Melittin is shown to affect the structure of the charged
phospholipid dipalmitoylphosphatidylglycerol (
DPPG). In the gel phase, the presence of
melittin leads to (i) an increased
lipid interchain vibrational coupling, (ii) a shift of the rectangular to hexagonal
lipid packing transition toward low temperatures, (iii) a very small conformational disordering effect, (iv) a decrease of the polarity or hydrogen bonding capability of the
lipid ester group surrounding, (v) an important decrease of the water content in the complexes where the remaining water has a more disordered structure than bulk water, and (vi) an interlamellar repeat distance of 79 A. All these observations are rationalized by the following model: adjacent bilayers of
DPPG are bridged by tetramers of
melittin through electrostatic interactions inducing surface charge neutralization and partial
dehydration of the complexes.
Melittin also affects the thermotropic behavior of
DPPG. When a small amount of the toxin is present, its affinity for charged
lipids is such that a phase separation occurs, the domains being stable enough to have their own gel to liquid-crystalline phase transition. In the fluid state, a deeper penetration into the
lipid matrix is proposed based on the downshift of the phase transition and the low vibrational interchain coupling. This study brings out general features of cationic species/anionic
lipid complexes. The charge neutralization leads to stronger interchain coupling, and electrostatic bridging of adjacent bilayers seems to be common. The hydrophobicity of the
peptide is a key factor in the modulation of the gel to liquid-crystalline phase transition and in its insertion in the fluid
lipid matrix.