Lateral diffusion of lipids in membranes by pulse saturation recovery electron spin resonance.

Abstract

Short-pulse saturation recovery electron spin resonance methods have been used to measure lateral diffusion of nitroxide-labeled lipids in multilamellar liposomal dispersions. Nitroxides with 14N and 15N isotopes introduced both separately and together were used. Differential equations have been written and solved for complex saturation recovery signals involving several superimposed exponentials. The time constants contain various combinations of the spin-lattice relaxation time (Tle) for both isotopes, Heisenberg exchange rates, and nuclear spin-lattice relaxation times (T1n). Signals of high quality were fitted by Monte Carlo variation of the amplitudes and time constants. The reliability of the approach was tested extensively by verifying that (i) the predicted number of exponentials agreed with the experimental number, (ii) relaxation parameters that were determined were independent of the observed hyperfino transition, (iii) the time constants were independent of saturating pulse length, (iv) Tle and T1n do not change when Heisenbeg exchange is changed by varying the concentration, and (v) Heisenberg exchange is indeed proportional to the concentration. It has been established that bimolecular collision rates over a wide range conditions can be reliably measured using the methodology described here. The methods depend on the favorable match of bimolecular collision rates at micromolar concentrations to nitroxide spinlattice relaxation probabilities.