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Abstract

Calibrations are given to extract orientation order parameters from pseudo-powder electron paramagnetic resonance line shapes of 14N-nitroxide spin labels undergoing slow rotational diffusion. The nitroxide z-axis is assumed parallel to the long molecular axis. Stochastic-Liouville simulations of slow-motion 9.4-GHz spectra for molecular ordering with a Maier–Saupe orientation potential reveal a linear dependence of the splittings, 2Amax and 2Amin, of the outer and inner peaks on order parameter Szz that depends on the diffusion coefficient DR⊥ which characterizes fluctuations of the long molecular axis. This results in empirical expressions for order parameter and isotropic hyperfine coupling: Szz=s1×(Amax−Amin)−so and ao=13(fmaxAmax+fminAmin)+δao, respectively. Values of the calibration constants s1, so, fmax, fmin and δao are given for different values of DR⊥ in fast and slow motional regimes. The calibrations are relatively insensitive to anisotropy of rotational diffusion (DR//≥DR⊥), and corrections are less significant for the isotropic hyperfine coupling than for the order parameter.