Magnetic field (g-value) dependence of proton hyperfine couplings obtained from ESEEM measurements: Determination of the orientation of the magnetic axes of model heme complexes in glassy media

Electron spin echo envelope modulation (ESEEM) studies were utilized to characterize the coupling between protons of axially bound pyrazole ligands (PzH) and the unpaired electron of low-spin tetraphenylporphyrinatoiron(III) chloride. Samples were prepared in mixed-solvent glasses to maximize the resolution of the electron paramagnetic resonance (EPR) signals. X-band two-pulse ESEEM experiments at 4.2 K in deuterated solvent glasses demonstrated that this coupling results in 0.2-0.7 MHz shifts of the nu(alpha) + nu(beta) proton sum combination peak from twice the Larmor frequency. These shifts have been investigated across most of the EPR absorption spectrum of [TPPFe(PzH)(2)]Cl-+(-). Two-pulse ESEEM spectra were simulated at different magnetic field positions. Combination peaks were observed from both distant protons (DP) (beta-pyrrole, ortho-phenyl, and beta-pyrazole) and near protons (NP) (alpha-H of the pyrazole ligands). For the simulations, the orientation of the nearest four protons of the pyrazole ligands with respect to the g-tensor of the complex, the Fe-III-proton distance, and g-strain were taken as input parameters. Comparison of the experimental data and computer simulations, in terms of magnetic field dependence of both frequency and intensity data, allows for the determination of the orientation of the hyperfine coupling tensor of the protons in coordinates of the g-tensor principal axes. For the DP peak, the magnetic field dependence clearly shows that the maximum g-value is aligned with or close to the Fe-N-ax vector perpendicular to the plane of the porphyrinate. For the NP doublet, the results show that the alpha-H atoms of the axial pyrazole ligands, and thus the planes of those ligands, are aligned with the g(min) or g(xx) magnetic axis of the metal, and hence, the p(pi) orbital of the axial ligands are aligned with the g(yy) magnetic axis of low-spin Fe-III. Thus, in spite of the fact that the ''rhombicity'' defined by Blumberg and Peisach is much greater than the theoretical value of 2/3, the magnetic axes of this model heme complex correspond to a ''proper axis system'', with g(zz) > g(yy) > g(xx).