Conformational investigation of alpha,beta-dehydropeptides. Part. IV. Beta-turn in saturated and alpha,beta-unsaturated peptides Ac-Pro-Xaa-NHCH3: NMR and IR studies.

Solution conformations of three series of model peptides, homochiral Ac-Pro-L-Xaa-NHCH3 and heterochiral Ac-Pro-D-Xaa-NHCH3 (Xaa = Val, Phe, Leu, Abu, Ala) as well as alpha,beta-unsaturated Ac-Pro-delta Xaa-NHCH3 [delta Xaa = delta Val, (Z)-delta Phe, (Z)-delta Leu, (Z)-delta Abu] were investigated in CDCl3 and CH2Cl2 by 1H-, 13C-NMR, and FTIR spectroscopy. NH stretching absorption spectra, solvent shifts delta delta for NH (Xaa) and NHCH3 on going from CDCl3 to (CD3)2SO, diagnostic interresidue proton NOEs, and trans-cis isomer ratios were examined. These studies performed showed the essential difference in conformational propensities between homochiral peptides (L-Xaa) on the one hand and heterochiral (D-Xaa) and alpha,beta-dehydropeptides (delta Xaa) on the other. Former compounds are conformationally flexible with an inverse gamma-bend, a beta-turn, and open forms in an equilibrium depending on the nature of the Xaa side chain. Conformational preferences of heterochiral and alpha,beta-dehydropeptides are very similar, with the type-II beta-turn as the dominating structure. There is no apparent correlation between conformational properties and the nature of the Xaa side chain within the two groups. The beta-turn formation propensity seems to be somewhat greater in alpha,beta-unsaturated than in heterochiral peptides, but an estimation of beta-folded conformers is risky.