Structural versatility of peptides from C-alpha,alpha-dialkylated glycines: linear Ac3c homo-oligopeptides

The crystal-state molecular structures of five linear Ac3c homo-oligopeptides to the tetramer were determined by X-ray diffraction. The oligomers are H-(Ac3c)2-OMe, Fmoc-(Ac3c)2-OMe MeOH, Ac-(Ac3c)2-OMe, pBrBz-(Ac3c)3-OMe · H2O, and t-Boc-(Ac3c)4-OMe · 2H2O. The results indicate the propensity of the tri- and tetrapeptides to fold into type I beta-bends and distorted 3(10)-helices, respectively, in partial contrast to Aib, Ac5c, and Ac6c homo-peptides of comparable main-chain length, where regular type III beta-bends and 3(10)-helical structures were found. When the influence of the constraints produced by the intramolecular H bonds of the C(10)-type is absent, other less common structural features may be observed. The average geometry of the cyclopropyl group of the Ac3c residue is found to be asymmetric and the N-C(alpha)-C' bond angle significantly expanded from the regular tetrahedral value