Comparisons of β-Hairpin Propensity Among Peptides with Homochiral or Heterochiral Strands.

Assemblies of racemic β-sheet-forming peptides have attracted attention for biomedical applications because racemic forms of peptides can self-associate more avidly than do single enantiomers. In 1953, Pauling and Corey proposed "rippled β-sheet" modes of H-bond-mediated interstrand assembly for alternating L- and D-peptide strands; this structural hypothesis was complementary to their proposal of "pleated β-sheet" assembly for L-peptides. Although no high-resolution structure has been reported for a rippled β-sheet, there is strong evidence for the occurrence of rippled β-sheets in some racemic peptide assemblies. Here we compare propensities of peptide diastereomers in aqueous solution to form a minimum increment of β-sheet in which two antiparallel strands associate. β-Hairpin folding is observed for homochiral peptides with aligned nonpolar side chains, but no β-hairpin population can be detected for diastereomers in which one strand contains L residues and the other contains D residues. These observations suggest that rippled β-sheet assemblies are stabilized by interactions between β-sheet layers rather than interactions within these layers.