EPR lineshapes and dynamics of TEMPOL solubilized in the aqueous microphase of lecithin/cyclohexane/water reverse micelles

EPR spectral lineshapes of the spin probe TEMPOL(4-hydroxy-2,2,6,6- tetramethyl-1-piperidinyl-1-oxyl), dissolved in the aqueous microphase of egg-yolk lecithin/cyclohexane/water reverse micelles, have been studied as a function of the water/lecithin molar ration, R. Although the spectral profiles reveal in general that the spin probe undergoes a ‘fast’ reorientational motion in the aqueous phase, the actual lineshapes are found to be non-Lorentzian. It is shown that, for an accurate lineshape analysis, the motion-dependent Lorentzian linewidths have to be convoluted with an appropriate motion-invariant Gaussian broadening factor representing unresolved isotropic superhyperfine interactions. Fast reorientational correlation times (τ), assessed from our iterative lineshape fitting procedure, span the range from 6.0 × 10−10 to 3.0 × 10−10 s as the R-value is increased.