the molecule to absorb light in the deep red (670 nm, ε ≈ 60000 M -1 cm -1 ) and in the near-infrared following two-photon excitation (920 nm, σ 2 ≈ 650 GM). The conjugate is also able to generate singlet oxygen, with a quantum yield of 0.58 in DMSO. Promising results were obtained in cellular studies, demonstrating that the conjugate is internalized in HeLa cells at micromolar concentration and leads to 70% of cell death following 30 min irradiation at 660 nm. These results confirm the potential of the designed molecule as an imaging and therapeutic agent.

Type 2 diabetes mellitus (T2DM), one of the most common metabolic diseases, is characterized by insulin resistance and inadequate insulin secretion of β cells. Glycogen phosphorylase (GP) is the key enzyme in glycogen breakdown, and contributes to hepatic glucose production during fasting or during insulin resistance. Pharmacological GP inhibitors are potential glucose lowering agents, which may be used in T2DM therapy. A natural product isolated from the cultured broth of the fungal strain No. 138354, called 2,3-bis(4-hydroxycinnamoyloxy)glutaric acid (FR258900), was discovered a decade ago. In vivo studies showed that FR258900 significantly reduced blood glucose levels in diabetic mice. We previously showed that GP inhibitors can potently enhance the function of β cells. The purpose of this study was to assess whether an analogue of FR258900 can influence β cell function. BF142 (Meso-Dimethyl 2,3-bis[(E)-3-(4-acetoxyphenyl)prop-2-enamido]butanedioate) treatment activated the glucose-stimulated insulin secretion pathway, as indicated by enhanced glycolysis, increased mitochondrial oxidation, significantly increased ATP production, and elevated calcium influx in MIN6 cells. Furthermore, BF142 induced mTORC1-specific phosphorylation of S6K, increased levels of PDX1 and insulin protein, and increased insulin secretion. Our data suggest that BF142 can influence β cell function and can support the insulin producing ability of β cells., Competing Interests: The authors have declared that no competing interests exist.

The environmental monitoring of Ni is targeted at a threshold limit value of 0.34 μM, as set by the World Health Organization. This sensitivity target can usually only be met by time-consuming and expensive laboratory measurements. There is a need for inexpensive, field-applicable methods, even if they are only used for signaling the necessity of a more accurate laboratory investigation. In this work, bioengineered, protein-based sensing layers were developed for Ni detection in water. Two bacterial Ni-binding flagellin variants were fabricated using genetic engineering, and their applicability as Ni-sensitive biochip coatings was tested. Nanotubes of mutant flagellins were built by in vitro polymerization. A large surface density of the nanotubes on the sensor surface was achieved by covalent immobilization chemistry based on a dithiobis(succimidyl propionate) cross-linking method. The formation and density of the sensing layer was monitored and verified by spectroscopic ellipsometry and atomic force microscopy. Cyclic voltammetry (CV) measurements revealed a Ni sensitivity below 1 μM. It was also shown that, even after two months of storage, the used sensors can be regenerated and reused by rinsing in a 10 mM solution of ethylenediaminetetraacetic acid at room temperature.

The search for more biocompatible alternatives to Gd 3+ -based MRI agents, and the interest in 52 Mn for PET imaging call for ligands that form inert Mn 2+ chelates. Given the labile nature of Mn 2+ , high inertness is challenging to achieve. The strongly preorganized structure of the 2,4-pyridyl-disubstituted bispidol ligand L 1 endows its Mn 2+ complex with exceptional kinetic inertness. Indeed, MnL 1 did not show any dissociation for 140 days in the presence of 50 equiv. of Zn 2+ (37 °C, pH 6), while recently reported potential MRI agents MnPyC3A and MnPC2A-EA have dissociation half-lives of 0.285 h and 54.4 h under similar conditions. In addition, the relaxivity of MnL 1 (4.28 mm -1  s -1 at 25 °C, 20 MHz) is remarkable for a monohydrated, small Mn 2+ chelate. In vivo MRI experiments in mice and determination of the tissue Mn content evidence rapid renal clearance of MnL 1 . Additionally, L 1 could be radiolabeled with 52 Mn and the complex revealed good stability in biological media., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We report a detailed study of the thermodynamic stability and dissociation kinetics of lanthanide complexes with two ligands containing a cyclen unit, a methyl group, a picolinate arm, and two acetate pendant arms linked to two nitrogen atoms of the macrocycle in either cis (1,4-H 3 DO2APA) or trans (1,7-H 3 DO2APA) positions. The stability constants of the Gd 3+ complexes with these two ligands are very similar, with log K GdL values of 16.98 and 16.33 for the complexes of 1,4-H 3 DO2APA and 1,7-H 3 DO2APA, respectively. The stability constants of complexes with 1,4-H 3 DO2APA follow the usual trend, increasing from log K LaL = 15.96 to log K LuL = 19.21. However, the stability of [Ln(1,7-DO2APA)] complexes decreases from log K = 16.33 for Gd 3+ to 14.24 for Lu 3+ . The acid-catalyzed dissociation rates of the Gd 3+ complexes differ by a factor of ∼15, with rate constants ( k 1 ) of 1.42 and 23.5 M -1 s -1 for [Gd(1,4-DO2APA)] and [Gd(1,7-DO2APA)], respectively. This difference is magnified across the lanthanide series to reach a 5 orders of magnitude higher k 1 for [Yb(1,7-DO2APA)] (1475 M -1 s -1 ) than for [Yb(1,4-DO2APA)] (5.79 × 10 -3 M -1 s -1 ). The acid-catalyzed mechanism involves the protonation of a carboxylate group, followed by a cascade of proton-transfer events that result in the protonation of a nitrogen atom of the cyclen unit. Density functional theory calculations suggest a correlation between the strength of the Ln-O carboxylate bonds and the kinetic inertness of the complex, with stronger bonds providing more inert complexes. The 1 H NMR resonance of the coordinated water molecule in the [Yb(1,7-DO2APA)] complex at 176 ppm provides a sizable chemical exchange saturation transfer effect thanks to a slow water exchange rate of (15.9 ± 1.6) × 10 3 s -1 .

Toxicity concerns related to Gd(III)-based magnetic resonance imaging (MRI) agents prompted an intensive research toward their replacement by complexes of essential metal ions, like Mn(II). Here, we report a macrocyclic chelate, [Mn(PC2A-BP)], which possesses high thermodynamic stability (log K MnL = 14.86 and pMn=8.35) and kinetic inertness ( t 1/2 pH=7.4 = 286.2 h) as well as as remarkable relaxivity ( r 1p = 23.5 mM -1 s -1 , 0.49 T, 37 °C) in the presence of human serum albumin, allowing a significant MRI signal intensity increase in the vasculature even at low dose (25 μmol/kg) of the complex.

The 1,7-diacetate-4,10-diacetamide substituted 1,4,7,10-tetraazacyclododecane structural unit is common to several responsive Magnetic Resonance Imaging (MRI) contrast agents (CAs). While some of these complexes (agents capable of sensing fluctuations in Zn 2+ , Ca 2+ etc. ions) have already been tested in vivo, the detailed physico-chemical characterization of such ligands have not been fully studied. To fill this gap, we synthesized a representative member of this ligand family possessing two acetate and two n-butylacetamide pendant side-arms (DO2A2M nBu  = 1,4,7,10-tetraazacyclodoecane-1,7-di(acetic acid)-4,10-di(N-butylacetamide)), and studied its complexation properties with some essential metal and a few lanthanide(III) (Ln(III)) ions. Our studies revealed that the ligand basicity, the stability of metal ion complexes, the trend of stability constants along the Ln(III) series, the formation rates of the Ln(III) complexes and the exchange rate of the bound water molecule in the Gd(III) complex fell between those of Ln(DOTA) - and Ln(DOTA-tetra(amide)) 3+ complexes (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, DOTAM = 1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane). The only exception is the stability of Cu(DO2A2M nBu ) which was found to be only slightly lower than that of Cu(DOTA) 2- (log K CuL  = 19.85 vs. 21.98). This is likely reflects exclusive coordination of the negatively charged acetate donor atoms to the Cu 2+ ion forming an octahedral complex with the amides remaining uncoordinated. The only anomaly observed during the study was the rates of acid assisted dissociation of the Ln(III) complexes, which occur at a rate similar to those observed for the Ln(DOTA) - complexes. These data indicate that even though the Ln(DO2A2M nBu ) + complexes have lower thermodynamic stabilities, their kinetic inertness should be sufficient for in vivo use., Competing Interests: Declaration of competing interest The authors declare no conflict of interest., (Copyright © 2020 The Author(s). Published by Elsevier Inc. All rights reserved.)

Gold nanoparticles (AuNPs) are interesting for the design of new cancer theranostic tools, mainly due to their biocompatibility, easy molecular vectorization, and good biological half-life. Herein, we report a gold nanoparticle platform as a bimodal imaging probe, capable of coordinating Gd 3+ for Magnetic Resonance Imaging (MRI) and 67 Ga 3+ for Single Photon Emission Computed Tomography (SPECT) imaging. Our AuNPs carry a bombesin analogue with affinity towards the gastrin releasing peptide receptor (GRPr), overexpressed in a variety of human cancer cells, namely PC3 prostate cancer cells. The potential of these multimodal imaging nanoconstructs was thoroughly investigated by the assessment of their magnetic properties, in vitro cellular uptake, biodistribution, and radiosensitisation assays. The relaxometric properties predict a potential T 1 - and T 2 - MRI application. The promising in vitro cellular uptake of 67 Ga/Gd-based bombesin containing particles was confirmed through biodistribution studies in tumor bearing mice, indicating their integrity and ability to target the GRPr. Radiosensitization studies revealed the therapeutic potential of the nanoparticles. Moreover, the DOTA chelating unit moiety versatility gives a high theranostic potential through the coordination of other therapeutically interesting radiometals. Altogether, our nanoparticles are interesting nanomaterial for theranostic application and as bimodal T 1 - and T 2 - MRI / SPECT imaging probes.

Thanks to their versatile magnetic and luminescence features, lanthanide complexes have gained a central position in biomedical imaging as magnetic resonance imaging (MRI) contrast agents and optical imaging probes. In addition, appropriate chemical design allows modification of the magnetic relaxation properties of Gd III complexes and the optical properties of visible- or near-infrared (NIR)-emitting lanthanide chelates upon interaction with various biomarkers, which makes them ideal candidates for the creation of responsive agents. In this Forum Article, we demonstrate such design principles as well as the difficulties encountered in the context of neurotransmitter (NT) detection. Lanthanide(III) complexes of a macrocyclic ligand incorporating a benzophenone chromophore and a monoazacrown ether (Ln L 3 ) have been synthesized as responsive probes to monitor amino acid NTs either in MRI (Ln = Gd) or in NIR optical detection (Ln = Nd or Yb). The parameters characterizing the water exchange and rotational dynamics of the gadolinium(III) complex were assessed by 17 O NMR and 1 H NMRD. In the presence of zwitterionic NTs, the inner-sphere water molecule is replaced by the carboxylate function of the NTs in the gadolinium(III) complex, leading to a decrease of the longitudinal relaxivity from 6.7 to 2-2.5 mM -1 s -1 (300 MHz and 37 °C). The apparent affinity constants range from K a = 35 for γ-aminobutyric acid (GABA) to 80 M -1 for glycine and glutamate, and there is no selectivity with respect to hydrogen carbonate ( K a = 232; pH 7.4). The gadolinium(III) complex interacts with human serum albumin (HSA), resulting in a 60% increase in the relaxivity (20 MHz, 37 °C) in the absence of NTs. The HSA-bound complex, however, was revealed to be less responsive to NTs because of displacement of the Gd III -bound water by HSA, which was confirmed by the hydration number calculated from luminescence lifetimes of the HSA-bound europium(III) complex. The creation of an imaging agent suitable for NIR detection of NTs for an enhanced sensitivity in biological systems using the benzophenone (BP) moiety as the sensitizer of lanthanide luminescence was also attempted. Upon excitation at 300 nm of the BP chromophore in aqueous solutions of Nd L 3 and Yb L 3 , characteristic NIR emissions of Nd III and Yb III were observed because of 4 F 3/2 → 4 I J ( J = 9 / 2 - 13 / 2 ) and 2 F 5/2 → 2 F 7/2 transitions, respectively, indicating that this chromophore is a suitable antenna. Despite these promising results, luminescence titrations of Nd III and Yb III complexes with NTs were not conclusive because of chemical conversion of the ligand triggered by light, preventing quantitative analysis. The observed photochemical reaction of the ligand is strongly dependent on the nature of the lanthanide chelated; it is considerably slowed down in the presence of Nd III and Eu III .

In an effort to explore novel ligand scaffolds for stable and inert lanthanide complexation in magnetic resonance imaging contrast agent research, three chiral ligands containing a highly rigid (1 S ,2 S )-1,2-cyclobutanediamine spacer and different number of acetate and picolinate groups were efficiently synthesized. Potentiometric studies show comparable thermodynamic stability for the Gd 3+ complexes formed with either the octadentate (L3) 4- bearing two acetate or two picolinate groups or the heptadentate (L2) 4- analogue bearing one picolinate and three acetate groups (log K GdL = 17.41 and 18.00 for [Gd(L2)] - and [Gd(L3)] - , respectively). In contrast, their dissociation kinetics is revealed to be very different: the monohydrated [Gd(L3)] - is considerably more labile, as a result of the significant kinetic activity of the protonated picolinate function, as compared to the bishydrated [Gd(L2)] - . This constitutes an uncommon example in which lowering ligand denticity results in a remarkable increase in kinetic inertness. Another interesting observation is that the rigid ligand backbone induces an unusually strong contribution of the spontaneous dissociation to the overall decomplexation process. Thanks to the presence of two inner-sphere water molecules, [Gd(L2)] - is endowed with high relaxivity ( r 1 = 7.9 mM -1 s -1 at 20 MHz, 25 °C), which is retained in the presence of large excess of endogenous anions, excluding ternary complex formation. The water exchange rate is similar for [Gd(L3)] - and [Gd(L2)] - , while it is 1 order of magnitude higher for the trishydrated tetraacetate analogue [Gd(L1)] - ( k ex 298 = 8.1, 10, and 127 × 10 6 s -1 , respectively). A structural analysis via density functional theory calculations suggests that the large bite angle imposed by the rigid (1 S ,2 S )-1,2-cyclobutanediamine spacer could allow the design of ligands based on this scaffold with suitable properties for the coordination of larger metal ions with biomedical applications.

Many smart magnetic resonance imaging (MRI) probes provide response to a biomarker based on modulation of their rotational correlation time. The magnitude of such MRI signal changes is highly dependent on the magnetic field and the response decreases dramatically at high fields (>2 T). To overcome the loss of efficiency of responsive probes at high field, with fast-field cycling magnetic resonance imaging (FFC-MRI) we exploit field-dependent information rather than the absolute difference in the relaxation rate measured in the absence and in the presence of the biomarker at a given imaging field. We report here the application of fast field-cycling techniques combined with the use of a molecular probe for the detection of Zn 2+ to achieve 166 % MRI signal enhancement at 3 T, whereas the same agent provides no detectable response using conventional MRI. This approach can be generalized to any biomarker provided the detection is based on variation of the rotational motion of the probe., (© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

For the development of redox responsive MRI probes based on the MnIII/MnII couple, stable complexation of both reduced and oxidized forms of the metal ion and appropriate tuning of the redox potential in the biologically relevant range are key elements. The water soluble fluorinated Mn-porphyrin derivative Mn-3 satisfies both requirements. In aqueous solutions, it can reversibly switch between MnIII/MnII oxidation states. In the presence of ascorbic acid or β-mercaptoethanol, the MnIII form undergoes reduction, which is slowly but fully reversed in the presence of air oxygen. A UV-Vis kinetic study of MnIII/MnII reduction under oxygen-free conditions yielded second-order rate constants, k2, of 46.1 M-1 s-1 and 13.8 M-1 s-1 for the reaction with ascorbic acid and β-mercaptoethanol, respectively. This could correspond, in the absence of oxygen, to a half-life of a few minutes in blood plasma and a few seconds in circulating immune cells where ascorbic acid reaches 20-40 μM and a few mM concentrations, respectively. In contrast to expectations based on the redox potential, reduction with glutathione or cysteine does not occur. It is prevented by the coordination of the glutathione carboxylate group(s) to MnIII in the axial position, as was evidenced by NMR data. Therefore, MnIII-3 acts as an ascorbate specific turn-on MRI probe, which in turn can be re-oxidized by oxygen. The relaxivity increase from the oxidized to the reduced form is considerably improved at medium frequencies (up to 80 MHz) with respect to the previously studied Mn-TPPS4 analogues; at 20 MHz, it amounts to 150%. No in vitro cytotoxicity is detectable for Mn-3 in the typical MRI concentration range. Finally, 19F NMR resonances of MnIII-3 are relatively sharp which could open further opportunities to exploit such complexes as paramagnetic 19F NMR probes.

A molecular theranostic agent designed for photodynamic therapy (PDT) treatment in the near-infrared and for imaging tissue tumors with magnetic resonance imaging (MRI) is reported. It consists of a linear π-conjugated Zn(II) porphyrin dimer linked at each extremity to a GdDOTA-type complex. This agent has shown very promising potential for PDT applications with good singlet oxygen generation in DMSO and high linear absorption in the near-infrared (λ max = 746 nm, ε ≈ 10 5 M -1 cm -1 ). Moreover, this molecule has a propensity for two-photon excited PDT with high two-photon cross sections (∼8000 GM in 880-930 nm range), which should allow for deeper tumor treatments and higher spatial precision as compared to conventional one-photon PDT. Regarding the MRI contrast agent properties, the molecule has shown superior relaxivity (14.4 mM -1 s -1 at 40 MHz, 298 K) in comparison to clinical contrast agents and the ability to be internalized in cells, thanks to its amphiphilic character. Irradiation of HeLa cells using either one-photon (740 nm) or two-photon excitation (910 nm) has led in both cases to important cell death.

We propose quantitative assessment of zinc by combining nuclear and MR imaging. We use a cocktail of a Gd3+-complex providing a Zn2+-dependent MRI response and its 165Er3+ analogue allowing for concentration assessment. 165Er is readily obtained in a cyclotron and purified, which is indispensable for successful quantification of metal ions.

Self-aggregating calix[4]arenes carrying four DOTA ligands on the upper rim for stable complexation of paramagnetic Gd III -ions have already been proposed as MRI probes. In this work, we investigate the luminescence properties of Tb III -DOTA-calix[4]arene-4OPr containing four propyl-groups and compare them with those of the analog substituted with a phthalimide chromophore (Tb III -DOTA-calix[4]arene-3OPr-OPhth). We show that, given its four aromatic rings, the calix[4]arene core acts as an effective sensitizer of Tb-centered luminescence. Substituents on the lower rim can modulate the aggregation behavior, which in turn determines the luminescence properties of the compounds. In solid state, the quantum yield of the phthalimide derivative is almost three times as high as that of the propyl-functionalized analog demonstrating a beneficial role of the chromophore on Tb-luminescence. In solution, however, the effect of the phthalimide group vanishes, which we attribute to the large distance between the chromophore and the lanthanide, situated on the opposite rims of the calix[4]arene. Both quantum yields and luminescence lifetimes show clear concentration dependence in solution, related to the strong impact of aggregation on the luminescence behavior. We also evidence the variability in the values of the critical micelle concentration depending on the experimental technique. Such luminescent calix[4]arene platforms accommodating stable lanthanide complexes can be considered valuable building blocks for the design of dual MR/optical imaging probes.

Surface PEGylation of nanoparticles designed for biomedical applications is a common and straightforward way to stabilize the materials for in vivo administration and to increase their circulation time. This strategy becomes less trivial when MRI active porous nanomaterials are concerned as their function relies on water/proton-exchange between the pores and bulk water. Here we present a comprehensive study on the effects of PEGylation on the relaxometric properties of nanozeolite LTL (dimensions of 20 × 40 nm) ion-exchanged with paramagnetic Gd III ions. We evidence that as long as the surface grafting density of the PEG chains does not exceed the "mushroom" regime (conjugation of up to 6.2 wt % of PEG), Gd-LTL retains a remarkable longitudinal relaxivity (38 s -1 mM -1 at 7 T and 25 °C) as well as the pH-dependence of the longitudinal and transverse relaxation times. At higher PEG content, the more compact PEG layer (brush regime) limits proton/water diffusion and exchange between the interior of LTL and the bulk, with detrimental consequences on relaxivity. Furthermore, PEGylation of Gd-LTL dramatically decreases the leakage of toxic Gd III ions in biological media and in the presence of competing anions, which together with minimal cytotoxicity renders these materials promising probes for MRI applications.

In the search for Mn II MR and PET/MR imaging agents with optimal balance between thermodynamic stability, kinetic inertness, and relaxivity, two novel bifunctional Mn II chelators (BFMnCs) based on CDTA (trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid) were synthesized. A six-step synthesis, involving the buildup of a functionalized trans-1,2-diaminocyclohexane core, provided CuAAC-reactive 6a and 6b bearing an alkyne or azide substituent on the cyclohexane ring, respectively (CuAAC = Cu I -catalyzed azide-alkyne 1,3-dipolar cycloaddition). Thermodynamic, kinetic, and relaxometric studies were performed with 4-HET-CDTA (8a) as a "model chelator," synthesized in two steps from 6a. The protonation constants revealed that 8a is slightly less basic than CDTA and forms a Mn II complex of marginally lower thermodynamic stability (log K MnL = 13.80 vs 14.32, respectively), while the conditional stability constant is almost identical for both chelates (pMn = 8.62 vs 8.68, respectively). Kinetic assessment of the Cu II -mediated transmetalation of [Mn(4-HET-CDTA)] 2- showed that proton-assisted complex dissociation is slightly slower than for [Mn(CDTA)] 2- (k 1 = 297 vs 400 M -1 s -1 , respectively). Importantly, the dissociation half-life near physiological conditions (pH 7.4, 25 °C) underlined that [Mn(4-HET-CDTA)] 2- is ∼35% more inert (t 1/2 = 16.2 vs 12.1 h, respectively). Those findings may be accounted for by a combination of reduced basicity and increased rigidity of the ligand. Analysis of the 17 O NMR and 1 H NMRD data attributed the high relaxivity of [Mn(4-HET-CDTA)] 2- (r 1 = 4.56 mM -1 s -1 vs 3.65 mM -1 s -1 for [Mn(CDTA)] 2- ; 20 MHz, 25 °C) to slower rotational dynamics (τ R 298 = 105 ps). Additionally, the fast water exchange of the complex correlates well with the value reported for [Mn(CDTA)] 2- (k ex 298 = 17.6 × 10 7 vs 14.0 × 10 7 s -1 , respectively). Given the exquisite compromise between thermodynamic stability, kinetic inertness, and relaxivity achieved by [Mn(4-HET-CDTA)] 2- , appropriately designed CuAAC-conjugates of 6a/6b are promising precursors for the preparation of targeted, bioresponsive, or high relaxivity manganese-based PET/MR tracers ( 52g/55 Mn II ) and MR contrast agents (Mn II ).