Signature of n→π* interactions in α-helices.

The oxygen of a peptide bond has two lone pairs of electrons. One of these lone pairs is poised to interact with the electron-deficient carbon of the subsequent peptide bond in the chain. Any partial covalency that results from this n→π* interaction should induce pyramidalization of the carbon (C'(i)) toward the oxygen (O(i-1)). We searched for such pyramidalization in 14 peptides that contain both α- and β-amino acid residues and that assume a helical structure. We found that the α-amino acid residues, which adopt the main chain dihedral angles of an α-helix, display dramatic pyramidalization but the β-amino acid residues do not. Thus, we conclude that O(i-1) and C'(i) are linked by a partial covalent bond in α-helices. This finding has important ramifications for the folding and conformational stability of α-helices in isolation and in proteins.