Linear Oligopeptides .316. Conformational Characterization of Syndiotactic Homo-peptides From C-alpha,alpha-disubstituted Glycines

Terminally blocked, syndiotactic linear homo-peptides from C(alpha,alpha)-disubstituted glycines Iva and (alphaMe)Val have been prepared to the hexapeptide and tripeptide amide levels, respectively, by solution methods and fully characterized. The molecular and crystal structures of pBrBz-(D-Iva-L-Iva)2-OBu(t) methanol solvate, pBrBz-(D-Iva-L-Iva)2-D-Iva-OBu(t) methanol solvate, and Z-D-(alphaMe)Val-L-(alphaMe)Val-D-(alphaMe)Val-NHPr(i) (pBrBz = p-bromobenzoyl, Z = benzyloxycarbonyl) were determined by X-ray diffraction. While the Iva pentapeptide and the (alphaMe) Val tripeptide amide are folded in an (incipient) left-handed 3(10)-helical conformation, the Iva tetrapeptide adopts a double beta-bend conformation of the II'-III type. The FTIR absorption and H-1 NMR analyses support our contention that in chloroform solution, the longest syndiotactic homo-peptides may be folded in well developed 3(10)-helical structures. This is the first structural study reported on regularly alternating (D-L) peptides based on conformationally constrained alpha-amino acids.