Calibration of Membrane Viscosity of the Reconstituted Vesicles by Measurement of Rotational Diffusion of Bacteriorhodopsin

Membrane viscosity of the reconstituted vesicles was calibrated by rotational diffusion of bacteriorhodopsin (BR) in
dimyristoylphosphatidylcholine (DMPC) and egg phosphatidylcholine (PC) vesicles. Rotational diffusion of BR in the vesicles was measured by flash-induced absorption anisotropy decay. BR was, for the first time, reconstituted successfully into DMPC and egg PC vesicles. From the measurement of flash-induced absorption anisotropy decay of BR, the value of rotational diffusion coefficient D was obtained from each curve fitting by a global fitting procedure and, in turn, membrane viscosity η was estimated from D. The results showed that membrane viscosity is temperature-dependent. It was decreased as temperature increased, but a transition occurred in the region of the respective phase transition of DMPC and egg PC, respectively. The decrease of η was fast near the phase transition for DMPC and egg PC. Few effects of lipid/BR ratio and glycerol or sucrose in suspension medium on membrane viscosity were found.