CRYSTALLOGRAPHIC CHARACTERIZATION OF TRYPTOPHAN-CONTAINING PEPTIDE 3(10)-HELICES

The molecular and crystal structures of four peptides containing one L -Trp guest residue in Aib (alpha-aminoisobutyric acid or C-alpha-methy l alanine) host oligopeptide chains have been determined by X-ray diff raction. The peptides are Z-Aib-L-Trp-Aib-OMe, Z-(Aib)(2)-L-Trp-Aib-OM e, Z-(Aib)(3)-L-Trp-Aib-OtBu and Boc-(Aib)(3)-L-Trp-Aib-OMe. Right-han ded beta-turns and incipient and fully developed 3(10)-helices are for med in the crystal state by the tri-, tetra- and pentapeptides, respec tively. The Trp residue is easily accommodated in these folded structu res. The average geometry and preferred conformation for the Trp indol yl side chain are also discussed.