Structural versatility of peptides from C α,α dialkylated glycines: Linear Ac 3 c homo‐oligopeptides

The crystal‐state molecular structures of five linear Ac 3 c homo‐oligopeptides to the tetramer were determined by x‐ray diffraction. The oligomers are H‐(Ac 3 c) 2 ‐OMe, Fmoc‐(Ac 3 c) 2 ‐OMe MeOH, Ac‐(Ac 3 c) 2 ‐OMe, p BrBz‐(Ac 3 c) 3 ‐OMe · H 2 O, and t ‐Boc‐(Ac 3 c) 4 ‐OMe · 2H 2 O. The results indicate the propensity of the tri‐ and tetrapeptides to fold into type I β‐bends and distorted 3 10 ‐helices, respectively, in partial contrast to Aib, Ac 5 c, and Ac 6 c homo‐peptides of comparable main‐chain length, where regular type III β‐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 Ac 3 c residue is found to be asymmetric and the NC α C′ bond angle significantly expanded from the regular tetrahedral value.