CD AND FOURIER-TRANSFORM IR SPECTROSCOPIC STUDIES OF PEPTIDES .2. DETECTION OF BETA-TURNS IN LINEAR PEPTIDES

Comparative CD and Fourier transform ir (FTIR) spectroscopic data on N-Boc protected linear peptides with or without the (Pro-Gly) beta-turn motif (e.g., Boc-Tyr-Pro-Gly-Phe-Leu-OH and Boc-Tyr-Gly-Pro-Phe-Leu-OH) are reported herein. The CD spectra, reflecting both backbone and aromatic contributions, were not found to be characteristic of the presence of beta-turns. In the amide I region of the FTIR spectra, analyzed by self-deconvolution and curve-fitting methods, the beta-turn band showed up between 1639 and 1633 cm(-1) in trifluoroethanol (TFE) but only for models containing the (Pro-Gly) core. This band was also present in the spectra in chloroform but absent in dimethylsulfoxide. These findings, in agreement with recent ir data on cyclic models and 3(10)-helical polypeptides and proteins in D2O [see S. J. Prestrelski, D. M. Byler, and M. P. Thompson (1991), International Journal of Peptide and Protein Research, Vol. 37, pp. 508-512; H. H. Mantsch, A. Perczel, M. Hollosi, and G. D. Fasman (1992), FASEB Journal, Vol. 6, p. A341; H. H. Mantsch, A. Perczel, M. Hollosi, and G. Fasman (1992), Biopolymers, Vol. 33, pp. 201-207; S. M. Miick, G. V. Martinet, W. R. Fiori, A. P. Todd, and G. L. Millhauser (1992), Nature, Vol. 359, pp. 653-655], suggest that the amide I band, with a major contribution from the acceptor C=O of the 1 <-- 4 intramolecular H bond of beta-turns, appears near or below 1640 cm(-1),rather than above 1660 cm(-1). In TFE, bands between 1670 and 1660 cm(-1) are mainly due to ''free'' carbonyls, that is, C= O's of amides that are solvated but not involved in the characteristic H bonds of periodic secondary structures or beta-turns. (C) 1994 John Wiley and Sons, Inc.