MICROSCOPIC VERSUS MACROSCOPIC DIFFUSION IN MODEL MEMBRANES BY ELECTRON-SPIN-RESONANCE SPECTRAL-SPATIAL IMAGING

The macroscopic and the microscopic diffusion coefficients of a phospholipid spin label (16-PC) in the model membrane 1-palmitoyl-2-oleoyl-sn-glycero-phosphatidylcholine have been measured simultaneously in the same sample utilizing the new technique of spectral-spatial electron spin resonance imaging. The macroscopic diffusion coefficient D(macro) for self-diffusion of 16-PC spin label is obtained from imaging the concentration profiles as a function of time, and it is (2.3 +/- 0.4) x 10(-8) cm2/s at 22-degrees-C. The microscopic diffusion coefficient D(micro) for relative diffusion of the spin probes is obtained from the variation of the spectral line broadening with spin label concentration, which is due to spin-spin interactions. D(micro) is found to be substantially greater than D(macro) for the same sample at the same conditions, and is estimated to be at least (1.0 +/- 0.4) x 10(-7) cm2/s. Possible sources for their difference are briefly discussed in terms of the models used for D(micro).