PORPHYRIN AND LIGAND PROTONS AS INTERNAL LABELS FOR DETERMINATION OF LIGAND ORIENTATION IN ESEEMS OF LOW-SPIN D(5) 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 proton sum frequency peak(s), I(v(+)), in the ESEEM spectra of low -spin ferriheme complexes provide single-crystal-like information conc erning the orientation of the g tensor in samples in frozen glassy med ia. 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 s hifts from twice the H-1 Larmor frequency of the observed signals were measured at various points across the EPR spectrum and compared to th e 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 pl ane of the porphyrinate, and it was found that g(zz) 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 orientati ons of g(xx) and g(yy) differ, depending on the orientation of the (pa rallel) axial ligands with respect to the porphyrinate nitrogens: For the bis(imidazole) complex, for which the axial ligands nearly eclipse opposite porphyrinate nitrogens (phi = 7 degrees) in the crystalline state, g(xx) and g(yy) are aligned at +/-45 degrees to the normal to t he plane of the axial ligands, while for the bis(4-(dimethylamino)pyri dine) complex, for which the axial ligands lie in parallel planes near ly bisecting the porphyrinate nitrogens (phi = 41 degrees) in the crys talline state, g(yy) is aligned along the plane of the axial ligands. The significance of these results with respect to the concept of count errotation of the g tensor with rotation of axial ligands and the inte rpretation of the in-plane magnetic anisotropy of heme proteins measur ed by NMR techniques is discussed.