Conformational behavior of C?,?-diphenyl glycine: Extended conformation in tripeptides containing consecutive D?g residues

Recent studies on the conformational preferences of the Dphig (C(alpha,alpha)-diphenylglycine) residue showed that this C(alpha,alpha)-disubstituted glycine has a structural versatility. In fact, depending on the nature of the following or preceding residue, Dphig can assume either folded or extended conformations. We have carried out the analysis of the conformational preferences of the Dphig residue in tripeptides containing consecutive Dphig residues. The crystal structures of Z-Dphig-Dphig -Dphig-OMe (a; Z = benzyloxycarbonyl; OMe = methyl ester), Z-Aib-Dphig-Dphig-OMe (b; Aib = alpha-aminoisobutyric acid), and Z-Ac(3)c-Dphig-Dphig-OMe (c; Ac(3)c = alpha-amino-cyclopropan carboxylic acid), are here reported. The Dphig residues adopt the fully extended conformation in the three tripeptides examined. Together with our previous findings on Dphig containing peptides, the structures of the peptides here examined, indicate that the presence of adjacent Dphig residue in the sequence further stabilizes the extended conformation.