Vibrational analysis of peptides, polypeptides, and proteins. XVIII. Conformational sensitivity of the ?-helix spectrum: ?I- and ?II-Poly(L-alanine) (original) (raw)

The q-h e l i x (6 =-70,47O, $ =-35.75O) is a structure having the same n and h as the (standard) aI-helix (4 =-57.37", $ =-47.49'). Its conformational angles are commonly found in proteins. Using an improved a-helix force field, we have compared the vibrational frequencies of these two structures. Despite the small conformational differences, there are significant predicted differences in frequencies, particularly in the amide A, amide I, and amide I1 bands, and in the conformation-sensitive region below 900 cm-'. This analysis indicates that aII-helices are likely to be present in bacteriorhodopsin [Krimm, S.