A study of α-helix hydration in polypeptides, proteins, and viruses using vibrational Raman optical activity

A vibrational Raman optical activity (ROA) study, supplemented by protein X-ray crystal structure data, of (α-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 α-helix. One type, associated with a positive ROA band at ∼1300 cm-1, dominates in hydrophobic environments and appears to be unhydrated; the other, associated with a positive ROA band at ∼1340 cm-1, dominates in hydrophilic environments and appears to be hydrated. Evidence is presented to support the hypothesis that unhydrated α-helix corresponds to the canonical conformation αc and hydrated α-helix to a more open conformation αo stabilized by hydrogen bonding of a water molecule or a hydrophilic side chain to the peptide carbonyl. α-Helical poly(L-lysine) and poly(L-ornithine) in aqueous solution and poly(L-alanine) in dichloracetic acid display both bands, but α-helical poly(L-glutamic acid) in aqueous solution and poly(γ-benzyl L-glutamate) in CHCl3 display only the ∼1340 cm-1 band and so may exist purely as αo due to enhanced stabilization of this conformation by particular side chain characteristics. The ROA spectrum of poly(β-benzyl L-aspartate) in CHCl3 reveals that it exists in a single left-handed α-helical state more analogous to αo than to αc.