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

Abstract

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 IR 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, IR 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 IR 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.degree. C) favor the .alpha.-helical form, while low temperatures (8.degree. C) lead to populations of both .beta. and .alpha.-helical strcutures. 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.