Raman amide bands of type‐II β‐turns in cyclo‐(VPGVG)₃ and poly‐(VPGVG), and implications for protein secondary‐structure analysis

The cyclopentadecapeptide, c(VPGVG) 3 , a model structure for protein type‐II β‐turns [W. J. Cook et al. (1980) J. Am. Chem. Soc. 102 , 5502–5505], has been investigated by laser Raman spectroscopy. Data obtained from both normal and deuterated crystals identify amide I, III, I′, and III′ bands characteristic of the β(II)turn. The structurally related polypentapeptide poly(VPGVG) in normal and deuterated forms has also been investigated, and exhibits the same Raman amide bands as c(VPGVG) 3 . The coacervate of poly(VPGVG), obtained by heating the solution to 40°C, likewise exhibits a Raman spectrum very similar to that of the c(VPGVG) 3 crystal. Raman spectra thus indicate closely similar secondary structures for crystalline c(VPGVG) 3 and aqueous poly(VPGVG), and provide an empirical basis for interpreting the conformation sensitive amide bands of globular proteins in terms of β(II)turn structures that may be present. An important conclusion from the present findings is that the amide I Raman profile of a protein may not be sufficient in general to distinguish turns from helix and sheet secondary structures, since the major amide I peaks of the β(II)turn at 1676 and 1652 cm −1 overlap, respectively, with amide I profiles generated by β‐sheet and α‐helix conformations.