PEPTIDES FROM CHIRAL C-ALPHA-ALPHA-DISUBSTITUTED GLYCINES - ON THE HELICAL SCREW SENSE OF ISOVALINE PEPTIDES

The preferred conformation of three N-alpha-acetylated Aib/ Iva host/g uest pentapeptide esters and their N-alpha-benzyloxycarbonylated synth etic precursors, prepared by solution methods and fully characterized, were examined in chloroform solution using FT-IR absorption and H-1-N MR and in the crystal state by X-ray diffraction. All these peptides a re folded in a 3(10)-helix structure, irrespective of the experimental conditions used in the conformational analysis. In the crystal state the screw sense preference of the helical structure that is formed see ms to be governed by the position of the single Iva residue in the pep tide main chain, the ethyl side-chain disposition, and the nature of t he N-alpha-blocking group.