Helix Conformation of a Small Peptide Melittin in a Methanol-Water Mixed Solvent Studied by NMR

Temperature dependence of the alpha-helix conformation of bee venom melittin in methanol-water mixed solvents has been examined by NMR, in order to elucidate conformation stability and a phase diagram. At high methanol concentration of 100 - ca. 80 wt.%, melittin forms a full alpha-helix conformation in the temperature range from 25 degrees C to 60 degrees C. At intermediate methanol concentration of ca. 80 - ca. 25 wt.%, it undergoes a thermal transformation from a full alpha-helix to a partial alpha-helix. In solutions of low methanol concentrations of ca. 25 - 0 wt.%, partial alpha-helix monomers and their self-aggregated conformers coexist at low temperatures, and the relative number of the monomers increases with increase in temperature. The monomers turn to a random coil state at high temperatures only below ca. 10 wt. % methanol concentrations. The thermal transitions are discussed from the viewpoint of stability of intra-molecular hydrogen bonds and intermolecular hydrophobic interactions.