Conformational Behaviour of Cα,α-Diphenylglicine. Extended Conformation Tripeptides containing Consecutive Dφg Residues

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