Linear oligopeptides. Part 316. Conformational characterization of syndiotactic homo-peptides from Cα,α-disubstituted glycines

Abstract

Terminally blocked, syndiotactic linear homo-peptides from C^α,α-disubstituted glycines Iva and (αMe)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-lva-L-lva)₂-OBu^t methanol solvate, pBrBz-(D-lva-L-lva)₂-D-lva-OBu^t methanol solvate, and Z-D-(αMe)Val-L-(aMe)Val-D-(aMe)Val-NHPrⁱ(pBrBz =p-bromobenzoyl, Z = benzyloxycarbonyl) were determined by X-ray diffraction. While the Iva pentapeptide and the (αMe) Val tripeptide amide are folded in an (incipient) left-handed 3₁₀-helical conformation, the Iva tetrapeptide adopts a double β-bend conformation of the II′–III type. The FTIR absorption and ¹H NMR analyses support our contention that in chloroform solution, the longest syndiotactic homo-peptides may be folded in well developed 3₁₀-helical structures. This is the first structural study reported on regularly alternating (D–L) peptides based on conformationally constrained α-amino acids.