Conformational Behaviour of Cα,α-Diphenylglicine. Folded versus 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 (peptidesbandc). In contrast, the Dϕg residue adopts the more frequently observed fully extended conformation when the following residue adopts a semi-extended conformation (peptidea). These findings are in agreement with the theoretical calculations on Ac-Dϕg-Aib-NHCH3 and Ac-Aib-Dϕg-NHCH3 also reported in this work.