A restatement of melittin-induced effects on the thermotropism of zwitterionic phospholipids

Abstract

Perturbations induced by melittin on the thermotropism of dimyristoyl-, dipalmitoyl-, distearoylphosphatidylcholine and natural sphingomyelin are investigated and rationalized from data obtained by fluorescence polarization, differential scanning calorimetry and Raman spectroscopy. Depending on the technique and/or experimental conditions used, the observed effects differ at the same lipid to protein molar ratio, due to partial binding of melittin. The binding is more efficient for tetrameric than for monomeric melittin, but in both cases its affinity is weaker for phosphatidylcholine dispersions in the gel phase than for sonicated vesicles. For temperatures T greater than or equal to Tm efficient binding occurs whatever the initial state of the lipids is. One can summarize the effects induced by melittin on the transition temperature as follows: No upward shift is observed on synthetic phosphatidylcholines when lipid degradation is avoided. This is achieved by using highly purified melittin, phospholipase inhibitors, and/or non-hydrolysable lipids. Melittin monomer does not change Tm. When melittin tetramer is stabilized, it decreases Tm by 10-15 deg. C. The transition broadens, and is finally abolished for Ri less than or equal to 2. Very similar results are found for natural sphingomyelin. Fluorescence polarization indicates similar changes in order and dynamics of the acyl chains for all lipid studied. For T less than or equal to Tm, fluorescence and Raman show that melittin decreases the amount of CH2 groups in 'trans' conformation and the intermolecular order of the chains. According to fluorescence data, there is an increase of the rigid-body orientational order at T greater than or equal to Tm, while from Raman the positional intermolecular order decreases without significant change in the CH2 groups 'trans'/'gauche' ratio.