Interactions of 2,2,2‐trifluoroethanol with melittin

Melittin dissolved in 42% trifluoroethanol‐water at pH 2 has been shown to be α‐helical between residues 6 and 12 and between residues 13 and 25, with the two helical regions separated by a bend at the Leu13 residue. The inter‐helix angle was found to be 154 ± 3° at 0 °C and 135 ± 3° at 25 °C. The dominant conformation of the peptide is thus similar to those observed by previous workers for the peptide in a variety of media. At 25 °C, intermolecular nuclear Overhauser effects arising from nuclear spin dipole‐dipole interactions between melittin hydrogens and fluorines of the solvent are essentially those expected for a system that is homogeneous as regards concentration and translational diffusion of the peptide and fluoroalcohol components. However, at 0 °C, peptide‐trifluoroethanol cross‐relaxation terms are negative, a result consistent with the conclusion that fluoroalcohol molecules associate with the peptide for times (～1 ns) that are long compared to the time of a typical peptide‐fluoroalcohol diffusive encounter (～0.2 ns). Such interactions may be responsible for the reduction of the translational diffusion coefficient of trifluoroethanol produced by dissolved peptides. Copyright © 2009 John Wiley & Sons, Ltd.