Helix-stabilizing nonpolar interactions between tyrosine and leucine in aqueous and TFE solutions: 2D-H-1 NMR and CD studies in alanine-lysine peptides

Interactions between side chains spaced (i, i + 3) and (i, i + 3) may expla in the context dependence of helix propensities observed in different syste ms. Nonpolar residues with these spacings occur frequently in protein helic es and stabilize isolated peptide helices. Here (i, i + 3) and (i, i + 3) n onpolar interactions between Tyr and Leu in different solution conditions a re studied in detail in alanine-based peptides using 2D H-1 NMR and CD spec troscopy. Helix contents analyzed using current models for helix-coil trans itions yield interaction energies which demonstrate significant helix stabi lization in aqueous 1 M NaCl solutions by Tyr-Leu or Leu-Tyr pairs when spa ced (i, i + 4) and, to a smaller extent, when spaced (i, i + 3), comparable to those estimated for other residue pairs. The interactions persist in so lutions containing TFE, a helix-stabilizing solvent believed to diminish hy drophobic interactions, but not in helix-destabilizing 6 M urea. H-1 NMR re sonances for all peptides and solution conditions except in 6 M urea were c ompletely assigned. NMR data indicate that the N-terminal residues are more helical and that the N-acetyl group participates in helix formation. The t wo (i, i + 3) spaced pairs show the same pattern of NOE cross-peaks between the Tyr and Leu side chains, as do the two (i, i + 3) pairs in 1 M NaCl as well in TFE solutions, and correspond well with that expected for the spec ific Tyr-Leu pair with side-chain contacts in protein helices.