RESUMO
Knowledge of the secondary structure of antifreeze peptides (AFPs) and glycopeptides (AFGPs) is crucial to understanding the mechanism by which these molecules inhibit ice crystal growth. A polyproline type II helix is perhaps the most widely accepted conformation for active AFGPs; however, random coil and alpha-helix conformations have also been proposed. In this report we present vibrational spectroscopic evidence that the conformation of AFGPs in solution is not random, not alpha-helical, and not polyproline type II. Comparison of AFGP amide vibrational frequencies with those observed and calculated for beta and gamma-turns in other peptides strongly suggests that AFGPs contain substantial turn structure. Computer-generated molecular models were utilized to compare gamma-turn, beta-turn, and polyproline II structures. The gamma-turn motif is consistent with observed amide frequencies and results in a molecule with planar symmetry with respect to the disaccharides. This intriguing conformation may provide new insight into the unusual properties of AFGPs.