Characterization of beta-turns in cyclic hexapeptides in solution by Fourier transform IR spectroscopy

The beta-turn represents a structural element frequently encountered in globular proteins. However, in spite of various theoretical and experimental studies the ir signature bands of pure beta-turns are still not established beyond doubt. Although considerable information exists now on the ir spectra of alpha-helical and beta-sheet structures, the lack of knowledge concerning turn structures in general, and that of beta-turns in particular, presents a major uncertainty in the estimation of global protein secondary structures from ir spectroscopic data. To obtain more specific information about the characteristic amide bands in beta-turns, we report herein an ir spectroscopic analysis of a series of five cyclic pseudo-hexapeptides known to form beta-turns from previous CD and nmr studies [A. Perczel, M. Hollósi, B. M. Foxman, and G. D. Fasman (1991) Journal of the American Chemical Society, Volume 113, pp. 9772-9784]. We show here that in these cyclic peptides the amide groups involved in beta-turns that comprise a ten-membered hydrogen-bonded ring (and represent the first H-bond pair in a beta-sheet), give rise to characteristic amide I bands in the range 1638-1646 cm-1, with the exact position depending on the solvent and the nature of the side-chain substituents.