A multifrequency ESR study of the complex dynamics of membranes

A combined 250 and 9 GHz ESR study was performed on membrane vesicles compo sed 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 r epresent a " fast time-scale " such that the overall restricted motion of t he 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 th e 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 in creasing motional rates, whereas its effects on the overall lipid motion ar e modest. This leads to a clearer characterization of the dynamic structure of the cholesterol-rich liquid-ordered phase as compared to the liquid cry stalline phase. This study thus shows the significant potential of a multif requency ESR approach to the complex dynamics of membranes.