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

Polarized ir spectra of oriented films of alpha-helical poly(L-alanine) (al pha-PLA) have been obtained as a function of residual solvent dichloroaceti c acid (DCA). The amide A, B, II, and V regions exhibit multiple bands whos e structure depends on the residual DCA content, and those associated with the alpha(I)-PLA structure have been identified. A calculation of the relev ant cubic anharmonic force constants indicates that, contrary to previous a ssignments, the overtone of amide II(A) is in Fermi resonance with the NH s tretch fundamental, whose unperturbed frequency we now find to be at 3314 c m(-1), significantly higher than the previously suggested 3279 cm(-1). The presence of a structure in addition to the standard alpha(I)-PLA is indicat ed by our analysis. (C) 1999 John Wiley & Sons, Inc.