Analysis of the Electronic Structure of Fe -Porphyrins and of the Molar Rati Fe 3+/ Fe2+ in Fe-Ascorbate by Mössbauer and EPR Spectroscopy

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Abbasher M., Gismelseed
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University of Khartoum
For the understanding of the structure/function relation of iron enzymes three corresponding model have been investigated. The iron (III) "picket-fence" porphyrin complex [Fe (III) - (TPpivP)-(OSO2CF3) (H2O) compound was characterized by its EPR, magnetic am Mössbauer properties. The EPR data yield , which corresponds to a mixture of 85% spin sextet 6A1 and 15% spin quartet 4A2 in the last Kramers doublet. This mixture is somewhat stronger than in typical high-spin iron porphyrins. The susceptibility measurements in an external field of 1.5 T show that the effective magnetic moment varies from 4.0µB to 5.6µB in the temperature range 2-300 K. Mössbauer spectra were recorded at temptress varying from 4.2 to 300 K in fields of 0-6.21 T. They exhibit temperature quadruple splitting, 2 mms-l, which lies be the -values characteristic for high-spin (S=5/2) and intermediate-spin (S=3/2) porphyrins magnetic. The magnetic hyperfine patterns of the measured Mössbauer spectra, in the low and fast relaxation limit, are successfully simulated within the 10-state model for the spin mixture between 6A1 and 4A2, using parameters which have been derived from susceptibility and EPR data within the sane model. The applicability of the usual spin (S=5/2) Hamiltonian analysis and its relation to the 10-state model are discussed. Spin-spin and spin-lattice relaxation effects are explicitly accounted for in the intermediate relaxation regime. Mössbauer spectroscopy has also been used to probe the five-coordinate anionic ferrous heme [Fe(CH3 CO2)TPpivP]- . This species is a synthetic model for the P-460 center of hydroxylamine oxidoreductase from Nitrosomonas europaea. Mössbauer spectra were recorded at selected temperatures from 1.6 to 300 K and applied magnetic field of 0-6.21T. The zero-field results of =l.05 mm s-l and = 4.25 mm s-1 at 4.2K are consistent with a five-coordinate high-spin ferrous ions and the small temperature dependence of the quadruple splitting suggests a well-isolated ground state. High-field Mössbauer data were analyzed by using a S=2 spin Hamiltonian. Mössbauer data, obtained from the oxidized form of the Model, are analyzed along the same lines with the complementary data obtained from the EPR experiment. The fit gave D=7.5cm-1, and g=2.0 which are consistent with a purely high-spin ferric ion.
Analysis of the Electronic Structure of Fe -Porphyrins and of the Molar Ratio Fe 3+/ Fe2+ in Fe-Ascorbate by Mössbauer and EPR Spectroscopy