Helical screw sense of peptide molecules: The pentapeptide system (Aib)(4)/L-Val[L-(alpha Me)Val] in the crystal state

The crystal-state preferred conformations of six N-alpha-blocked pentapeptide esters, each containing four helicogenic, achiral alpha-aminoisobutyric acid (Aib) residues followed by one chiral L-valine (L-Val) or C-alpha-methyl-L-valine [(alpha Me)Val] residue at the C-terminus, have been assessed by x-ray diffraction analysis. In all of the compounds the -(Aib)(4)- sequence is folded in a regular 3(10)-helical conformation. In the four pentapeptides characterized by the L-(alpha-Me)Val residue two conformationally distinct molecules occur in the asymmetric unit. Conversely only one molecule is observed in the asymmetric unit of two pentapeptides with the C-terminal L-Val residue. In the L-Val based peptides the helical screw sense of the -(Aib)(4)- sequence is right-handed, whereas in the L-(alpha Me)Val- analogues both right- and left-handed helical screw senses concomitantly occur in the two crystallographically independent molecules. (C) 1998 John Wiley & Sons, Inc.