CHAIN DYNAMICS AND THE SIMULATION OF ELECTRON-SPIN-RESONANCE SPECTRA FROM ORIENTED PHOSPHOLIPID-MEMBRANES

A model previously developed for describing the dynamics of flexible a lkyl chains that is based on Flory's rotational isomeric state approxi mation is adapted and applied to the analysis of electron spin resonan ce (ESR) spectra obtained from a phospholipid spin label in a macrosco pically aligned phospholipid membrane, In this model, rotation around each C-C bond of the labeled alkyl chain is characterized by three ine quivalent minima, with one end of the chain fixed to mimic the phospho lipid headgroup, and with the dynamic effects of the nitroxide label e xplicitly included. This model is integrated with that for the overall rotation of the phospholipid in the mean orientational potential of t he aligned membrane, and it is incorporated into the stochastic Liouvi lle equation which describes the ESR line shape in the presence of the se dynamic processes. The analysis is simplified by introducing the fa ct that the relatively rapid internal modes of motion can be treated b y motional narrowing theory and a time scale separation can be made wi th respect to the much slower overall motions of the phospholipid. A s eries of ESR spectra from the spin label 16-PC in the lipid dimyristoy lphosphatidylcholine were obtained over a range of temperatures (35-65 degrees C) in the L-alpha phase for various orientations of the norma l to the bilayer plane relative to the magnetic field. Very good agree ment with experiment is obtained from this model by using least square s fitting procedures for the overall motional dynamics. One finds an o rder parameter of [D-00(2)] that is constant throughout the phase and the perpendicular component for rotational diffusion, R-perpendicular to, that ranges from about 1-3 x 10(7)s(-1) (which corresponds to the ESR slow motional regime). Fits to the ESR spectra were also obtained from a simple but standard model wherein a single overall rotational d iffusion tensor is used to describe the combined effects of internal a nd overall dynamics. These fits were almost as good, but they lead to a much larger R-perpendicular to approximate to 3-6 x 10(8)s(-1) and a smaller [D-00(2)] = 0.1, since these parameters now include the compo site effects of both types of processes. New ESR experiments are propo sed to provide more critical tests of these models.