Measurement of rotational dynamics by the simultaneous nonlinear analysis of optical and EPR data.

In the preceding companion article in this issue, an optical dye and a nitroxide radical were combined in a new dual function probe, 5-SLE. In this report, it is demonstrated that time-resolved optical anisotropy and electron paramagnetic resonance (EPR) data can be combined in a single analysis to measure rotational dynamics. Rigid-limit and rotational diffusion models for simulating nitroxide EPR data have been incorporated into a general non-linear least-squares procedure based on the Marquardt-Levenberg algorithm. Simultaneous fits to simulated time-resolved fluorescence anisotropy and linear EPR data, together with simultaneous fits to experimental time-resolved phosphorescence anisotropy decays and saturation transfer EPR (ST-EPR) spectra of 5-SLE noncovalently bound to bovine serum albumin (BSA) have been performed. These results demonstrate that data from optical and EPR experiments can be combined and globally fit to a single dynamic model.