Conformational Behaviour of Calpha,alpha-Diphenylglicine. Extended Conformation Tripeptides containing Consecutive Dphig Residues

Recent studies on the conformational preferences of the D?g (C?,?-diphenylglycine) residue showed that this C?,?-disubstituted glycine has a structural versatility. In fact, depending on the nature of the following or preceding residue, D?g can assume either folded or extended conformations. We have carried out the analysis of the conformational preferences of the D?g residue in tripeptides containing consecutive D?g residues. The crystal structures of Z-D?g-D?g-D?g-OMe (a; Z = benzyloxycarbonyl; OMe = methyl ester), Z-Aib-D?g-D?g-OMe (b; Aib = ?-aminoisobutyric acid), and Z-Ac3c-D?g-D?g-OMe (c; Ac3c = ?-amino-cyclopropan carboxylic acid), are here reported. The D?g residues adopt the fully extended conformation in the three tripeptides examined. Together with our previous findings on D?g containing peptides, the structures of the peptides here examined, indicate that the presence of adjacent D?g residue in the sequence further stabilizes the extended conformation.