ESR study of the liposome membrane physical parameters in the heating-cooling cycles

Changes of dynamic and structural parameters of egg yolk lecithin (EYL) liposome membranes in the heating-cooling cycles have been studied using the ESR spin probe method. The investigations were conducted in the range of temperatures from -18 degrees C to +60 degrees C. It has been found that in the range of temperatures -15 degrees C to +45 degrees C in both the heating and the cooling run the spectroscopic parameters changed practically along the same curve (reversible changes). However, after exceeding this range of temperatures one of the parameters (partition coefficient of the spin probe 2,2,6,6--tetramethylpiperidine -1-oxyl; TEMPO) changed along a closed curve, showing the phenomenon of thermal hysteresis. In the heating process the TEMPO content in liposome membranes was smaller than this in the cooling process. We assume that during the heating, the lipid molecules of the outer liposome layers dissolve in the aqueous medium. In the cooling process they can aggregate and form new liposomes, what in turn increases the surface of liposome membranes, accessible for the TEMPO probe (active surface).