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Magnetic circular dichroism studies of Ni(III) complexes of relevance to Ni-containing hydrogenases
648
Abstracts
M15
M a g n e t i c Circular Diehroism Studies of Ni(III) Complexes of Relevance to Ni-Containing Hydrogenases
Paul K. Ross and Marsha G. Little
Department of Chemistry, PO Box 842006, Virginia Commonwealth University, Richmond, VA 23284 A mononuclear Ni is believed to be the catalytic site for the reversible oxidation of H2 in the enzyme hydrogenase. Spectroscopic data on the resting state of the enzyme indicate the nickel is in the +3 oxidation state and is coordinated by 3+1 sulfur atoms and 2±1 nitrogen or oxygen atoms. We are interested in more fully characterizing the geometry and ligation of the Ni in the enzyme site. To this end we have undertaken a spectroscopic study of a variety of Ni model complexes using a combination of variable temperature magnetic circular dichroism and EPR spectroscopies. We report here magnetic circular dichroism (MCD) spectra of three series of Ni(III) complexes. The first of the series, [Ni(III)(PS3*)(t-butylpyridine)] and [Ni(III)(PS3*)(N-methylimidazole)] synthesized by Millar [1] are based on the sterically hindered PS3* ligand (PS3*= tris(6-phenyl-2mercaptophenyl)phosphine). These complexes have trigonal-bipyramidal geometry with equatorial thiolates. The MCD spectra are dominated by S-Ni and P-Ni charge-transfer transitions and no d-d transitions have been definitively assigned. The second group, consisting of the complexes [Ni(cyclam)(SO4)](CIO4) and [Ni(cyclam)(Cl)2](CIO4) have square planar geometry with equatorial nitrogen coordination. The MCD spectra of these complexes exhibit one resolvable d-d transition at 690 nm (C1) or 810 nm (SO4). This transition has been definitively assigned based on the sign of the MCD signal. The last series consists of [Ni(tacn)2](CIO4)3 and [Ni(ttcn)2](C104)3. which have 6-coordinate octahedral geometry with amine or thioether ligation. Comparison of the MCD spectra with the absorption spectra allow preliminary assignment of several transitions. These recent results of MCD spectra of Ni(III) complexes will be presented and implications to the enzyme spectra will be discussed. . D.-H. Nguyen, S. Beatty, K.-S. Ai, J. Franolic, M. Millar, Abstracts of
the Fourth International Conference of the Molecular Biology of Hydrogenases, August 14-19, 1994.
Abstracts
M15
M a g n e t i c Circular Diehroism Studies of Ni(III) Complexes of Relevance to Ni-Containing Hydrogenases
Paul K. Ross and Marsha G. Little
Department of Chemistry, PO Box 842006, Virginia Commonwealth University, Richmond, VA 23284 A mononuclear Ni is believed to be the catalytic site for the reversible oxidation of H2 in the enzyme hydrogenase. Spectroscopic data on the resting state of the enzyme indicate the nickel is in the +3 oxidation state and is coordinated by 3+1 sulfur atoms and 2±1 nitrogen or oxygen atoms. We are interested in more fully characterizing the geometry and ligation of the Ni in the enzyme site. To this end we have undertaken a spectroscopic study of a variety of Ni model complexes using a combination of variable temperature magnetic circular dichroism and EPR spectroscopies. We report here magnetic circular dichroism (MCD) spectra of three series of Ni(III) complexes. The first of the series, [Ni(III)(PS3*)(t-butylpyridine)] and [Ni(III)(PS3*)(N-methylimidazole)] synthesized by Millar [1] are based on the sterically hindered PS3* ligand (PS3*= tris(6-phenyl-2mercaptophenyl)phosphine). These complexes have trigonal-bipyramidal geometry with equatorial thiolates. The MCD spectra are dominated by S-Ni and P-Ni charge-transfer transitions and no d-d transitions have been definitively assigned. The second group, consisting of the complexes [Ni(cyclam)(SO4)](CIO4) and [Ni(cyclam)(Cl)2](CIO4) have square planar geometry with equatorial nitrogen coordination. The MCD spectra of these complexes exhibit one resolvable d-d transition at 690 nm (C1) or 810 nm (SO4). This transition has been definitively assigned based on the sign of the MCD signal. The last series consists of [Ni(tacn)2](CIO4)3 and [Ni(ttcn)2](C104)3. which have 6-coordinate octahedral geometry with amine or thioether ligation. Comparison of the MCD spectra with the absorption spectra allow preliminary assignment of several transitions. These recent results of MCD spectra of Ni(III) complexes will be presented and implications to the enzyme spectra will be discussed. . D.-H. Nguyen, S. Beatty, K.-S. Ai, J. Franolic, M. Millar, Abstracts of
the Fourth International Conference of the Molecular Biology of Hydrogenases, August 14-19, 1994.