Combined effect of the ΔPhe or ΔAla residue and the p ‐nitroanilide group on a didehydropeptides conformation

Two series of dehydropeptides of the general formulae Boc‐Gly‐X‐Phe‐ p ‐NA, Boc‐Gly‐Gly‐X‐Phe‐ p ‐NA, Gly‐X‐Gly‐Phe‐ p ‐NA·TFA, and Boc‐Gly‐X‐Gly‐Phe‐ p ‐NA, with X = Δ Z Phe and ΔAla, were studied with NMR in DMSO and CDCl 3 ‐DMSO, and with CD in MeOH, MeCN, and TFE. The NMR spectra measured in DMSO suggest that peptides with the ΔPhe residue next to Phe are folded whereas peptides with Gly between ΔPhe and Phe are less ordered. NMR spectra of ΔAla‐containing peptides indicate that these peptides are flexible and their conformational equilibria are populated by many different conformations. The CD spectra show that conformational properties of the peptides studied are distinctly influenced by a mutual position of the dehydroamino acid residue and the p ‐NA group. They indicate that all dehydropeptides with the ΔPhe residue, Boc‐Gly‐ΔAla‐Phe‐ p ‐NA, and Boc‐Gly‐Gly‐ΔAla‐Phe‐ p ‐NA adopt ordered conformations in all solvents studied, presumably of the β‐turn type. The last two peptides exhibit surprising chiroptical properties. Their spectra show exciton coupling‐like couplets in the region of the p ‐NA group absorption. This shape of CD spectra suggests a rigid, chiral conformation with a fixed disposition of the p ‐NA group. The CD spectra indicate that Boc‐Gly‐ΔAla‐Gly‐Phe‐ p ‐NA and Gly‐ΔAla‐Gly‐Phe‐ p ‐NA·TFA are unordered, independently of the solvent. © 2007 Wiley Periodicals, Inc. Biopolymers 89: 220–234, 2008. This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com