Effect of Melittin on Phase Transition and Fluidity of a Phospholipid Model Membrane

The interaction of melittin, the major protein component of bee venom, with dimyristoyl phosphatidylcholine ( DMPC ) liposomes was investigated by Electron Spin Resonance (ESR) spectroscopy using high resolution amphiphilic spin probe perdeutero-di-t-butyl nitroxide (PDDTBN). For pure DMPC liposomes, ESR signal due to the probe dissolved in the fluid hydrophobic environment provided by the interior of the phospholipid bilayer (lipid site) is observed around 24°C which corresponds to the main gel to liquid crystalline phase transition of pure DMPC liposomes. Howewer with the addition of melittin, onset temperatures of lipid lines shift towards higher temperatures. For lipidrmelittin molar ratios of 32:1 and 19:1, lipid line appears at 28°C, and for lipid:melittin molar ratios of 14:1 and 10:1, lipid site appears around 30.5°C. The results are discussed in terms of boundary lipids. Variation with temperature of both partition coefficient and peak to peak linewidth of high field lipid lines obtained by spectral deconvolution, indicate that in the liquid crystalline phase fluidity of the DMPC bilayers decreases when melittin concentration is increased.