Structure and solvation of melittin in hexafluoroacetone/water

Intermolecular 1H{19F} and 1H{1H} nuclear Overhauser effects have been used to explore interaction of solvent components with melittin dissolved in 50% hexafluoroacetone trihydrate (HFA)/water. Standard nuclear Overhauser effect experiments and an analysis of C(alpha)H proton chemical shifts confirm that the conformation of the peptide in this solvent is (alpha)-helical from residues Ala4 to Thr11 and from Leu13 to Arg24. The two helical regions are not collinear; the interhelix angle (144 (plus-minus) 20°) found in this work is near that observed in the solid state and previous NMR studies. Intermolecular NOEs arising from interactions between spins of the solvent and the solute indicate that both fluoroalcohol and water molecules are strongly enough bound to the peptide that solventsolute complexes persist for >=2 ns. Preferential interactions of HFA with many hydrophobic side chains of the peptide are apparent while water molecules appear to be localized near hydrophilic side chains. These results indicate that interactions of both HFA and water are qualitatively different from those present when the peptide is dissolved in 35% hexafluoro-2-propanol/water, a chemically similar helix-supporting solvent system.