Conformational behaviour of Cα,α-diphenylglycine: Folded vs. extended structures in DφG-containing tripeptides

The crystal structures of three fully protected tripeptides containing the Dφg residue (Cα,α-diphenylglycine) in the central position are reported, namely Z-Gly-Dφg-Gly-OMe (a), Z-Gly-Dφg-Aib-OMe (b) and Z-Aib-Dφg-Aib-OMe (c). The molecular conformations are quite unusual because the Dφg residue adopts a folded conformation in the 310-helical region when the following residue adopts a folded conformation of opposite handedness (peptides b and c). In contrast, the Dφg residue adopts the more frequently observed fully extended conformation when the following residue adopts a semi-extended conformation (peptide a). These findings are in agreement with the theoretical calculations on Ac-Dφg-Aib-NHCH3and Ac-Aib-Dφg-NHCH3 also reported in this work. © 1998 European Peptide Society and John Wiley & Sons, Ltd.