TURN TENDENCY OF MODEL ALPHA,BETA-DEHYDROPEPTIDES

We examined the conformations of a series of model peptides Ac-Pro-DEL TAXaa-NHMe, where DELTAXaa = DELTAAla, (Z)-DELTAAbu, (E)-DELTAAbu, DEL TAVal, (Z)-DELTALeu and (Z)-DELTAPhe, using NMR, FTIR and CD spectrosc opies and X-ray crystallography. The peptides studied adopt, as a rule , a betaII-turn conformation in inert solvents, unfolded ones in polar media and a betaI-like turn in crystals. There are two exceptions: (1 ) Ac-Pro-(Z)-DELTAPhe-NHMe in water seems to have an ordered structure (s) but different from the betaII-turn, which, however, is preserved i n the solid state; (2) in Ac-Pro-DELTAAla-NHMe, the DELTAAla moiety be ing fully extended in solution and in crystal is rigid and does not pa rticipate in the formation of any beta-turn. DELTAAla is then unique a mong the alpha,beta-dehydroamino acids investigated, while the remaini ng residues are capable of forming beta-turns, but do not provide comp elling conformational constraints, with the DELTAPhe residue being the most rigid.