Dependence of melittin structure on its interaction with multivalent anions and with model membrane systems

The conformation and the aggregation state of melittin were investigated in aqueous solutions having different pH values and ionic composition as well as upon interaction with phospholipids. While circular dichroism could only show the existence, in aqueous solution, of either a random-coil or of a right-handed helical conformation, high resolution 1H and 13C n.m.r. spectra, together with the results of photon correlation spectroscopy, produced evidence in favour of a number of different well-defined structural states, depending on anion concentration and charge and on pH. In pure water at neutral pH melittin appeared to exist as a flexible random-cell monomer; in dilute NaCl a monomeric form was found which was still essentially unordered, but presented a pronounced rigidity of structure, and could be approximated to a prolate ellipsoid. When divalent anions were present (or when high ionic strengths were reached even with monovalent anions) melittin molecules associated into a compact disc-like tetramer; by 31P n.m.r., correlations could be established between the binding of phosphate ions and the variations in the structure or in the aggregation state of the polypeptide chains. As alkaline pH a helical tetramer was also found, different, however, from that formed on the presence of divalent anions at neutral pH. Upon binding to phospholipids, melittin molecules can be visualized, similarly to what happens in aqueous phosphate solutions, as consisting essentially of a bent right-handed helix, with a grouping of polar residues along one face of the molecule. The glutamine and lysine residues were strongly immobilized, while there was no n.m.r. evidence for any self-aggregation of the peptide; the ability of melittin to induce dichromate efflux from phospholipid vesicles was in fact higher when the peptide was in the monomeric state than in the tetrameric one.