Contributions to the Gaussian line broadening of the proxyl spin probe EPR spectrum due to magnetic-field modulation and unresolved proton hyperfine structure

A simple expression is derived to compute the total Gaussian linewidth of a Voigt line that is broadened by sinusoidal magnetic-field modulation as follows: Delta H-pp(G)(H-m)(2) = Delta H-pp(G)(0)(2) + kappa(2)H(m)(2), where Delta H-pp(G)(H-m) is the Gaussian linewidth observed with an modulation amplitude H-m/2 and Delta H-pp(G)(0) is the Gaussian line-width in the limit of zero modulation. The field modulation contributes an additional Gaussian broadening of kappa H-m, where kappa is a constant, which adds in quadrature to Delta H-pp(G)(0) to give the total Gaussian linewidth, Denoting the overall linewidth of the Voigt line in the absence of modulation broadening by Delta H-pp(0)(0), it is shown, both by analytical means and by spectral simulation, that the constant kappa is equal to 1/2 in the limit of H-m much less than Delta H-pp(0)(0); however, using values of H-m as large as Delta H-pp(0)(0) leads to only minor departures from kappa = 1/2. The formulation is valid both for Lorentzian and Voigt lines and is tested for 2,2,5,5-tetramethylpyrrolidin-1-oxyl-3-carboxylic acid (3-carboxy proxyl) in CCl4 and in aqueous buffer. This spin probe was studied because the proxyl group is the only major spin-probe moiety whose Gaussian linewidth had not been characterized in the literature. For 3-carboxy proxyl, it is found that Delta H-pp(G)(0) = 1.04 +/- 0.01 G independent of solvent polarity. Precision values of the N-14 hyperfine coupling constant for 3-carboxy proxyl at 9.5 degrees C are as follows: 14.128 +/- 0.001 G in CCl4 and 16.230 +/- 0.002 G in aqueous buffer. The temperature dependence of Delta H-pp(G)(0) and the N-14 hyperfine coupling constant are reported as empirical equations. Results of the present work taken together with previously published data permits accurate correction for the effects of inhomogeneous broadening due to unresolved hyperfine structure and modulation broadening for the majority of spin probes in common use. (C) 1998 Academic Press.