| Abstract
| - 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 extendedamide 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 appearsto 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 thatunhydrated α-helix corresponds to the canonical conformation αc and hydrated α-helix to a more openconformation αo stabilized by hydrogen bonding of a water molecule or a hydrophilic side chain to thepeptide carbonyl. α-Helical poly(l-lysine) and poly(l-ornithine) in aqueous solution and poly(l-alanine) indichloracetic 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 toenhanced stabilization of this conformation by particular side chain characteristics. The ROA spectrum ofpoly(β-benzyl l-aspartate) in CHCl3 reveals that it exists in a single left-handed α-helical state moreanalogous to αo than to αc.
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