TEMPERATURE AND IONIC-STRENGTH DEPENDENT LIGHT-SCATTERING OF DMPG DISPERSIONS

The temperature dependence of the intensity of light scattered by aque ous dispersions of the anionic lipid DMPG (dimyristoyl phosphatidylgly cerol) was studied at different ionic strengths. The lipid main transi tion, gel-liquid crystal, is clearly detected by a sharp decrease in l ight scattering. As expected, the temperature of the main transition ( T-m) was found to increase with the increase of the ionic strength. Fo r low ionic strength, a DMPG second temperature transition, termed the 'post-transition', can be monitored by both an increase in light scat tering and a decrease in conductivity. Zimm plot analysis indicates th at below T-m the liposomes tend to aggregate, and show a negative seco nd virial coefficient A(2), and particles of large molecular weight. A t the main phase transition, parallel to the decrease in light scatter ing, there is an increase in the sample conductivity, A(2) becomes pos itive and the particle molecular weight decreases, indicating that the vesicles become disaggregated. Moreover, at the post-transition (T-po st) A(2) becomes very small, perhaps slightly negative, and the molecu lar weight increases again. Both the main and the post transitions wer e 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 incre ased ionization at T-m could be related to the melting of the hydrocar bon chains, the mechanism for the possible counterions recondensation at T-post is far less clear. (C) 1997 Elsevier Science Ireland Ltd.