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 an d ab initio computations. The molecule does not exist in a vacuum and class ical approaches of quantum mechanics cannot be directly applied. Thus, the dipolar Onsager, ionic, and conductor-like screening (COSMO) continuum solv ent models were used for calculation of geometries, harmonic vibrational fr equencies, and Raman intensities. Reasonable agreement was found between ex periment and the spectra, enabling one to assign most of the observed exper imental bands. The results suggest that bulk water significantly increases conformational flexibility of the dipeptide by lowering the energy differen ces 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 conf ormer search. The model that red to significant savings in computer time wa s also successfully applied for mono-and trialanine molecules.