Raman optical activity (ROA) is a relatively new method combining the variability of scattering experiments with the structural sensitivity of chiral spectroscopy. Typically, ROA can be employed to determine absolute configuration (AC) of organic compounds, chiral metal complexes, and conformation of biologically relevant chiral molecules in solution. The present review covers the latest theoretical and experimental studies documenting the possibilities and limitations of the technique to probe molecular structure. The quantum-chemical apparatus necessary for spectral interpretation is introduced, and example applications provided, including recent data on possible extensions of the ROA spectroscopy to a more diverse systems. Josef Kapitán received his PhD from the Charles University in Prague in 2006 such as in the eld of Raman optical activity (ROA), with Petr Bouř and Vladimír Baumruk. As a postdoc, he stayed with Laurence D. Barron and Lutz Hecht at the University of Glasgow, simulating Raman optical activity of biopolymers and developing ultraviolet ROA instrument for pre-resonant studies. Since 2010 he is leading a laboratory of Raman spectroscopy at Palacký University in Olomouc. His scientic interest involves instrumentation of optical spectroscopy targeted to elucidation of peptide and protein structure.
The ability to detect chirality gives stereochemically attuned nanosensors the potential to revolutionize the study of biomolecular processes. Such devices may structurally characterize the mechanisms of protein-ligand binding, the intermediates of amyloidogenic diseases and the effects of phosphorylation and glycosylation. We demonstrate that single nanoparticle plasmonic reporters, or nanotags, can enable a stereochemical response to be transmitted from a chiral analyte to an achiral benzotriazole dye molecule in the vicinity of a plasmon resonance from an achiral metallic nanostructure. The transfer of chirality was verified by the measurement of mirror image surface enhanced resonance Raman optical activity spectra for the two enantiomers of both ribose and tryptophan. Computational modelling confirms these observations and reveals the novel chirality transfer mechanism responsible. This is the first report of colloidal metal nanoparticles in the form of single plasmonic substrates displaying an intrinsic chiral sensitivity once attached to a chiral molecule.
Accessible values of the φ and ψ torsional angles determining peptide main chain conformation are traditionally displayed in the form of Ramachandran plots. The number of experimental methods making it possible to determine such conformational distribution is limited. In the present study, Raman optical activity (ROA) spectra of Ac-Ala-NHMe were measured and fit by theoretical curves. This revealed the most favored conformers and a large part of the potential energy surface (PES) of this model dipeptide. Such experimental PES compares well to quantum chemical computations, whereas molecular dynamics (MD) modeling reproduces it less faithfully. The surface shape is consistent with the temperature dependence of the spectra, as observed experimentally and predicted by MD. Despite errors associated with spectral modeling and the measurement, the results are likely to facilitate future applications of ROA spectroscopy.
Nicotinic
acid and pyridoxine, adsorbed on electrochemically prepared copper
substrates, were studied by Surface-Enhanced Infrared Absorption (SEIRA)
spectroscopy. Assignment of the bands in SEIRA spectra to appropriate
vibrational modes was based on results of DFT calculations. The role
of dissolved gas in deposition solution was elucidated for adsorption
of nicotinic acid. Temperature dependence of SEIRA spectral features
of both nicotinic acid and pyridoxine was studied in the temperature
range 15–50 °C. In order to evaluate the temperature-induced
changes of SEIRA spectra of both two adsorbates, the bands were fitted
with Voigt profiles and their heights and widths were compared. Principal
Component Analysis (PCA) and Partial Least Squares (PLS) regression
were applied. It was found out that heating even to 50 °C does
not induce any observable changes in molecular structure of adsorbates.
However, noticeable changes of band intensities and widths were detected
and assigned to the partial reorientation of adsorbed molecules, to
the changes of intermolecular interactions within the adsorbed layers,
and to a minor extent, to the other changes of metal–adsorbate
systems properties, for example, to the changes of surface moisture.
The changes of metal reflectivity are considered to be the minor source
of spectral variation.
New molecular tweezers based on bis-Tröger’s base with methoxycarbonyl groups on its pincers was prepared. These groups were converted into hydroxymethyl groups, which were interconnected by a linker to give the bridged molecular tweezers, a cavitand. The cavitand was studied and its ability to bind nitrobenzene was compared with similar bis-Tröger’s base molecular tweezers.
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