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

Abstract

The proton sum frequency peak(s), I(ν+), in the ESEEM spectra of low-spin ferriheme complexes provide single-crystal-like information concerning the orientation of the g tensor in samples in frozen glassy media. In this work we have investigated two model heme complexes, [OEPFe(imidazole)2]+ and [OEPFe(4-(dimethylamino)pyridine)2]+ (OEP = octaethylporphyrinate). Both experimental intensities and frequency shifts from twice the 1H Larmor frequency of the observed signals were measured at various points across the EPR spectrum and compared to the expected spectra, simulated using the known crystal structure data, isotropic hyperfine coupling constants, and g strain. In each case the z magnetic axis direction was defined as perpendicular to the mean plane of the porphyrinate, and it was found that gzz is the largest g value in both cases. The in-plane magnetic axis directions could also be determined from the ESEEM data, and it was found that the orientations of gxx and gyy differ, depending on the orientation of the (parallel) axial ligands with respect to the porphyrinate nitrogens: For the bis(imidazole) complex, for which the axial ligands nearly eclipse opposite porphyrinate nitrogens (φ = 7°) in the crystalline state, gxx and gyy are aligned at ±45° to the normal to the plane of the axial ligands, while for the bis(4-(dimethylamino)pyridine) complex, for which the axial ligands lie in parallel planes nearly bisecting the porphyrinate nitrogens (φ = 41°) in the crystalline state, gyy is aligned along the plane of the axial ligands. The significance of these results with respect to the concept of counterrotation of the g tensor with rotation of axial ligands and the interpretation of the in-plane magnetic anisotropy of heme proteins measured by NMR techniques is discussed.