No abstract

Raman and Raman optical activity (ROA) spectra of molecules reflect not only molecular structure and conformation but also the dynamics and interactions with the solvent. For polar, biologically relevant molecules in aqueous environment, this often complicates the band assignment and interpretation of the spectra. In the present study, implicit dielectric and explicit solvent models are compared with respect to the influence of the choice of solvent model on the spectral shape. Lactamide and 2-aminopropanol were selected as model compounds, and the Raman and ROA spectra were measured for both enantiomers. Geometries of explicitly solvated clusters were derived from quantum-mechanical calculations, classical (MD), and Car-Parrinello (CPMD) molecular dynamics. The results indicate that although the dielectric model reasonably well reproduces the main spectral features, more faithful intensity profiles, including the inhomogeneous band broadening, are obtained from the explicit MD and CPMD clusters. Additionally, the CPMD clusters are capable of reproducing most spectral features better than the classical dynamics, provided the simulation time is long enough to allow for a complete sampling of the conformational space. The hydrogen-bonded water molecules of the first hydration shell significantly influence the spectral intensities, whereas the effect of loosely attached or distant solvent molecules is minor. In order to average the signal, however, a relatively large number of MD geometries need to be considered, as was also exemplified by simulations of the ROA spectrum of the achiral molecule glycine. An explicit solvent modeling of sizable systems thus requires extensive computations, which became possible only recently due to the development of efficient analytical computational techniques.

The natural optical activity of two chiral amino acids, alanine and proline, has been calculated using Hartree−Fock and density-functional theory with the Becke three-parameter Lee−Yang−Parr (B3LYP) functional employing analytical response theory. The dependence of the optical activity on the molecular conformation in the gas phase was investigated. In the case of proline, calculations were also carried out for the protonated and deprotonated molecules. The increase of the optical rotation of proline with increasing pH, found experimentally, is reproduced by our calculations. The optical rotation of both amino acids is found to be very sensitive to the molecular geometry, to the extent that it changes sign for the different conformers. For alanine, the sign of the optical rotation varies with the rotation of the amino or carbonyl groups.