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.