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Abstract: Interaction of the nonionic surfactant Hecameg® with the plasma protein Bovine Serum Albumin (BSA), and its effect on protein conformation, has been studied using spectroscopic techniques such as steady-state and time-resolved fluorescence and circular dichroism. A weak interaction of the surfactant with BSA is reflected by changes in the intrinsic fluorescence of BSA in either steady-state or time-resolved measurements. The fluorescence intensity data allowed us to determine the corresponding binding curve, which suggests a sequential binding mechanism, in which the surfactant first occupies the hydrophobic sites of the inner protein cavity and then, condenses onto the surface hydrophobic sites of BSA via a cooperative mechanism. Additional fluorescence data obtained by synchronous, three-dimensional and anisotropy experiments show that the surfactant mainly interacts with the tryptophan residues of BSA, which seem to experience motional restriction as a result of this interaction. Time-resolved fluorescence data, which were analyzed using the modified Stern–Volmer equation, also support the above mechanism. Finally, far-UV circular dichroism studies indicated that the secondary structure of the protein remains almost unaltered even for BSA to surfactant molar ratio as high as 1 to 100. [Copyright &y& Elsevier]

Atomic-scale reproducibility and tunability endorse magnetic molecules as candidates for spin qubits and spintronics. A major challenge is to implant those molecular spins into circuit geometries that may allow one, two, or a few spins to be addressed in a controlled way. Here, the formation of mechanically bonded, magnetic porphyrin dimeric rings around carbon nanotubes (mMINTs) is presented. The mechanical bond places the porphyrin magnetic cores in close contact with the carbon nanotube without disturbing their structures. A combination of spectroscopic techniques shows that the magnetic geometry of the dimers is preserved upon formation of the macrocycle and the mMINT. Moreover, the metallic core selection determines the spin location in the mMINT. The suitability of mMINTs as qubits is explored by measuring their quantum coherence times ( T m ). Formation of the dimeric ring preserves the T m found in the monomer, which remains in the μs scale for mMINTs. The carbon nanotube is used as vessel to place the molecules in complex circuits. This strategy can be extended to other families of magnetic molecules. The size and composition of the macrocycle can be tailored to modulate magnetic interactions between the cores and to introduce magnetic asymmetries (heterometallic dimers) for more complex molecule-based qubits.

We show that H-bonded host-guest systems associate in ionic liquids (ILs), pure salts with melting point below room temperature, in which dipole-dipole electrostatic interactions should be negligible in comparison with dipole-charge interactions. Binding constants (K a ) obtained from titrations of four H-bonded host-guest systems in two organic solvents and two ionic liquids yield smaller yet comparable K a values in ionic liquids than in organic solvents. We also detect the association event using force spectroscopy, which confirms that the binding is not solely due to (de)solvation processes. Our results indicate that classic H-bonded host-guest supramolecular chemistry takes place in ILs. This implies that strong H-bonds are only moderately affected by surroundings composed entirely of charges, which can be interpreted as an indication that the balance of Coulombic to covalent forces in strong H-bonds is not tipped towards the former.

Symmetry breaking and amplification processes have likely played a fundamental role in the development of homochirality on earth. Such processes have not been much studied for inorganic matter at the nanoscale. Here, we show that the balance between left- and right-handed intrinsically chiral metal clusters can be broken by adsorbing a small amount of a chiral molecule in its ligand shell. We studied the amplification of enantiomeric excess of the Au 38 (2-PET) 24 cluster (2-PET = 2-phenylethylthiolate). By exchanging a small fraction of the achiral 2-PET ligand by chiral R-1,1'-binaphthyl-2,2'-dithiol (R-BINAS), a mixture of species is obtained composed of anticlockwise (A) and clockwise (C) versions of Au 38 (2-PET) 24 and Au 38 (2-PET) 22 (R-BINAS) 1 . At 70 °C, the system evolves towards the anticlockwise clusters at the expense of the clockwise antipode. It is shown that the interplay between the diastereospecific ligand exchange, which introduces selectivity but does not change the A/C ratio, and the fast racemization of the Au 38 (2-PET) 24 is at the origin of this observation.

Herein we report the transformation of [Au 25 (SR) 18 ] 0 into Au 28 (SR) 21 induced by ligand exchange reaction. In contrast to other reported cluster transformations, which proceed at elevated temperature and large excess of incoming ligands, the transformation reported here occurs under mild conditions (room temperature, very low thiol excess) with a chiral ligand. A difference of one methyl group between incoming and leaving thiol is sufficient to induce the transformation. To the best of our knowledge the Au 28 (SR) 21 cluster has not been isolated before.

Over recent years, the field of thiolate-protected gold nanoclusters has made remarkable progress. The successful determination of the structure of some of these clusters by X-ray crystallography was a milestone in this field. X-ray crystallography is arguably the most important technique in the field up to now, and it enabled the study of structure evolution as a function of cluster size. It also shed light on the structure of the Au-S interface. Recently, it has been realized that thiolate-protected gold clusters are very dynamic systems. Metal atoms and ligands can exchange easily between clusters. Furthermore, the adsorbed ligands bear conformational dynamics. Such dynamic effects call for experimental methods that can cope with it. Future efforts in this field will be directed toward applications of thiolate-protected clusters, and many of them will rely on dissolved clusters. Therefore, structure determination in solution is an important issue, though it is very challenging. The structure of the metal core and the Au-S interface is not expected to change in solution with respect to the crystal. However, the structure of the adsorbed ligand itself is sensitive to the environment and may be different in the solid state and in solution, as has been shown in fact in the past. It is this (dynamic) structure of the ligand that determines the interaction between the cluster and its environment, which is crucial, for example, for sensing applications. Vibrational spectroscopy is a promising technique to characterize thiolate-protected clusters in different environments. A vibrational spectrum is sensitive to structure (conformation) although this information is often "hidden" in the spectrum, requiring detailed analysis and support from theory to be deciphered. Compared to other techniques like UV-vis spectroscopy and mass spectrometry, vibrational spectroscopy was not extensively used in the field of thiolate-protected clusters, but we believe that the technique will be very valuable for the future developments in the field. We have used vibrational spectroscopy to investigate thiolate-protected gold clusters for mainly two lines of research. In the first, we studied in detail the low energy region of the vibrational spectrum, in particular the Au-S vibrational modes, in order to understand the structure sensitivity. It emerges that the Au-S vibrational spectrum is indeed sensitive to the structure of the interface but also to other factors, especially the organic part of the thiol, in a complex way. The ability to directly correlate structure, from X-ray crystallography, and vibrational spectra for thiolate-protected clusters, should lead to a database that will help in the future the structure determination of the Au-S interface by vibrational spectroscopy for systems where direct structure determination is not possible, for example, for flat surfaces. A second line of research focused on the determination of the structure of the adsorbed ligands for dissolved clusters. Such information is mostly extracted by the comparison of theoretical and calculated spectra for different conformers. In this respect, vibrational circular dichroism (VCD) is particularly powerful as it strongly depends on the conformation, more than conventional infrared spectroscopy. VCD can be applied to chiral nonracemic compounds, and it is a sensitive probe for chirality. Using this method, it was possible to demonstrate that a cluster can transfer its chirality to achiral thiolate ligands. In this Account, we summarize the possibilities and challenges of vibrational spectroscopy in the field of thiolate-protected clusters.

Introducing one or two alkynyl-iron moieties onto a carbo[6]helicene results in organometallic helicenes (2 a,b) that display strong chiroptical activity combined with efficient redox-triggered switching. The neutral and oxidized forms have been studied in detail by electronic and vibrational circular dichroism, as well as by Raman optical activity (ROA) spectroscopy. The experimental results were analyzed and spectra were assigned with the help of first-principles calculations. In particular, a recently developed method for ROA calculations under resonance conditions has been used to study the intricate resonance effects on the ROA spectrum of mono-iron ethynylhelicene 2 a., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

One of the most attractive applications of carbon nanomaterials is as catalysts, due to their extreme surface-to-volume ratio. The substitution of C with heteroatoms (typically B and N as p- and n-dopants) has been explored to enhance their catalytic activity. Here we show that encapsulation within weakly doping macrocycles can be used to modify the catalytic properties of the nanotubes towards the reduction of nitroarenes, either enhancing it (n-doping) or slowing it down (p-doping). This artificial regulation strategy presents a unique combination of features found in the natural regulation of enzymes: binding of the effectors (the macrocycles) is noncovalent, yet stable thanks to the mechanical link, and their effect is remote, but not allosteric, since it does not affect the structure of the active site. By careful design of the macrocycles' structure, we expect that this strategy will contribute to overcome the major hurdles in SWNT-based catalysts: activity, aggregation, and specificity.

We describe the synthesis of rotaxane-type species composed of macrocyclic porphyrin rings mechanically interlocked with SWCNT threads. The formation of mechanically interlocked SWCNTs (MINTs) proceeds with chiral selectivity, and was confirmed by spectroscopic and analytical techniques and adequate control experiments, and corroborated by high-resolution electron microscopy. From a thorough characterization of the MINTs through UV-vis-NIR absorption, fluorescence, Raman, and transient absorption spectroscopy we analyse in detail the electronic interactions of the porphyrins and the SWCNTs in the ground and excited states.

Several steps of chiral induction have been detected in poly(phenylacetylene)s among their different hierarchical levels of chirality by vibrational circular dichroism, namely, (i) from the stereogenic centers to the innermost polyacetylene helical covalent backbone (helix int ), (ii) from this to the external helix (helix ext ) formed by the side phenyl pendants that form a complementary helix or counter-helix, and (iii) from this pendant helix to the helical solvation sphere (helix solv. ), the last one being observed along this work. The pendant to polyene backbone chiral induction determines the helical structure adopted by the polymer and therefore the solvation helix. This helical structure is promoted by two mechanisms: steric effects and hydrogen bonding. An important finding concerns the demonstration by VCD of how an achiral solvent becomes chirally organized owing to the template effect of the covalent polymer helices, an effect that is silent to other structural techniques such as ECD or AFM and that hence significantly broadens the scope of these previous analyses.

Due to their outstanding electronic and mechanical properties, single-walled carbon nanotubes (SWCNTs) are promising nanomaterials for the future generation of optoelectronic devices and composites. However, their scarce solubility limits their application in many technologies that demand solution-processing of high-purity SWCNT samples. Although some non-covalent functionalization approaches have demonstrated their utility in extracting SWCNTs into different media, many of them produce short-lived dispersions or ultimately suffer from contamination by the dispersing agent. Here, we introduce an unprecedented strategy that relies on a cooperative clamping process. When mixing (6,5)SWCNTs with a dinucleoside monomer that is able to self-assemble in nanorings via Watson-Crick base-pairing, a synergistic relationship is established. On one hand, the H-bonded rings are able to associate intimately with SWCNTs by embracing the tube sidewalls, which allows for an efficient SWCNT debundling and for the production of long-lasting SWCNT dispersions of high optical quality along a broad concentration range. On the other, nanoring stability is enhanced in the presence of SWCNTs, which are suitable guests for the ring cavity and contribute to the establishment of multiple cooperative noncovalent interactions. The inhibition of these reversible interactions, by just adding, for instance, a competing solvent for hydrogen-bonding, proved to be a simple and effective method to recover the pristine nanomaterial with no trace of the dispersing agent.

The encapsulation of viologen derivatives into metallic single-walled carbon nanotubes (SWNTs) results in the opening of a band gap, making the SWNTs semiconducting. Raman spectroscopy, thermogravimetric analysis, and aberration-corrected high-resolution transmission electron microscopy confirm the encapsulation process. Through the fabrication of field-effect transistor devices, the change of the electronic structure of the tubes from metallic to semiconducting upon the encapsulation is confirmed. The opening of a gap in the band structure of the tubes was not detected in supramolecular controls., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Decades after the birth of supramolecular chemistry, there are many techniques to measure noncovalent interactions, such as hydrogen bonding, under equilibrium conditions. As ensembles of molecules rapidly lose coherence, we cannot extrapolate bulk data to single-molecule events under non-equilibrium conditions, more relevant to the dynamics of biological systems. We present a new method that exploits the high force resolution of optical tweezers to measure at the single molecule level the mechanical strength of a hydrogen bonded host-guest pair out of equilibrium and under near-physiological conditions. We utilize a DNA reporter to unambiguously isolate single binding events. The Hamilton receptor-cyanuric acid host-guest system is used as a test bed. The force required to dissociate the host-guest system is ∼17 pN and increases with the pulling rate as expected for a system under non-equilibrium conditions. Blocking one of the hydrogen bonding sites results in a significant decrease of the force-to-break by 1-2 pN, pointing out the ability of the method to resolve subtle changes in the mechanical strength of the binding due to the individual H-bonding components. We believe the method will prove to be a versatile tool to address important questions in supramolecular chemistry.

The combination of a bis-alkynyl-helicene moiety with two iron centers leads to novel electroactive species displaying unprecedented redox-triggered chiroptical switching. Upon oxidation, strong changes of vibrational modes (either local or extended coupled modes) are detected by vibrational circular dichroism and Raman optical activity. Remarkably, the sign of the optical rotation at 1.54 µm (that is, at wavelengths typically used for telecommunications) changes upon oxidation while the topology and stereochemistry of the helicene remain unchanged., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A complete chiroptical characterization of the supramolecular polymers formed by tricarboxamides and is performed using ECD, VCD and CPL dichroic techniques. The helical aggregates show an intense CPL signal and their absolute P- or M-configuration is assigned with the help of theoretical calculations.

The relationships between the chiroptical activity and the vibrational normal modes of epichlorohydrin have been investigated on the basis of a nonredundant internal coordinate definition not reported until now. These coordinates were verified by comparing, for the lower energy conformers, the diagonal quadratic force constants and were found to display similar values among conformers and to be consistent with the molecular structure of epichlorohydrin and its vibrational circular dichroism (VCD) spectrum. Boltzmann population factors were used to calculate the weighted sum of the individual VCD and IR spectra of the three lower energy conformers, which accurately fitted the experimental spectra of (R)-epichlorohydrin. The electric and magnetic transition dipole moments of the 24 vibrational normal modes were calculated for the most stable conformers. The combined analysis of these vectors and the normal mode description, given in terms of the potential energy distribution, allowed us to investigate the role of the functional groups (methylene, chlorine) and the type of internal coordinates (stretching, bending, etc.) in the chiroptical activity of the vibrations.

By reformulating Raman and ROA invariants we provide ground for the definition of robust modes in ROA spectroscopy. Introduction of two parameters defining robustness helps characterization and assignment of ROA bands. Application and use of robustness parameters to [n]helicenes and oxirane/thiirane derivatives are presented.

We present a complete Raman spectroscopic study in two structurally well-defined diradical species of different lengths incorporating oligo p-phenylene vinylene bridges between two polychlorinated triphenylmethyl radical units, a disposition that allows sizeable conjugation between the two radicals through and with the bridge. The spectroscopic data are interpreted and supported by quantum chemical calculations. We focus the attention on the Raman frequency changes, interpretable in terms of: (i) bridge length (conjugation length); (ii) bridge conformational structure; and (iii) electronic coupling between the terminal radical units with the bridge and through the bridge, which could delineate through-bond spin polarization, or spin delocalization. These items are addressed by using the "oligomer approach" in conjunction with pressure and temperature dependent Raman spectroscopic data. In summary, we have attempted to translate the well-known strategy to study the electron (charge) structure of π-conjugated molecules by Raman spectroscopy to the case of electron (spin) interactions via the spin delocalization mechanism.

The helical organization of oligo-p-phenylene-based organogelators has been investigated by atomic force microscopy, circular and vibrational circular dichroism, and Raman techniques. Whilst OPPs with more than two phenyl rings in the core self-assemble into left-handed helices, that with a biphenyl core shows an inversion of the supramolecular helicity depending on the formation conditions through the atropisomerism of the biphenyl central unit. The results presented herein outline a new example of kinetically controlled modulation of supramolecular helicity., (Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Protein denaturation by surfactants has received increased attention in the last years due to its implications in topics such as pharmaceutics, cosmetics, paints, or biotechnology. This phenomenon is highly dependent on the physicochemical (structural) properties of the denaturing agents. In this work, we have measured for the first time the Raman optical activity (ROA) of bovine serum albumin (BSA) in the presence of three surfactants (anionic, cationic, and neutral), which has allowed us to detect new spectroscopic insights of the protein-surfactant interaction that conventional Raman spectroscopy cannot. Our work proposes two new groups of ROA marker bands to explore the unfolding of BSA induced by surfactants, which are related to "polar" (amide I and III modes) and "apolar" (methylene bending and phenyl breathing modes) protein sections. The appearance of the former groups is related to the initial attack of the surfactant, while the second groups relate to the hydrophobic unfolding.

In this study we designed a new class of symmetrical facial oligothiophene amphiphiles, which could be obtained in fewer steps than for previously reported analogues, but still possess the specific substituent sequence to control their backbone curvature. This novel design allows the late-stage introduction of hydrophilic groups, aiding both purification and ease of structure variation. Following the new synthetic scheme, symmetrical ter- and sexi-thiophenes were synthesized, analysed and their properties were compared to their non-symmetrical analogues. Surprisingly, the self-assembly behaviour in water, aggregate morphologies and photo-physical properties turned out to be significantly different despite the same ratio of hydrophilic and hydrophobic substituents. The new substitution pattern resulted in a drastic decrease of the critical aggregation concentration and an increase of the aggregate size. The symmetrical positioning of the substituents also heavily influenced the photo-physical properties. The changes were observed as large blue shifts in the absorption and emission spectra in water when compared to similar regio-regular oligothiophene amphiphiles.

In 1990 the article on the first Raman spectrum of a protein using a circularly polarized light as excitation radiation, authored by the group of Prof. L. Barron, was published in ChemComm. In this viewpoint we analyze this pioneering work and we used it as a starting point to highlight, after 20 years since its publication, the most outstanding contributions in the application of the Raman Optical Activity to proteins and biological systems.