Simulation of the Raman optical activity of L-alanyl-L-alanine

Raman and Raman optical activity (ROA) spectra of zwitterionic dipeptide L-alanyl-L-alanine (AA) were measured and compared to previous experiments and ab initio computations. The molecule does not exist in a vacuum and classical approaches of quantum mechanics cannot be directly applied. Thus, the dipolar Onsager, ionic, and conductor-like screening (COSMO) continuum solvent models were used for calculation of geometries, harmonic vibrational frequencies, and Raman intensities. Reasonable agreement was found between experiment and the spectra, enabling one to assign most of the observed experimental bands. The results suggest that bulk water significantly increases conformational flexibility of the dipeptide by lowering the energy differences among its conformers. However, detailed conformational analysis was not attempted due to the limited accuracy of the calculations. Supposedly, the nearly extended conformation of the zwitterion prevails under experimental conditions. A simplistic polar model of ROA was used in a preliminary conformer search. The model that red to significant savings in computer time was also successfully applied for mono-and trialanine molecules.