CONFORMATIONAL INVESTIGATION OF ALPHA,BETA-DEHYDROPEPTIDES .5. 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-X aa-NHCH3 and heterochiral Ac-Pro-D-Xaa-NHCH3 (Xaa = Phe, Val, Leu, Abu , Ala) as well as alpha,beta-dehydro Ac-Pro-DELTAXaa-NHCH3 [DELTAXaa = (Z)-DELTAPhe,DELTAVal, (Z)-DELTALeu, (Z)-DELTAAbu] 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 a ll 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% dichlorom ethane-cyclohexane, represented by class B spectra, and retain this co nformation in water as well as in fluorinated alcohols but not always to a full extent. The CD spectra of the unsaturated peptides in 2% dic hloromethane-cyclohexane, although they cannot be assigned to any comm on spectral class, must be attributed to the betaII-turn conformation as determined for these compounds by NMR and IR spectroscopy. The CD s pectra of dehydropeptides exhibit a considerable solvent dependence an d suggest unordered structures in water. (C) Munksgaard 1993.