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In the vast family of two-dimensional (2D) van der Waals materials, 2D magnets have unique advantages in the experimental realization of numerous theoretical works and fundamental exploration of high-performance spintronic devices in the 2D limit. Fe3GeTe2 (FGT), as a typical 2D metallic ferromagnet, is a promising candidate to achieve functional devices at room temperature due to its relatively high Curie temperature (TC) and strong perpendicular magnetic anisotropy (PMA). Here, we investigated self-modulated magnetic properties of FGT nanoflakes in an ambient environment by anomalous Hall effect (AHE). Unique double square-shaped hysteresis loops are observed in oxidized Fe3GeTe2 nanoflakes induced by ambient effects. We propose a phenomenological model based on antiferromagnetic/ferromagnetic coupling in the FGT nanoflakes with oxidized surface layers to elucidate this observation. Compared with a single square-shaped hysteresis loop of well-preserved FGT, double square-shaped hysteresis loops exhibit four-resistance plateaus, which can be regarded as a kind of media for information storage or sensing, showing potential applications in next-generation spintronic devices. [ABSTRACT FROM AUTHOR]

Research has been conducted on Escherichia coli lactose permease which transports protons and lactose across the plasma membrane. The effect of the electrochemical gradient on the permease has been investigated and the results indicate the importance of the reaction-induced spectroscopy in the transmembrane transport study.

The electron spin-echo envelope modulation (ESEEM) technique is used to investigate the formation mechanism of the mesoporous material MCM-41. The ESEEM technique is an effective tool for probing changes in the local environment of surfactant-like spin-probes during the formation of MCM-41.

Issues concerning the use of the spin-probe 5-doxyl stearic acid to reveal the mechanism of formation of the hexagonal mesoporous material MCM-41 are reported.

The magnetic heating effect under alternating magnetic fields (AMFs) has recently gained attention in catalysis due to its potential to greatly boost catalytic activities by providing localized heating around magnetic nanoparticles. However, nanoparticles still suffer from low magnetic heating efficiency due to their low magnetic anisotropy and thermal fluctuation, which is a key barrier in the wide application of AMF-assisted catalysis. Herein, by introducing the pinning effect of ferromagnetic/antiferromagnetic (FM/AFM) coupling, NiO/NiOOH (AFM/FM) core-shell nanoparticles exhibit significantly enhanced oxygen evolution reaction activity and resistance to thermal fluctuations under AMF, compared to NiOOH nanoparticles. Notably, magnetized NiO/NiOOH nanoparticles provide an overpotential of 186 mV at 10 mA cm -2 , outperforming unmagnetized ones (218 mV) under the same conditions, further emphasizing the prominent role of the pinning effect in enhanced magnetic heating efficiency. This work provides valuable inspiration to design advanced magnetic catalysts and meet the challenge of the development of AMF-assisted catalysis.

Topological spin textures are of great significance in magnetic information storage and spintronics due to their high storage density and low drive current. In this work, the transformation of magnetic configuration from chaotic labyrinth domains to uniform stripe domains was observed in MnFe 1- x Co x Ge magnets. This change occurs due to the noncollinear magnetic structure switching to a uniaxial ferromagnetic structure with increasing Co content, as identified by neutron diffraction results and Lorentz transmission electron microscopy (L-TEM). Of utmost importance, a hexagonal lattice of high-density robust type-II magnetic bubble lattice was established for x = 0.8 through out-of-plane magnetic field stimulation and field-cooling. The dimensions of the type-II magnetic bubbles were found to be tuned by the sample thickness. Therefore, the stabilization of complex magnetic spin textures, associated with enhanced uniaxial ferromagnetic interaction and magnetic dipole-dipole interaction in MnFe 1- x Co x Ge through magnetic structure manipulation, as further confirmed by the micromagnetic simulations, will provide a convenient and efficient strategy for designing topological spin textures with potential applications in spintronic devices.

Electrically generated spin-orbit torque (SOT) has emerged as a powerful pathway to control magnetization for spintronic applications including memory, logic, and neurocomputing. However, the requirement of external magnetic fields, together with the ultrahigh current density, is the main obstacle for practical SOT devices. In this paper, we report that the field-free SOT-driven magnetization switching can be successfully realized by interfacial ion absorption in perpendicular Ta/CoFeB/MgO multilayers. Besides, the tunable SOT efficiency exhibits a strong dependence on interfacial Ti insertion thicknesses. Polarized neutron reflection measurements demonstrate the existence of canted magnetization with Ti inserted, which leads to deterministic magnetization switching. In addition, interfacial characterization and first-principles calculations reveal that B absorption by the Ti layer is the main cause behind the enhanced interfacial transparency, which determines the tunable SOT efficiency. Our findings highlight an attractive scheme to a purely electric control spin configuration, enabling innovative designs for SOT-based spintronics via interfacial engineering.

Molecular adsorption on solids is inevitable and has significant influences on the wettability of materials, while the tuning mechanism of the wettability from molecular adsorption is yet to be understood. Using molecular dynamics (MD) simulations, we comprehensively studied the relation between the wettability of the TiO 2 surface and the adsorption of water and carboxylic acid molecules. Our results reveal that the increasing amount of surface hydroxyl groups from the decomposition adsorption of H 2 O increases the hydrophilicity of TiO 2 , providing molecular-level evidence for the previously proposed mechanism of photo-induced hydrophilicity. By contrast, the surface wettability becomes tunable with water contact angles changing from 0 to ∼130° through length adjustment of the adsorbed carboxylic acids. The TiO 2 surface is hydrophilic with the adsorption of short-alkyl-chain carboxylic acids (e.g., HCOOH) and becomes hydrophobic when longer-alkyl-chain carboxylic acids (H(CH 2 ) n COOH, n > 2) are present. Furthermore, long-alkyl-chain acids also increase surface oleophilicity, while the adsorption of HCOOH and CH 3 COOH significantly enhances the oleophobicity of TiO 2 . Water molecules can also more easily penetrate the space between oily contaminants and adsorbed short acid molecules, thereby further increasing its self-cleaning capacity. The present simulations not only reveal the mechanism of wettability caused by molecular adsorption but also provide a promising method to create materials with controllable wettability and high self-cleaning efficiency.

The exploration of solid-state sodium superionic conductors with high sodium-ion conductivity, structural and electrochemical stability, and grand interface compatibility has become the key to the next-generation energy storage applications with high energy density and long cycling life. Among them, halide-based compounds exhibit great potential with the higher electronegativity of halogens than that of the sulfur element. In this work, combined with first-principles calculation and ab initio molecular dynamic simulation, the investigation of trivalent metal iodide-based Na superionic conductors C 2/ m -Na 3 XI 6 (X = Sc, Y, La, and In) was conducted, including the fast ion transport mechanism, structural stability, and interface electrochemical compatibility with electrode materials. Along with the tetrahedral-center saddle site-predominant three-dimensional octahedral-tetrahedral-octahedral diffusion network, C 2/ m -Na 3 XI 6 possesses the merits of high Na ionic conductivities of 0.36, 0.35, and 0.20 mS cm -1 for Na 3 ScI 6 , Na 3 YI 6 , and Na 3 LaI 6 , respectively. Benefiting from its structural stabilities, C 2/ m -Na 3 XI 6 exhibits lower interface reaction energy and better electrochemical compatibility in contact with both Na metal and high-voltage poly-anion (fluoro)phosphate cathode materials than those of sulfide-based ones. Our theoretical work provides rational design principles for screening and guiding iodide-based C 2/ m -Na 3 XI 6 (X = Sc, Y, La, and In) as promising Na superionic conductor candidates used in all-solid-state energy storage applications.

Reconstructing the surface-electronic-structure of catalysts for efficient electrocatalytic activity is crucial but still under intense exploration. Herein, we introduce a double-cation gradient etching technique to manipulate the electronic structure of perovskite LaCoO 3 . With the gradient dissolution of cations, the surface was reconstructed, and the perovskite/spinel heterostructure V-LCO/Co 3 O 4 (V-LCO refers to LaCoO 3 with La and Co vacancies) can be realized. Its surface-electronic-structure is effectively regulated due to the heterogeneous interface effect and abundant vacancies, resulting in a significantly enhanced activity for oxygen evolution reaction (OER). The V-LCO/Co 3 O 4 exhibits low electrochemical activation energy and 2 orders of magnitude higher carrier concentrations (1.36 × 10 21 cm -3 ) compared with LCO (6.03 × 10 19 cm -3 ). Density functional theory (DFT) calculation unveils that the directional reconstruction of surface-electronic-structure enables the d -band center of V-LCO/Co 3 O 4 to a moderate position, endowing perfect adsorption strength for oxo groups and thus promoting the electrocatalytic activity.

Incorporation of cationic groups into polymers represents one of the most widely used strategies to prepare antibacterial materials. Sulfonium, as a typical cationic moiety, displays potent antibacterial efficacy in the form of small molecules, however, has long underperformed in polymeric systems. Herein, we developed a series of alternating polysulfoniums, where the hydrophobicity of each alternating unit can be accurately tuned by altering the monomer precursors. Excellent antibacterial activity against a broad spectrum of clinically relevant bacteria, including Methicillin-resistant Staphylococcus aureus , can be obtained in the optimal compositions with minimum bactericidal concentrations in the range of 1.25-10 μg/mL, as well as negligible hemolytic effect at polymer concentrations even up to 10000 μg/mL. Bacteria do not readily develop resistance to polysulfoniums due to the antibacterial action is possibly the membrane disrupting mechanism. This work demonstrates sulfonium-based polymers with well-defined sequences can function as a promising candidate to combat drug-resistant bacterial infection.

It is known that few Co-based superconducting compounds have been found compared with their Fe- or Ni-based counterparts. In this study, we have found superconductivity of 4 K in the LaCoSi compound for the first time. The combined analysis of neutron and synchrotron X-ray powder diffractions reveals that LaCoSi exhibits an isostructure with the known Fe-based LiFeAs superconductor, which is the first "111" Co-based superconductor. First-principles calculation shows that LaCoSi presents a quasi-two-dimensional band structure that is also similar to that of LiFeAs. The small structural distortion may be more conducive to the emergence of superconductivity in the LaCoSi compound, which provides a direction for finding new Co-based superconducting compounds.

Soft porous crystals (SPCs) with both crystallinity and flexibility have evolved as emerging materials for lots of applications. However, the development of purely organic SPCs (SPOCs) with advanced functionalities significantly lags behind. Herein, we report the construction of an emission-tunable SPOC with a rationally designed squaraine derivative (named as SPOC-SQ). SPOC-SQ is featured with a squaraine core and four peripheries with electron donor-π-acceptor (D-π-A) characteristics, which facilitates the formation of porous crystal framework stabilized by π-π interactions and H bonds and at the same time provides structural flexibility through phenyl rotations. This SPOC can be easily obtained from its dichloromethane (DCM) solution and exhibits reversible stimuli-responsive single-crystal-to-single-crystal (SCSC) structural transformation, accompanied by bright and tunable emission. In addition, this activated SPOC (SPOC-SQ-a) selectively recognizes and absorbs acetylene (C 2 H 2 ) over other gases without destroying the single crystallinity, enabling the single-crystal XRD analysis of the structural transformation. Close inspection of single-crystal XRD results of SPOC-SQ-C 2 H 2 facilitates the understanding of the host-guest interactions. More interestingly, upon interacting with C 2 H 2 , a one-dimensional (1D) channel is formed in the crystal to adopt C 2 H 2 , which proves the SCSC process and provides molecular-level insights into the gate-opening process. Furthermore, C 2 H 2 adsorption dynamics can be monitored in real time by tracking the fluorescence wavelength changes of SPOC-SQ framework. Thus, the unique gate-opening sorption attribute of SPOC-SQ-a crystals toward C 2 H 2 enables its potential applications for gas separation.

The efforts toward the experimental realization of spin-polarized transports in ideal materials or platforms, such as the magnetized graphene or various quantum Hall states, is a research hotspot in spintronics. Magnetic van der Waals materials open the door for exploring various physical phenomena, technologies, and integrating novel spintronic devices seamlessly within the 2D limit. Here, we demonstrate magnetic proximity effect in chromium trichloride (CrCl 3 )/bilayer graphene (BLG) heterostructures by low-temperature transport measurements. An effective exchange field induced in BLG has been demonstrated by the Zeeman spin Hall effect via nonlocal measurements. Furthermore, the exchange field modulates the quantum Hall ground state of BLG and thus favors the formation of a canted antiferromagnetic (CAF) phase in an external perpendicular magnetic field ( B ⊥ ). Asymmetric nonlocal magneto-transport behaviors are also observed at opposite B ⊥ directions, due to the asymmetric modulation on the exchange field by external B ⊥ directions. Our work suggests that the 2D magnetic van der Waals materials and graphene hybrid systems offer a unique platform for quantum Hall ferromagnetism physics.

A large linear negative thermal expansion (NTE) and expanded NTE temperature range (Δ T NTE ) were obtained in magnetoelastic CrTe 1- x Se x (0 ≤ x ≤ 0.15) compounds. For CrTe compound, its thermal expansion coefficient of volume (α V ) was calculated to be -28.8 ppm K -1 with the temperature ranging from 280 to 340 K. Substituting Te with Se atoms, the NTE behavior and magnetic properties can be well manipulated. With increasing Se in CrTe 1- x Se x (0 ≤ x ≤ 0.15) compounds, the Δ T NTE increases from 60 K (280-340 K for x = 0), to 80 K (240-320 K for x = 0.05), to 95 K (200-295 K for x = 0.1), and finally to 100 K (170-270 K for x = 0.15). Furthermore, a linear NTE remains independent of temperature for samples with x ≤ 0.1. The relationship between tunable NTE and magnetic properties was analyzed in detail, indicating that the NTE in CrTe 1- x Se x compounds originates from the magnetovolume effect (MVE).

Despite the recent developments on the construction of point-of-care testing (POCT) devices, it is still a big challenge to build portable POCT tools for simple, sensitive, selective, and quantitative determination of disease-related biomarkers. With this in mind, we developed a simple and user-friendly POCT tool for onsite analysis of DNA adenine methyltransferase (Dam) activity by using DNA tetrahedra-based hydrogel to trap glucose-producing enzymes for target recognition and signal transduction. The enzyme-encapsulated DNA hydrogel and the substrate of enzyme were separately modified on papers and then combined onto a commercial glucose test strip for the sensitive evaluation of Dam activity via using a personal glucose meter (PGM) for quantitative signal readout. Taking advantage of the great amount of enzyme entrapped in DNA hydrogel and the high signal amplification ability of enzyme, this POCT tool is capable of highly sensitive and selective determination of Dam activity with a direct detection limit down to 0.001 U/mL, which is superior to that of most previously reported biosensors. Furthermore, this device can also be applied to screen inhibitor and analyze Dam activity in spiked serum samples, indicating the great potential for clinical practice and diagnostic applications. Additionally, all the reactions for Dam assay are performed on paper, which is simple and deliverable to end-users for medical diagnostics at home or in-field.

Composites of graphene quantum dots (GQDs) and reduced graphene oxide (rGO) with unique three-dimensional (3D) structure are prepared and their catalytic activities for reduction of nitroarenes are explored. We demonstrate that the 3D GQDs/rGO composites are more active in nitroarene reduction than GQDs and rGO. Some of them are even more active than the Ag-embedded calcium alginate (Ag/CA) or Au-embedded calcium alginate (Au/CA) catalysts. Interestingly, their catalytic property is closely related to the ratio of GQDs to rGO in the 3D GQDs/rGO composites and GQDs-to-rGO mass ratio of 1/4 exhibits the highest catalytic activity. Raman spectra of the composites show that GQDs-to-rGO ratio is related to the number of the surface/edge defects, indicating that the sites of defect and edges are active sites. In addition, the catalytic performance of the 3D GQDs/rGO composites is also contributed by their unique 3D network structures that are beneficial for the reactant adsorption and product diffusion. Given also the long cycling duration and the easy recovery from the reaction system, 3D GQDs/rGO composites are potential applicable metal-free catalytic system for nitorarene reduction., Competing Interests: The authors declare no competing financial interest.

Well-defined graphene quantum dots (GQDs) are crucial for their biological applications and the construction of nanoscaled optoelectronic and electronic devices. However, as-synthesized GQDs reported in many works assume a very wide lateral size distribution; thus, their apparent properties cannot truthfully reflect intrinsic properties of the well-defined GQDs, and more importantly, the applications of GQDs will be affected and limited as well. In this work, we demonstrated that different sized GQDs with a narrow size distribution could be obtained via gel electrophoresis of the crude GQDs prepared through a photo-Fenton reaction of graphene oxide (GO). It is illustrated that the photoluminesce (PL) emissions of the well-defined GQDs originated mainly from the peripheral carboxylic groups and conjugated carbon backbone planes through fluorescence and UV-vis spectroscopies. More importantly, our findings challenge the notion that the excitation wavelength dependent PL property of the as-synthesized GQDs is the intrinsic property of the size-defined GQDs. Preliminary data at the cellular level indicated that the small sized GQDs exhibit weaker quenching DNA dye ability but higher toxicity to the cells compared to that of the as-synthesized GQDs. This discovery is essential to explore applications of the GQDs in pharmaceutics and to understand the origin of the optoelectronic properties of GQDs.

The high oxygen storage/release capability of the catalyst Ru is used to manipulate the interfacial electronic structure in spintronic materials to obtain perpendicular magnetic anisotropy (PMA). Insertion of an ultrathin Ru layer between the CoFeB and Ta layers in MgO/CoFeB/Ta/MgO films effectively induces PMA without annealing. Ru plays a catalytic role in Fe-O-Ta bonding and isolation at the metal-oxide interface to achieve moderate interface oxidation. In contrast, PMA cannot be obtained in the sample with a Mg insertion layer or without an insertion layer because of the lack of a catalyst. Our work would provide a new approach toward catalyst-induced PMA for future CoFeB-based spintronic device applications.

A deep understanding of the interaction of a graphene oxide (GO) sheet with cells at the molecular level may expedite its biomedical application and predict its new functions and adverse effects. Herein we inspect the interaction between micrometer-sized GO (mGO), commonly used in biomedical research, and cells at the molecular level through a variety of techniques. A major finding is that, instead of direct cellular penetration, the mGO sheets can stimulate the cellular response by interacting with the membrane protein and the membrane. Specifically, it is illustrated that even within a short exposure time the mGO sheets can induce the formation of vacuoles in the cytosolic compartment and enhance the cell permeability. The vacuolization is only observed in the cells that strongly express aquaporin (AQP1), indicating the specific interaction of the mGO with AQP1. Moreover, inhibition of the AQP1 activity prevents the formation of vacuoles, revealing that the interaction of the mGO with AQP1 occurs most probably at the vestibule of AQP1 at the extracellular side. Additionally, though the cell permeability was enhanced, it only improves the penetration of small molecules, not for macromolecules such as proteins. These findings are potentially valuable in cancer therapy because AQPs are strongly expressed in tumor cells of different origins, particularly aggressive tumors, and it will also be beneficial for drug transport across barrier membranes.

Eight triphenylamine (TPA)-based Schiff bases that exhibit different aggregation-induced emission (AIE) or aggregation-caused quenching (ACQ) behavior in tetrahydrofuran (THF)/water mixtures have been synthesized and characterized. The photophysical properties in solution, aqueous suspension, film, and the crystalline state along with their relationships were comparatively investigated. The single-crystal structures of 1-8 indicate that compact π···π stacking or excimers induce fluorescence quenching of 1, 2, 5, and 7. However, the existence of J aggregates or multiple intra- and intermolecular interactions restrict the intramolecular vibration and rotation, enabling compounds 3, 4, 6, and 8 to exhibit good AIE character. The size and growth process of particles with different water fractions were studied using scanning electron microscopy, which demonstrated that smaller uniformly dispersed nanoparticles in the THF/water mixtures favor fluorescence emission. The above results suggest that the combined effects of multiple forces caused by structural variation have a great influence on their molecular packing, electronic structure, and aggregation-induced fluorescence properties. In addition, piezofluorochromic experiments verified the potential applications of 4 and 6.

DNA i-motif structures have been found in telomeric, centromeric DNA and many in the promoter region of oncogenes; thus they might be attractive targets for gene-regulation processes and anticancer therapeutics. We demonstrate in this work that i-motif structures can be stabilized by graphene quantum dots (GQDs) under acidic conditions, and more importantly GQDs can promote the formation of the i-motif structure under alkaline or physiological conditions. We illustrate that the GQDs stabilize the i-motif structure through end-stacking of the bases at its loop regions, thus reducing its solvent-accessible area. Under physiological or alkaline conditions, the end-stacking of GQDs on the unfolded structure shifts the equilibrium between the i-motif and unfolded structure toward the i-motif structure, thus promoting its formation. The possibility of fine-tuning the stability of the i-motif and inducing its formation would make GQDs useful in gene regulation and oligonucleotide-based therapeutics.

Graphene quantum dots (GQDs) are great promising in various applications owing to the quantum confinement and edge effects in addition to their intrinsic properties of graphene, but the preparation of the GQDs in bulk scale is challenging. We demonstrated in this work that the micrometer sized graphene oxide (GO) sheets could react with Fenton reagent (Fe(2+)/Fe(3+)/H(2)O(2)) efficiently under an UV irradiation, and, as a result, the GQDs with periphery carboxylic groups could be generated with mass scale production. Through a variety of techniques including atomic force microscopy, X-ray photoelectron spectroscopy, gas chromatography, ultraperformance liquid chromatography-mass spectrometry, and total organic carbon measurement, the mechanism of the photo-Fenton reaction of GO was elucidated. The photo-Fenton reaction of GO was initiated at the carbon atoms connected with the oxygen containing groups, and C-C bonds were broken subsequently, therefore, the reaction rate depends strongly on the oxidization extent of the GO. Given the simple and efficient nature of the photo-Fenton reaction of GO, this method should provide a new strategy to prepare GQDs in mass scale. As a proof-of-concept experiment, the novel DNA cleavage system using as-generated GQDs was constructed.

The exploration of efficient DNA intercalative agents (intercalators) is essential for understanding DNA scission, repair, and signal transduction. In this work, we explored systematically the graphene oxide (GO) interaction with DNA molecules using fluorescence spectroscopic (FL) and circular dichroism (CD) studies, gel electrophoresis, and DNA thermal denaturation. We demonstrated that the GO nanosheets could intercalate efficiently into DNA molecules. Significantly, we illustrated that the scission of DNA by GO sheets combining with copper ions could take place pronouncedly. The scission of DNA by the GO/Cu(2+) system is critically dependent on the concentrations of GO and Cu(2+) and their ratio. DNA cleavage ability exhibited by the GO with several other metal ions and the fact that GO/Cu(2+)-cleaved DNA fragments can be partially relegated suggest that the mechanism of DNA cleavage by the GO/metal ion system is oxidative and hydrolytic. The result reveals that the GO/Cu(2+) could be used as a DNA cleaving system that should find many practical applications in biotechnology and as therapeutic agents.

Graphene oxide (GO), having a large specific surface area and abundant functional groups, provides an ideal substrate for study enzyme immobilization. We demonstrated that the enzyme immobilization on the GO sheets could take place readily without using any cross-linking reagents and additional surface modification. The atomically flat surface enabled us to observe the immobilized enzyme in the native state directly using atomic force microscopy (AFM). Combining the AFM imaging results of the immobilized enzyme molecules and their catalytic activity, we illustrated that the conformation of the immobilized enzyme is mainly determined by interactions of enzyme molecules with the functional groups of GO.

Well-defined AB2 Y-shaped miktoarm star polypeptide copolymer, PZLL-b-(PBLG)2, was synthesized via a combination of ring-opening polymerization (ROP) of alpha-amino acid N-carboxyanhydride (NCA) and click chemistry, where PZLL is poly(epsilon-benzyloxycarbonyl-L-lysine) and PBLG is poly(gamma-benzyl-L-glutamate). First, two types of primary-amine-containing initiators, N-aminoethyl 3,5-bis(propargyloxyl)-benzamide and 3-azidopropylamine, were synthesized and employed for the ROP of NCA, leading to the formation of dialkynyl-terminated PZLL and azide-terminated PBLG, dialkynyl-PZLL and PBLG-N3, respectively. The subsequent copper(I)-catalyzed cycloaddition reaction between dialkynyl-PZLL and slightly excess PBLG-N3 led to facile preparation of PZLL-b-(PBLG)2 Y-shaped miktoarm star polypeptide copolymer. The excess PBLG-N3 was scavenged off by reacting with alkynyl-functionalized Wang resin. The obtained Y-shaped miktoarm star polypeptide copolymer was characterized by gel permeation chromatograph (GPC), Fourier transform-infrared spectroscopy (FT-IR), and (1)H NMR. Moreover, after the hydrolysis of protecting benzyl and benzyloxycarbonyl groups of PZLL-b-(PBLG)2, water-soluble pH-responsive Y-shaped miktoarm star polypeptide copolymer, PLL-b-(PLGA)2, was obtained, where PLL is poly(L-lysine) and PLGA is poly(L-glutamic acid). It can self-assemble into PLGA-core micelles at acidic pH and PLL-core micelles at alkaline pH, accompanied with the coil-to-helix transition of PLGA and PLL sequences, respectively. The spontaneous pH-responsive supramolecular assembly of PLL-b-(PLGA)2 miktoarm star polypeptide copolymer has been investigated via a combination of (1)H NMR, laser light scattering (LLS), transmission electron microscopy (TEM), and circular dichroism (CD) spectroscopy.

A series of well-defined poly(ethylene oxide)- b-poly(2-(diethylamino)ethyl methacrylate) (PEO- b-PDEA) diblock copolymers containing PEO block of identical chain length and PDEA block with varying degrees of polymerization (DP, in the range of 32-154) were prepared via atom transfer radical polymerization (ATRP) employing a PEO-based macroinitiator (DP = 113). Upon a pH-jump from 3 to 12 under highly efficient stopped-flow mixing conditions, PEO- b-PDEA copolymers spontaneously form spherical micelles of increasing sizes and aggregation numbers ( N agg) with increasing PDEA chain lengths. Stopped-flow light scattering technique was used to probe the pH-induced micellization kinetics of PEO- b-PDEA copolymers, aiming to elucidate the PDEA chain-length effects on the unimer-to-micelle transition process. Upon a stopped-flow pH-jump from 3 to 12, the obtained dynamic traces can be well-fitted with double exponential functions. The calculated fast and slow characteristic relaxation times (tau 1 and tau 2) can be ascribed to the formation of quasi-equilibrium micelles (fast process) and subsequent relaxation into final equilibrium micelles (slow process), respectively. For PEO 113- b-PDEA 32 and PEO 113- b-PDEA 61, tau 2 is almost independent of polymer concentrations, suggesting that the relaxation from quasi-equilibrium micelles into final equilibrium micelles mainly proceeds via insertion/expulsion of unimer chains. Upon increasing the DP of pH-responsive PDEA block to 89, 117, and 154, the obtained slow relaxation time, tau 2, tends to decrease with increasing polymer concentrations, suggesting that the slow process is dominated by the micelle fusion/fission mechanism. The apparent activation energy ( E a) associated with tau 2 has also been determined from temperature-dependent micellization kinetics for five PEO- b-PDEA copolymers. It was found that during micellization, copolymers with longer PDEA blocks exhibit much lower E a compared to those with shorter blocks. Thus, we observed experimentally for the first time that increasing the hydrophobic block length in double hydrophilic block copolymers (DHBCs) can transform the mechanism of the slow process from unimer insertion/expulsion to micelle fusion/fission.

A pyrene end-labeled double hydrophilic diblock copolymer, poly(2-(diethylamino)ethyl methacrylate)-b-poly(2-(dimethylamino)ethyl methacrylate) (Py-PDEA-b-PDMA), was synthesized by sequential monomer addition via oxyanionic polymerization using a 1-pyrenemethanol-based initiator. This diblock copolymer exhibits reversible pH-responsive micellization behavior in aqueous solution, forming PDEA-core micelles stabilized by the soluble PDMA block at neutral or alkaline pH. Taking advantage of the pyrene probe covalently attached to the end of the PDEA block, the pH-induced micellization kinetics of Py-PDEA-b-PDMA was monitored by stopped-flow light scattering using a fluorescence detector. Upon a pH jump from 4.0 to 9.0, both the scattered light intensity and excimer/monomer fluorescence intensity ratios (IE/IM) increase abruptly initially, followed by a more gradual increase to reach plateau values. Interestingly, the IE/IM ratio increases abruptly within the first 10 ms: a triple exponential function is needed to fit the corresponding dynamic trace, leading to three characteristic relaxation time constants (tau(1,fluo) < tau(2,fluo) < tau(3,fluo)). On the other hand, dynamic traces for the scattered light intensity can be well-fitted by double exponential functions: the resulting time constants tau(1,scat) and tau(2,scat) can be ascribed to formation of the quasi-equilibrium micelles and relaxation into their final equilibrium state, respectively. Most importantly, tau(1,scat) obtained from stopped-flow light scattering is in general agreement with tau(2,fluo) obtained from stopped-flow fluorescence. The fastest process (tau(1,fluo) approximately 4 ms) detected by stopped-flow fluorescence is ascribed to the burst formation of small transient micelles comprising only a few chains, which are too small to be detected by conventional light scattering. These nascent micelles undergo rapid fusion and grow into quasi-equilibrium micelles and then slowly approach their final equilibrium state. The latter two processes can be detected by both techniques.

The interaction of the soluble methane monooxygenase regulatory component (MMOB) and the active site-bearing hydroxylase component (MMOH) is investigated using spin and fluorescent probes. MMOB from Methylosinus trichosporium OB3b is devoid of cysteine. Consequently, site-directed mutagenesis was used to incorporate single cysteine residues, allowing specific placement of the probe molecules. Sixteen MMOB Cys mutants were prepared and labeled with the EPR spin probe 4-maleimido-2,2,6,6-tetramethyl-1-piperidinyloxy (MSL). Spectral evaluation of probe mobility and accessibility to the hydrophilic spin-relaxing agent NiEDDA showed that both properties decrease dramatically for a subset of the spin labels as the complex with MMOH forms, thereby defining the likely interaction surface on MMOB. This surface contains MMOB residue T111 thought to play a role in substrate access into the MMOH active site. The surface also contains several hydrophilic residues and is ringed by charged residues. The surface of MMOB opposite the proposed binding surface is highly charged, consistent with solvent exposure. Probes of both of the disordered N- and C-terminal regions remain highly mobile and exposed to solvent in the MMOH complex. Spin-labeling studies show that residue A62 of MMOB is located in a position where it can be used to monitor MMOH-MMOB complex formation without perturbing the process. Accordingly, steady-state kinetic assays show that it can be changed to Cys (A62C) and labeled with the fluorescent probes 6-bromoacetyl-2-dimethylaminonaphthalene (BADAN) or 5-((((2-iodoacetyl)amino)ethyl)amino)naphthalene-1-sulfonic acid (1,5-IAEDANS) without loss of the ability of MMOB to promote turnover. The BADAN fluorescence is partially quenched and red shifted as the complex with MMOH forms, allowing affinity measurements. It is shown that the high affinity of labeled MMOB (K(D) = 13.5 nM at pH 6.6, 25 degrees C) for the oxidized MMOH decreases substantially with increasing pH and increasing ionic strength but is nearly unaffected by addition of nonionic detergents. Similarly, the fluorescence anisotropy of the 1,5-IAEDANS-labeled A62C-MMOH complex is perturbed by salts but not nonionic detergents. This suggests that the MMOB-MMOH complex is stabilized by electrostatic interactions consistent with the characteristics of the proposed binding surface. Reduction of MMOH results in a 2-3 order of magnitude decrease in the affinity of the BADAN-labeled A62C-MMOB-MMOH complex, consistent with previous indications of structural change associated with reduction of the active site dinuclear iron cluster. Utilizing BADAN-labeled MMOB, the association and dissociation rate constants for the MMOB-MMOH binding reaction were determined and found to be consistent with a two-step process, possibly involving rapid association followed by a slower conformational change. The latter may be related to the regulation of substrate access into the active site of MMOH.

The effects of the C-terminal region of the B component (MMOB) of soluble methane monooxygenase (sMMO) from Methylosinus trichosporium OB3b on steady-state turnover, the transient kinetics of the reaction cycle, and the properties of the sMMO hydroxylase (MMOH) active site diiron cluster have been explored. MMOB is known to have many profound effects on the rate and specificity of sMMO. Past studies have revealed specific roles for the well-folded core structure of MMOB as well as the disordered N-terminal region. Here, it is shown that the disordered C-terminal region of MMOB also performs critical roles in the regulation of catalysis. Deletion mutants of MMOB missing 5, 8, and 13 C-terminal residues cause progressive decreases in the maximum steady-state turnover number, as well as lower apparent rate constants for formation of the key reaction cycle intermediate, compound Q. It is shown that this latter effect is actually due to a decrease in the rate constant for formation of an earlier intermediate, probably the hydroperoxo species, compound P. Moreover, the deletions result in substantial uncoupling at or before the P intermediate. It is proposed that this is due to competition between slow H(2)O(2) release from one of the intermediates and the reaction that carries this intermediate on to the next step in the cycle, which is slowed by the mutation. Electron paramagnetic resonance (EPR) studies of the hydroxylase component (MMOH) in the mixed valent state suggest that complexation with the mutant MMOBs alters the electronic properties of the diiron cluster in a manner distinct from that observed when wild-type MMOB is used. Active site structural changes are also suggested by a substantial decrease in the deuterium kinetic isotope effect for the reaction of Q with methane thought to be associated with a decrease in quantum tunneling in the C-H bond breaking reaction. Thus, the surface interactions between MMOH and MMOB that affect substrate oxidation and its regulation appear to require the complete MMOB C-terminal region for proper function.