Hydrogen bonding interactions between glutamine and asparagine in alpha-helical peptides.

We have measured the strength of a Gln-Asn side-chain-side-chain interaction spaced i, i+4 in an alpha-helix by placing pairs of interacting residues in the middle, N and C termini of Ala-based peptides. Experimental helicities for peptides containing Gln-Asn spaced i, i+4, as measured by circular dichroism, are considerably higher than those of a peptide with Gln-Asn spaced i, i+5, and that predicted by a modified form of Lifson-Roig helix-coil transition theory that does not include side-chain interactions. A model that includes side-chain-side-chain interactions successfully fits the experimental data and gives a free energy of interaction of between -0.4 and -0.7 kcal mol-1. This favourable interaction is evident in a statistical survey of alpha-helices from a set of non-homologous crystal structures. The Asn-Gln orientation has an interaction energy of less than that of Gln-Asn and close to zero. This difference shows that the free energy of interaction is sensitive to the geometry of the interacting groups. Because i, i+4 interactions can occur only when the side-chain chi1 angle of residue i is trans and that of residue i+4 is gauche+, and because side-chains are free to rotate in peptides, we have corrected interaction free energies from this and other studies to remove the conformational entropy cost of placing them in these conformations so that they are comparable with studies of hydrogen bonding in mutant proteins. The corrected DeltaG is -1 kcal mol-1, which is slightly lower than that reported for hydrogen bonds in folded proteins; however, this value is similar to that for hydrophobic i, i+4 interactions in peptides. We conclude that even in highly mobile, surface-exposed regions, hydrogen bonds can significantly stabilise proteins, provided that their geometric requirements can be achieved.