Unifying description of the effect of membrane proteins on lipid order. Verification for the melittin/dimyristoylphosphatidylcholine system.

The effect of melittin on lipid order in dimyristoylphosphatidylcholine bilayers is investigated by means of Raman spectroscopy and fluorescence anisotropy using diphenylhexatriene as fluorescence probe. In the fluid lipid phase, the Raman results indicate a slight increase in the conformational order of the lipid chains, and the fluorescence anisotropy results indicate a considerable increase in the rigid-body orientational order of the lipid chains. These results are contrasted with the reported decrease of the deuterium magnetic resonance order parameter. A consistent interpretation of the complete set of experimental data is presented according to which proteins induce a tilt of the preferred axes of lipid orientation and increase the orientational order with respect to these axes. The values of the tilt angle and the orientational order parameter at the surface of proteins are determined from the experimental data within a continuum model of lipid-protein interaction. The same values are obtained for melittin, Ca/Mg-ATPase, and cytochrome c oxidase, suggesting that different membrane proteins affect the lipid order in the same way.