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

Abstract

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 ν R + ν 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) ( -pyrrole, orthophenyl, and -pyrazole) and near protons (NP) (R-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 R-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 π 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 .

Role of Calcium in Metalloenzymes: Effects of Calcium Removal on the Axial Ligation Geometry and Magnetic Properties of the Catalytic Diheme Center in MauG

“…Highly anisotropic low-spin (HALS) hemes have been known since the early work of Griffith ( 27, 28 ), Blumberg and Peisach ( 29, 30 ), Taylor ( 31 ), Bohan ( 32 ), Loew ( 33 ), Walker ( 26, 34-36 ) and many others (for reviews see ref. ( 35, 37, 38 )).…”

Porphyrin and Ligand Protons as Internal Labels for Determination of Ligand Orientation in ESEEMS of Low-Spin d5 Complexes in Glassy Media: ESEEM Studies of the Orientation of the g Tensor with Respect to the Planes of Axial Ligands and Porphyrin Nitrogens of Low-Spin Ferriheme Systems

“…The nearest protons (NP) to the low-spin Fe(III) center, the α-H of the pyrazole ligands, which are at 3.1−3.2 Å from the metal, provided the information concerning the orientation of g xx : The magnetic field dependence of the intensity of the NP doublet signal was shown to be consistent with the minimum g value, g xx being aligned along the direction of the α-H of the pyrazole ligands, or, that is, g xx lies along the nodal plane of the (as we had assumed, coparallel) pyrazoles we suspected that the axial ligand dihedral angles of 45° and 54° found for the two independent molecules in the unit cell may not represent the structure of the complex in frozen, glassy solutions, and thus we were left with no clear proof as to how the orientation of the in-plane g tensor related to the nitrogens of the porphyrin ring.…”

“…As will be shown, g yy is aligned along the nodal plane of the 4-NMe 2 Py ligands (90° different than in [TPPFe(PzH) 2 ] + 9 ), while g xx and g yy are aligned at angles of ±45° to the normal of the plane of the imidazole ligands. In distinction to the previous study, for the 4-(dimethylamino)pyridine complex all information concerning the orientation of the g tensor (both g zz and g xx ) has been obtained from the NP peak(s). In the companion work, we have explored the theoretical basis for the counterrotation of the g tensor with axial ligand rotation and have shown that the angular dependence may not be simply linear and that corotation can be observed in some cases.…”

Structure and spin density of ferric low-spin heme complexes determined with high-resolution ESEEM experiments at 35 GHz

“…The distribution of the unpaired electron within the 3d orbitals of Fe(III) can be obtained from the g values measured in a continuous-wave (CW) EPR spectrum [4][5][6], but determining the orientation of these orbitals in disordered samples requires more effort. Dipolar interactions between electron and proton nuclear spins depend on the position of the proton relative to the iron; therefore, one can use the proton hyperfine couplings obtained by ENDOR [7][8][9][10][11][12] or ESEEM [8,[13][14][15][16][17] to determine the relative orientations between the g matrix, heme molecular frame, and axial ligands. The main difficulty with this approach is that typically there are many protons in the active site and proton signals often cannot be unambiguously assigned.…”