Conformational Behaviour of C-alpha,alpha-Diphenylglicine. Folded versus Extended Structures in Dphig-containing Tripeptides

The crystal structures of three fully protected tripeptides containing the D phi g residue (C-alpha,C-alpha-diphenylglycine) in the central position are reported, namely Z-Gly-D phi g-Gly-OMe (a), Z-Gly-D phi g-Aib-OMe (b) and Z-Aib-D phi g-Aib-OMe (c). The molecular conformations are quite unusual because the D phi g 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 D phi g 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-D phi g-Aib-NHCH3 and Ac-Aib-D phi g-NHCH3 also reported in this work.