A vibrational Raman optical activity (ROA) study, supplemented by protein X-ray crystal structure data, of alpha-helices in polypeptides, proteins, and viruses has suggested that ROA bands in the extended amide III spectral region may be used to distinguish between two types of right-handed alpha-helix. One type, associated with a positive ROA band at similar to1300 cm(-1), dominates in hydrophobic environments and appears to be unhydrated; the other, associated with a positive ROA band at similar to1340 cm-1, dominates in hydrophilic environments and appears to be hydrated. Evidence is presented to support the hypothesis that unhydrated alpha-helix corresponds to the canonical conformation at, and hydrated a-helix to a more open conformation alpha(c) stabilized by hydrogen bonding of a water molecule or a hydrophilic side chain to the peptide carbonyl. alpha-Helical poly(L-lysine) and poly(L-ornithine) in aqueous solution and poly(L-alanine) in dichloracetic acid display both bands, but a-helical poly(L-glutamic acid) in aqueous solution and poly(gamma-benzyl L-glutamate) in CHCl3 display only the similar to1340 cm-1 band and so may exist purely as alpha(o) due to enhanced stabilization of this conformation by particular side chain characteristics. The ROA spectrum of polyo-(beta-benzyl L-aspartate) in CHCl3 reveals that it exists in a single left-handed alpha-helical state more analogous to alpha(o) than to alpha(c).