Conformational investigation of alpha, beta-dehydropeptides. V*. Stability of reverse turns in saturated and alpha, beta-unsaturated peptides Ac-Pro-Xaa-NHCH3: CD studies in various solvents.

Conformations of three series of model peptides: homochiral Ac-Pro-L-Xaa-NHCH3 and heterochiral Ac-Pro-D-Xaa-NHCH3 (Xaa = Phe, Val, Leu, Abu, Ala) as well as alpha, beta-dehydro Ac-Pro-delta Xaa-NHCH3 [delta Xaa = (Z)-delta Phe, delta Val,(Z)-delta Leu,(Z)-delta Abu] were investigated by CD spectroscopy in 2% dichloromethane-cyclohexane, trifluoroethanol, water, and occasionally in other solvents. The spectra of homochiral peptides show a significant solvent dependence. Folded structures are present in 2% dichloromethane-cyclohexane and unordered ones occur in water. The folded conformers are of the inverse gamma-turn type for all the peptides but Ac-Pro-L-Phe-NHCH3 for which the type-I beta-turn is preferred. The changes in the spectra of the heterochiral peptides are limited. The compounds adopt the type-II beta-turn in 2% dichloromethane-cyclohexane, represented by class B spectra, and retain this conformation in water as well as in fluorinated alcohols but not always to a full extent. The CD spectra of the unsaturated peptides in 2% dichloromethane-cyclohexane, although they cannot be assigned to any common spectral class, must be attributed to the beta II-turn conformation as determined for these compounds by NMR and IR spectroscopy. The CD spectra of dehydropeptides exhibit a considerable solvent dependence and suggest unordered structures in water.