Characterization of β‐turns in cyclic hexapeptides in solution by fourier transform IR spectroscopy

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Abstract
The β‐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 β‐turns are still not established beyond doubt. Although considerable information exists now on the ir spectra of β‐helical and β‐sheet structures, the lack of knowledge concerning turn structures in general, and that of β‐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 β‐turns, we report herein an ir spectroscopic analysis of a series of five cyclic pseudo‐hexapeptides known to form β‐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 β‐turns that comprise a ten‐membered hydrogen‐bonded ring (and represent the first H‐bond pair in a β‐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. © 1993 John Wiley & Sons, Inc.