Vibrational analysis of peptides,polypeptides, and proteins. II. β-Poly(L-alanine) and β-poly(L-alanylglycine)

The normal vibration frequencies of poly(L -alanine) and poly(L -alanylglycine) in the antiparallel chain-pleated sheet structure have been calculated, using the force field for polyglycine I from the previous paper (Biopolymers 15 , 2439–2464) plus additional force constants for the methyl group. The agreement with observed ir and Raman bands is very good. This substantiates the excellent transferability of the force field, since polyglycine I was shown to have a rippled-sheet structure. The amide I and amide II mode splittings are very well accounted for by transition dipole coupling, showing that subtle structural differences are sensitively manifested through this mechanism.