Structural versatility of peptides containing C-alpha,alpha-dialkylated glycines: an X-ray diffraction study of six 1-aminocyclopropane-1-carboxylic acid rich peptides

The molecular and crystal structures of six fully blocked, Ac3c-rich peptides to the tetramer level were determined by X-ray diffraction. The peptides are Fmoc-(Ac3c)2-OMe-CH3OH, Ac-(Ac3c)2-OMe, t-Boc-Ac3c-L-Phe-OMe, pBrBz-(Ac3c)3-OMe . H2O, Z-Gly-Ac3c-Gly-OTmb . (CH3)2CO, and t-Boc-(Ac3c)4-OMe . 2H2O. Type-I (I') beta-bends and distorted 3(10)-helices were found to be typical of the tri- and tetrapeptides, respectively. In the dipeptides, too short to form beta-bend conformations, other less common structural features may be observed. The average geometry of the cyclopropyl moiety of the Ac3c residue is asymmetric and the N-C alpha-C' bond angle is significantly expanded from the regular tetrahedral value. A comparison with the structural preferences of other extensively investigated C alpha, alpha-dialkylated alpha-amino acids is made and the implications for the use of the Ac3c residue in conformational design are examined.