A multifrequency ESR study of the complex dynamics of membranes

A combined 250 and 9 GHz ESR study was performed on membrane vesicles composed of pure lipid (DPPC) and of DPPC:cholesterol in a 1:1 molar ratio using the end chain labeled lipid, 16-PC. It is shown that the 250 GHz spectra represent a " fast time-scale " such that the overall restricted motion of the lipid in the membrane is frozen out, but it is sensitive to the internal dynamics of the end chain. The 9 GHz spectra are, however, sensitive to the overall motion as well. This combined study thus permitted the separation of both types of motion. It is shown that the addition of cholesterol has a large effect on the end chain dynamics by restricting its motion while increasing motional rates, whereas its effects on the overall lipid motion are modest. This leads to a clearer characterization of the dynamic structure of the cholesterol-rich liquid-ordered phase as compared to the liquid crystalline phase. This study thus shows the significant potential of a multifrequency ESR approach to the complex dynamics of membranes.