A quantitative anharmonic analysis of the amide A band in α-helical poly(L-alanine)

Abstract

Polarized ir spectra of oriented films of α-helical poly(L-alanine) (α-PLA) have been obtained as a function of residual solvent dichloroacetic acid (DCA). The amide A, B, II, and V regions exhibit multiple bands whose structure depends on the residual DCA content, and those associated with the α(I)-PLA structure have been identified. A calculation of the relevant cubic anharmonic force constants indicates that, contrary to previous assignments, the overtone of amide II(A) is in Fermi resonance with the NH stretch fundamental, whose unperturbed frequency we now find to be at 3314 cm-1, significantly higher than the previously suggested 3279 cm-1. The presence of a structure in addition to the standard α(I)-PLA is indicated by our analysis.