Interactions of 2,2,2-trifluoroethanol with melittin

Melittin dissolved in 42% trifluoroethanol-water at pH 2 has been shown to be alpha-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 degrees at 0 degrees C and 135 +/- 3 degrees at 25 degrees 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 degrees 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 degrees C, peptide-trifluoroethanol cross-relaxation terms are negative, a result consistent with the conclusion that fluoroalcohol molecules associate with the peptide for times (approximately 1 ns) that are long compared to the time of a typical peptide-fluoroalcohol diffusive encounter (approximately 0.2 ns). Such interactions may be responsible for the reduction of the translational diffusion coefficient of trifluoroethanol produced by dissolved peptides.