Temperature and ionic strength dependent light scattering of DMPG dispersions

Abstract

The temperature dependence of the intensity of light scattered by aqueous dispersions of the anionic lipid DMPG (dimyristoyl phosphatidylglycerol) was studied at different ionic strengths. The lipid main transition, gel-liquid crystal, is clearly detected by a sharp decrease in light scattering. As expected, the temperature of the main transition (Tm) was found to increase with the increase of the ionic strength. For low ionic strength, a DMPG second temperature transition, termed the `post-transition', can be monitored by both an increase in light scattering and a decrease in conductivity. Zimm plot analysis indicates that below Tm the liposomes tend to aggregate, and show a negative second virial coefficient A2, and particles of large molecular weight. At the main phase transition, parallel to the decrease in light scattering, there is an increase in the sample conductivity, A2 becomes positive and the particle molecular weight decreases, indicating that the vesicles become disaggregated. Moreover, at the post-transition (Tpost) A2 becomes very small, perhaps slightly negative, and the molecular weight increases again. Both the main and the post transitions were found to be reversible. In contrast to the main transition, the post-transition could not be detected by spin labels placed either at the membrane surface, or in the bilayer core. While the mechanism of increased ionization at Tm could be related to the melting of the hydrocarbon chains, the mechanism for the possible counterions recondensation at Tpost is far less clear.