Electron spin resonance study of phospholipid membranes employing a comprehensive line-shape model

Abstract

The ESR spectra of the 1-myristoyl-2-[6-(4,4-dimethyloxazolidine-N-oxyl)myristoyl]-sn-glycero-3-phosphocholine spin-label in highly oriented, fully hydrated bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine were studied as a function of temperature and magnetic field orientation. The oriented spectra show clear indications of slow motional components (rotational correlation times > 3 ns) even in the fluid phase (T > 23.degree. C), indicating that motional narrowing theory is not applicable to the structural analysis. The spectra were simulated by a comprehensive line-shape model that incorporates trans-gauche isomerization in addition to restricted anisotropic motion of the lipid long molecular axis and that is valid in all motional regimes. In the gel (L.beta.'') phase the spine-label chains are found to be tilted at 28.degree. with respect to the normal of the orienting plane. In the intermediate (P.beta.'') phase there is a continous distribution of tilt angles between 0.degree. and 25.degree.. In the fluid (L.alpha.) phase there is no net tilt of the lipid chains. The chains rotate at an intermediate rate about their long axis in the fluid phase (.tau.R,.dblvert. = 1.4-6.6 ns for T = 50-25.degree. C), but the reorientation of the chain axis is much slower (.tau.R,.perp. = 13-61 ns for T = 50-25.degree. C), whereas trans-gauche isomerization (at the C-6 position) is rapid (.tau.J .ltoreq. 0.2 ns). Below the chain melting transition both chain reorientation and chain rotation are at the ESR rigid limit (.tau.R .gtoreq. 100 ns), and trans-gauche isomerization is in the slow-motion regime (.tau.J = 3.7-9.5 ns for T = 22-2.degree. C). The chain order parameter increases continuously with decreasing temperature in the fluid phase (SZZ = 0.47-0.61 for T = 50-25.degree. C), increases abruptly on going below the chain melting transition, and then increases continuously in the intermediate phase (SZZ = 0.79-0.85 for T = 22-14.degree. C) to an approximately constant value in the gel phase (SZZ .simeq. 0.86 for T = 10-2.degree. C). The trans rotational isomer population (corresponding to the C-6 position) remains approximately constant in the fluid phase (nt .simeq. 0.52 for T = 50-25.degree. C) and decreases abruptly at the chain melting transition to a constant value in the intermediate and gel phases (nt .simeq. 0.44 for T = 22-2.degree. C). The results were compared with those obtained from 2H NMR spectroscopy of dimyristoylphosphatidylcholine deuterium labeled at the same position of the sn-2 chain. Most of the parameters governing the chain order and dynamics are in good accord, except that the gauche population is considerably higher for the spin-labeled chain and its isomerization rate is much faster in the low-temperature phases. The implications for the interpretation of spin-labeled measurements in membranes are discussed with particular reference to the well-known discrepancy in the order gradients along the chains, obtained by the 2 methods.