Infrared spectroscopic study of the secondary structure of melittin in water, 2-chloroethanol, and phospholipid bilayer dispersions.

The conformations of melittin, an amphipathic polypeptide consisting of 26 amino acid residues, and its hydrophobic (residues 1--19) and hydrophilic (residues 20--26) fragments were examined in various solvent systems, including H2O, 2H2O, 2-chloroethanol, and 1,2-dimyristoylphosphatidylcholine (DMPC) multilayers, by infrared spectroscopy. Water and 2-chloroethanol were used as reference solvents for characterizing the amide I and II vibrational frequencies of the polypeptide in systems reflecting unordered, beta-structure, or alpha-helical forms. In DMPC bilayer assemblies both melittin and its hydrophobic fragment F1 exhibit alpha-helical conformations. In contrast, infrared spectra for the hydrophilic F2 fragment are suggestive of a beta conformation with perhaps spectral contributions from random-coil configurations. The alpha-helical conformation of intact melittin in DMPC multilayer dispersions remains unchanged as the bilayer passes from the gel to liquid-crystalline state. For melittin-water solutions the infrared spectra monitor changes in population of specific conformations as the temperature is varied. Thus, for melittin concentrations in which tetramers are dominant high temperatures (31 degrees C) favor the alpha-helical form, while low temperatures (8 degrees C) lead to populations of both beta and alpha-helical structures. At lower melittin concentrations for which monomers persist, high temperatures favor an unordered polypeptide form, while low temperatures induce an alpha-helical conformation. Although peak-height intensity ratios AII/AI for the amide I and II regions are difficult to interpret rigorously, values of this parameter for aqueous solutions of melittin suggest a sensitivity to structural changes involving the aggregation properties of the polypeptide.