Measuring the strength of side-chain hydrogen bonds in peptide helices: the Gln.Asp (i, i + 4) interaction.

Whether hydrogen bonds between side chains are energetically significant in proteins and peptides has been controversial. A method is given here for measuring these interactions in peptide helices by comparing the helix contents of peptides with 1, 2, or 3 interactions. Results are given for the glutamine--aspartate (i, i + 4) hydrogen-bond interaction. The Gibbs energy of the interaction is -1.0 kcal/mol when aspartate is charged and -0.4(4) kcal/mol when it is protonated. Magnetic resonance experiments show that the aspartate carboxylate group interacts specifically with the trans amide proton (HE) of glutamine. The interaction is observed only when the glutamine residue is N-terminal to the aspartate and when the spacing is (i, i + 4). The same stereochemistry is found in protein structures, where the (i, i + 4) glutamine-aspartate interaction occurs much more frequently than other possible arrangements.