Helix-stabilizing interaction between tyrosine and leucine or valine when the spacing is i, i + 4

A helix-stabilizing interaction between tyrosine and leucine or valine has been found in alanine-based peptide helices when the spacing is i, i + 4. Control peptides have identical compositions but an i, i + 3 spacing. This is, to our knowledge, the first report of a helix-stabilizing interaction between two non-polar side-chains in an isolated helix. The results explain why, in an earlier study, leucine was found to have a helix propensity similar to that of alanine in an alanine-based peptide, whereas later work from another laboratory and our own has shown that alanine is markedly more helix-stabilizing than leucine in alanine-based peptides. The change in helix content resulting from the i, i + 4 Tyr-Leu interaction is comparable to the changes seen for other specific interactions between pairs of side-chains, such as ion-pair or Phe.His+ interactions.