VIBRATIONAL ANALYSIS OF THE DIPEPTIDES CONTAINING ALANINE AND SERINE

Fourier-transform infrared and Raman spectra of L-Ala-L-Ser, D-Ala-L-S er and L-Ser-L-Ala were recorded, and significant differences were fou nd between the spectra of these dipeptides. In order to interpret thes e differences, the normal vibrations of L-Ala-L-Ser, D-Ala-L-Ser, L-Se r-L-Ala and D-Ser-L-Ala were calculated from an empirical general vale nce force field and molecular geometries calculated by MM2 methods. In itial values of the force constants for the normal coordinate analysis were transferred from the known spectroscopic empirical force fields of alanine, serine and poly(L-alanine). Then, a partial refinement of force constants was performed to improve the fit to the experimental s pectra of L-Ala, L-Ser and some of their deuterated analogues. Differe nces were obtained between the calculated spectra of LL and DL derivat ives; the significant differences between the experimental spectra, ho wever, could be reproduced with limited success, suggesting that the l atter may be due in part to differences in crystal rather than in mole cular structures.