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Helix-Stabilizing Nonpolar Interactions between Tyrosine and Leucine in Aqueous and TFE Solutions: 2D- 1 H NMR and CD Studies in Alanine-Lysine Peptides

1998; American Chemical Society; Volume: 37; Issue: 49 Linguagem: Inglês

10.1021/bi9813678

1943-295X

S. Padmanabhan, M. Ángeles Jiménez, Douglas V. Laurents, M. Rico,

Chemical Synthesis and Analysis

Interactions between side chains spaced (i, i + 3) and (i, i + 4) may explain the context dependence of helix propensities observed in different systems. Nonpolar residues with these spacings occur frequently in protein helices and stabilize isolated peptide helices. Here (i, i + 3) and (i, i + 4) nonpolar interactions between Tyr and Leu in different solution conditions are studied in detail in alanine-based peptides using 2D 1H NMR and CD spectroscopy. Helix contents analyzed using current models for helix-coil transitions yield interaction energies which demonstrate significant helix stabilization in aqueous 1 M NaCl solutions by Tyr-Leu or Leu-Tyr pairs when spaced (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 solutions containing TFE, a helix-stabilizing solvent believed to diminish hydrophobic interactions, but not in helix-destabilizing 6 M urea. 1H NMR resonances for all peptides and solution conditions except in 6 M urea were completely assigned. NMR data indicate that the N-terminal residues are more helical and that the N-acetyl group participates in helix formation. The two (i, i + 4) 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 specific Tyr-Leu pair with side-chain contacts in protein helices.