Vibrational analysis of peptides, polypeptides, and proteins. 3. alpha-Poly(L-alanine).

Starting with a force field transferred from our earlier studies on beta-polypeptides, we have calculated the optically active normal vibration frequencies of alpha-helical poly(L-alanine) and poly(L-alanine-N-d). The 47/13 helical structure was used, and all atoms were included. Only small modifications to the force field were required, and most of these could be justified. The analysis indicates that amide II' is in Fermi resonance with one component of CH3 asymmetric bend, thus leading to a small modification of C-N and C=O stretching force constants. The agreement between calculated and observed Raman and infrared bands is quite good. This has encouraged ls to calculate the influence of small structural changes on the spectrum as a means of explaining the observed effects of temperature changes.