Crystal-state 3D-structural characterization of novel 3(10)-helical peptides

The crystal-state conformations of two octapeptides, pBrBz-(D-Iva)(8)-OtBu (SI) and Ac-[L-(alphaMe)Val](8)-OH (8II), the heptapeptide Z-[L-(alphaMe)Val](7)-OH (7), the hexapeptide Z-[L-(alphaMe)Leu](6)-OtBu (6) and the tetrapeptide alkylamide Z-(Aib)(2)-L-Glu(OMe)-L-Ala-L-Lol (5) were assessed by x-ray diffraction analyses. Two independent molecules are observed in the asymmetric unit of each L-(alphaMe)Val homo-peptide. All four homo-peptides are folded in a regular 3(10)-helical structure (only the C-terminal H-bonded conformation of the D-Iva octapeptide is distorted to a type-I beta-turn). The hydroxyl groups of the C-terminal carboxyl moieties of the two L-(alphaMe)Val homo-peptides participate in an oxy-analogue of the type-III beta-turn conformation. While the two L-(alphaMe)Val 3(10)-helices are right-handed, the D-Iva and L-(alphaMe)Leu helices are left-handed. The tetrapeptide alkylamide is 3(10)-helical at the N-terminus, but it is mixed 3(10)/alpha-helical at the C-terminus.