Linewidth analysis of spin labels in liquids - II. Experimental

This work demonstrates that homogeneous linewidths can be extracted from co ntinuous wave electron paramagnetic resonance spectra and that they quantit atively agree,vith the predictions of existing relaxation theory. We sugges t that relaxation theory can be used to predict experimental lineshapes pro vided that the simulations properly include sources of broadening. We have found that the rotational correlation times for spin labels in different pe rcentages of glycerol/water mixtures are best modeled by a power law treatm ent for the viscosity, similar to that for translational diffusion. The tra nslational diffusion coefficients themselves also have a power law dependen ce on the viscosity for glycerol/water mixtures. The linewidths were linear ly dependent upon both the oxygen and the spin label concentration. The hyp erfine splittings of all nuclei were observed to decrease linearly with inc reasing spin label concentration, completely at odds with existing theory w hich predicts a quadratic dependence upon concentration. The linear depende nce was independent of hyperfine splitting until the magnitude of the hyper fine splitting was less than the homogeneous linewidth. (C) 1999 Academic P ress.