CONFORMATIONAL BEHAVIOR OF C-ALPHA,ALPHA-DIPHENYLGLYCINE - FOLDED VS.EXTENDED STRUCTURES IN D-PHI-G-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 conformation s are quite unusual because the D phi g residue adopts a folded confor mation in the 3(10)-helical region when the following residue adopts a folded conformation of opposite handedness (peptides b and c). In con trast, the D phi g residue adopts the more frequently observed fully e xtended conformation when the following residue adopts a semi-extended conformation (peptide a). These findings are in agreement with the th eoretical calculations on Ac-D phi g-Aib-NHCH3 and Ac-Aib-D phi g-NHCH 3 also reported in this work. (C) 1998 European Peptide Society and Jo hn Wiley & Sons, Ltd.