Secondary structure of peptides: 19. Primary/secondary structure relationships of leucine/valine copolypeptides prepared from α-amino acid N-carboxyanhydrides

Abstract

l-Leucine -N- carboxyanhydride ( l-Leu-NCa) and l-valine -N- carboxyanhydride ( l-Val-NCA) were copolymerized under various conditions. In addition to the initiators benzylamine, triethylamine, pyridine), temperature, solvent and molar ratio of the co-monomers were varied. The molar composition of the isolated copolypeptides were determined from 1H n.m.r. spectra. The sequences were analysed by means of 15N n.m.r. spectroscopy and characterized by average block lengths. By means of 13C n.m.r. CP/MAS spectroscopy of the solid ‘as polymerized’ copolypeptides it was feasible to quantify the α-helix/β-sheet ratio of both Leu and Val units separately. In all copolypeptides with Leu/Val ratios 1 2 a substantial fraction of the Val units adopts the α-helical conformation. Upon reprecipitation the α-helix/β-sheet ratio changed, indicating that most ‘as polymerized’ samples possess a kinetically controlled, thermodynamically unstable secondary structure. In several samples the percentage of α-helical Val units reached 90% after reprecipitation. At constant Leu/Val ratios the percentage of α-helical Val units decreases with increasing blockiness of the sequence.