Doxyl nitroxide probes and the intrinsic flexibility gradient. A slow-motional line shape analysis study

The ESR spectra of seven doxyl-labeled stearic acids and stearic methyl esters dissolved in the liquid crystalline solvent Phase V have been examined over a wide range of temperature with the intent of addressing both the question of the intrinsic flexibility gradient of such probes in a gradient-free environment and questions related to the dynamics and ordering details of such systems. We find that slow-motional effects are observable in the ESR spectra at temperatures as high as 45°C and that with the full line shape analysis derived from the stochastic Liouville equation a wealth of information is obtainable from these systems. Our results support the model of ah intrinsic flexibility gradient originating at the center of mass of the probe; further, the spectra are not interpretable without invoking a dependence of solute dynamics on environment such as that arising from anisotropic viscosity.