The utility of multifrequency and very high frequency EPR for full determination of the rotational diffusion tensor was recently demonstrated by Freed and coworkers for selected nitroxides in deuterated toluene solvent. In this work we extend this approach to the nitroxides with complex superhyperfine (shf) structure such as 3-doxyl-17a-hydroxy-5`-androstane (probe 1) and 3-maleimido-PROXYL (probe2). O-xylene was used as a solvent. Even at a relatively long rotational correlation time, gw10-9 sec, the X-band EPR spectra do not reveal any motion effects because of a complex shf pattern. At 95 GHz (W-band), the range of rotational correlation times measured by EPR is extended by a factor of 10 towards short times compared with 9.5 GHz, producing a more pronounced motional effect on the linewidth at the same correlation times. The shf is purely resolved at W-band, but it contributes significantly to the inhomogeneous linewidth and makes direct estimation of g inaccurate. We use shf information from the X-band spectra in order to improve the accuracy of g determination. The linewidth parameters are extracted from W-band spectra via Levenberg-Marquardt optimization under the assumption of a fixed shf pattern. Microwave phase effects are directly included in the fitting algorithm. Results are analyzed in terms of anisotropic Brownian diffusion theory. There are many potential applications to motion in biological membranes and study of MRI contrast agents bound to membranes.
Showing the most recent 10 out of 16 publications
