Spin-label Order Parameter Calibrations for Slow Motion

Calibrations are given to extract orientation order parameters from pseudo-powder electron paramagnetic resonance line shapes of (14)N-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, [Formula: see text] and [Formula: see text] , of the outer and inner peaks on order parameter [Formula: see text] that depends on the diffusion coefficient [Formula: see text] which characterizes fluctuations of the long molecular axis. This results in empirical expressions for order parameter and isotropic hyperfine coupling: [Formula: see text] and [Formula: see text] , respectively. Values of the calibration constants [Formula: see text] , [Formula: see text] , [Formula: see text] , [Formula: see text] and [Formula: see text] are given for different values of [Formula: see text] in fast and slow motional regimes. The calibrations are relatively insensitive to anisotropy of rotational diffusion [Formula: see text] , and corrections are less significant for the isotropic hyperfine coupling than for the order parameter.