SIMULATION OF SATURATION TRANSFER ELECTRON-PARAMAGNETIC RESONANCE-SPECTRA FOR ROTATIONAL MOTION WITH RESTRICTED ANGULAR AMPLITUDE

We have simulated both conventional (V1) and saturation transfer (V2') electron paramagnetic resonance spectra for the case of Brownian rotational diffusion restricted in angular amplitude. Numerical solutions of the diffusion-coupled Bloch equations were obtained for an axially symmetric N-14 nitroxide spin label with its principal axis rotating within a Gaussian angular distribution of full width DELTAtheta at half maximum. Spectra were first calculated for a macroscopically oriented system with cylindrical symmetry (e.g., a bundle of muscle fibers or a stack of membrane bilayers), with the Gaussian angular distribution centered at theta0 with respect to the magnetic field. These spectra were then summed over theta0 to obtain the spectrum of a randomly oriented sample (e.g., a dispersion of myofibrils or membrane vesicles). The angular amplitude DELTAtheta was varied from 0-degrees, corresponding to no motion (order parameter = 1 ), to greater-than-or-equal-to 270-degrees, corresponding to isotropic motion (order parameter = 0). For each value of DELTAtheta, the rotational correlation time, tau(r), was varied from 10(-7) to 10(-2) s, spanning the range from maximal to minimal saturation transfer. We provide plots that illustrate the dependence of spectral parameters on DELTAtheta and tau(r). For an oriented system, the effects of changing DELTAtheta and tau(r) are easily distinguishable, and both parameters can be determined unambiguously by comparing simulated and experimental spectra. For a macroscopically disordered system, the simulated spectra are still quite sensitive to DELTAtheta, but a decrease in tau(r) produces changes similar to those from an increase in DELTAtheta. If DELTAtheta can be determined independently, then the results of the present study can be used to determine tau(r) from experimental spectra. Similarly, if tau(r) is known, then DELTAtheta can be determined.