A planar rotor model for cholestane spin label motion in phospholipid multibilayers with high order

Abstract

An EPR study of a macroscopically oriented multibilayer system of DMPC/cholesterol (2:1), order parameter S≈1, is presented. The conventional cholestane spin label 14N-CSL as well as its isotopically substituted analogue 15N-CSL were used in the experiments. The temperature range studied, −18 to 18° C, covered the rigid-limit and the slow-motion regime. The experimental spectra were simulated by a numerical solution of the stochastic Liouville equation using Padé approximant techniques. As the ordering of the CSL molecules in this bilayer system is virtually indistinguishable from perfect alignment of the long molecular axes along the director (the normal to the multibilayer plane) the planar rotor model was used to describe the reorientational motions. Thus the molecules were taken to possess only a single rotational degree of freedom about their long axes. The analysis of the slow-motion spectra reveals that the CSL molecules undergo a random rotational jump motion rather than small-step Brownian diffusion. The use of both the 14N-CSL and 15N-CSL greatly improves the reliability of the spectral simulations. Furthermore the use of the 15N-CSL molecules affords a unique mapping of the orientational distribution of the molecules in the rigid-limit spectra in this system, in contrast to the information obtained with the 14N-CSL spectra.