Through-space transfer of chiral information mediated by a plasmonic nanomaterial.

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.

Ramachandran Plot for Alanine Dipeptide as Determined from Raman Optical Activity.

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.

Transferability of anharmonic force fields in simulations of molecular vibrations.

Accurate simulations of vibrational molecular spectra require precise molecular force fields, at least with cubic and quartic anharmonic corrections beyond the harmonic limits. Generation of such force field terms becomes computationally prohibitive for larger molecules. In this work, an alternate possibility is explored, where approximate anharmonic force field components are obtained from molecular fragments. Transferability properties of the cubic and incomplete quartic fields are discussed and tested on model oligoproline molecules. Automatic transfer schemes including cubic, two and three atomic quartic force constants are developed and implemented. The results indicate that the main vibrational interactions in molecules are local and the anharmonic constants are mostly well amendable to the transfer. Exact anharmonic normal mode force fields of larger molecules compared very well to those obtained from smaller molecular parts. The most important changes in vibrational spectra caused by the anharmonic interactions could be reproduced with two and three atomic force field terms. The transfer scheme thus provides molecular anharmonic force fields without a significant loss of accuracy and brings significant savings of computer time and memory needed to generate molecular vibrational energies and spectra.