The vibrational Raman optical activity (ROA) spectrum of a polypeptide in a model beta-sheet conformation, that of poly(l-lysine), was measured for the first time, and the alpha-helix --> beta-sheet transition monitored as a function of temperature in H(2)O and D(2)O. Although no significant population of a disordered backbone state was detected at intermediate temperatures, some side chain bands not present in either the alpha-helix or beta-sheet state were observed. The observation of ROA bands in the extended amide III region assigned to beta-turns suggests that, under our experimental conditions, beta-sheet poly(L-lysine) contains up-and-down antiparallel beta-sheets based on the hairpin motif. The ROA spectrum of beta-sheet poly(L-lysine) was compared with ROA data on a number of native proteins containing different types of beta-sheet. Amide I and amide II ROA band patterns observed in beta-sheet poly(L-lysine) are different from those observed in typical beta-sheet proteins and may be characteristic of an extended flat multistranded beta-sheet, which is unlike the more irregular and twisted beta-sheet found in most proteins. However, a reduced isoform of the truncated ovine prion protein PrP(94-233) that is rich in beta-sheet shows amide I and amide II ROA bands similar to those of beta-sheet poly(L-lysine), which suggests that the C-terminal domain of the prion protein is able to support unusually flat beta-sheets. A principal component analysis (PCA) that identifies protein structural types from ROA band patterns provides a useful representation of the structural relationships among the polypeptide and protein states considered in the study.

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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.

A vibrational Raman optical activity (ROA) study of a series of alanine peptides in aqueous solution is presented. The seven-alanine peptide Acetyl-OOAAAAAAAOO-Amide (OAO), recently shown by NMR and UVCD to adopt a predominantly poly(l-proline II) (PPII) helical conformation in aqueous solution, gave an ROA spectrum very similar to that of disordered poly(l-glutamic acid) which has long been considered to adopt the PPII conformation, both being dominated by a strong positive extended amide III ROA band at approximately 1319 cm-1 together with weak positive amide I ROA intensity at approximately 1675 cm-1. A series of alanine peptides Ala2-Ala6 studied in their cationic states in aqueous solution at low pH displayed ROA spectra which steadily evolved toward that of OAO with increasing chain length. As well as confirming that alanine peptides can support the PPII conformation in aqueous solution, our results also confirm the previous ROA band assignments for PPII structure, thereby reinforcing the foundation for ongoing ROA studies of unfolded and partially folded proteins.