Mossbauer studies of spin coupled binuclear iron centers in proteins
The mixed valence centers of the diiron-oxo proteins uteroferrin and nitrosyl derivatives of hemerythrin have been studied by Mossbauer Spectroscopy. Simulations of data recorded in applied fields for the 4.2 K to 250 K temperature range have been performed with the intrinsic spin Hamiltonian $\rm {\cal H} = J{\bf S}\sb1\cdot{\bf S}\sb2 + \Sigma\sbsp{i=1}{2}\{{\bf S}\sb{i}\cdot\tilde D\sb{i}\cdot{\bf S}\sb{i} + \beta{\bf S}\sb{i}\cdot\tilde g\sb{i}\cdot{\bf H} + {\bf S}\sb{i}\cdot\tilde a\sb{i}\ \cdot\ {\bf I}\sb{i} + {\bf I}\sb{i}\cdot\tilde P\sb{i}\cdot {\bf I}\sb{i} - \beta\sb{n}g\sb{n}{\bf H}\cdot I\sb{i}\}.$ Values for the isotropic exchange constant J, zero field splitting parameters D$\sb{\rm i}$ and E$\sb{\rm i}$, electronic Zeeman tensors $\rm\tilde g\sb{i},$ magnetic hyperfine tensors $\rm\tilde a\sb{i},$ electric quadrupole splittings $\rm\Delta E\sb{Q}$ and isomer shifts, have been obtained for the two distinctly different iron sites of each protein. In both cases an intermediate coupling regime $\rm\Vert{J\over D\sb{i}}\Vert \ge 1$ was found and the zero field splitting strongly perturbs the eigenstates of the exchange interaction. From the spin Hamiltonian parameters, the electronic ground state and ligand field symmetry for the Fe$\sp{2+}$(S = 2) site of uteroferrin and for the $\rm\{FeNO\}\sp7(S = {3\over2})$ site of the nitric oxide derivative of hemerythrin were inferred. Knowledge of the electronic ground states allowed the interpretation of the magnetic hyperfine interactions with simple ligand field and molecular orbital models. Mossbauer spectra of two spin coupled iron-chromium complexes have also been analyzed in terms of effective spin Hamiltonians for their ground states. A highly effective genetic algorithm was used to search in the parameter space of the Hamiltonians.
Chemistry, Physical, Physics, Molecular, Physics, Atomic
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign