Conformation of dehydropeptides

Six model dipeptide methylamides containing dehydroaminobutyric acid (delta Abu) of the type Boc-X-delta z Abu-NHCH3 and Box-X-delta E Abu-NHCH3, X = Ala, Val, Phe (Boc = tert-butoxycarbonyl), have been synthesized and their solution conformations explored using 300 MHz 1H NMR and IR spectroscopy. Studies based on delineation of intramolecularly hydrogen bonded NH groups in CDCl3 and (CD3)2SO revealed that none of the NH groups is appreciably solvent shielded. Difference NOE (Nuclear Overhauser Effect) studies have also failed to detect the presence of any discernible turn structure in these peptides. These studies indicate that the conformational preferences of peptides containing, alpha, beta-dehydroaminobutyric acid are different from those of delta ZPhe and delta ZLeu. It appears that steric interactions due to the beta-substituent in the dehydroamino acid moiety play an important role. Unlike delta ZPhe and delta ZLeu, which have relatively large beta-substituents, phenyl and isopropyl, respectively, and stabilize a beta-turn, the beta-methyl group of delta ZAbu or delta EAbu is readily accommodated in extended conformation. Clearly, the size of beta-substituent in dehydroamino acid crucially influences the conformational preferences. Thus, it may be possible to use different dehydroamino acids to introduce variable but definite constraints in synthetic peptides.