GAMMA-TURN CONFORMATION INDUCED BY ALPHA,ALPHA-DISUBSTITUTED AMINO-ACIDS WITH A CYCLIC 6-MEMBERED SIDE-CHAIN

4-Aminotetrahydrothiopyran-4-carboxylic acid (Thp) is an unusual achir al cyclic alpha,alpha-disubstituted amino acid mimicking the natural M et residue. The conformational energy map computed for Ac-Thp-NHMe sho ws that the gamma-turn is the lowest minimum. H-1-NMR and IR studies p erformed on For-Thp-Leu-OMe (2a), a short peptide unable to give a 4.. .>1 H-bond, indicate that the gamma-turn is adopted in CDCl3 solution, whereas is not retained in(CD3)(2)SO. An analogous conformational beh aviour in solution has been observed for the strictly related For-Ac(6 )c-Leu-OMe (2b), containing 1-aminocyclohexane carboxylic acid (Ac(6)c ).