Conformational behaviour of C α,α ‐diphenylglycine: folded vs. extended structures in DϕG‐containing tripeptides

The crystal structures of three fully protected tripeptides containing the Dϕg residue (C α,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 3 10 ‐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‐NHCH 3 and Ac‐Aib‐Dϕg‐NHCH 3 also reported in this work. © 1998 European Peptide Society and John Wiley & Sons, Ltd.