BYU Authors: Bo Zhang, John N. Harb, Robert C. Davis
, Tim Miller, and Gerald D. Watt, published in Inorg. Chem.
Horse spleen ferritin (HoSF) containing 800−1500 cobalt or 250−1200 manganese atoms as Co(O)OH and Mn(O)OH mineral cores within the HoSF interior (Co−HoSF and Mn−HoSF) was synthesized, and the chemical reactivity, kinetics of reduction, and the reduction potentials were measured. Microcoulometric and chemical reduction of HoSF containing the M(O)OH mineral core (M = Co or Mn) was rapid and quantitative with a reduction stoichiometry of 1.05 ± 0.10 e/M forming a stable M(OH)2 mineral core. At pH 9.0, ascorbic acid (AH2), a two-electron reductant, effectively reduced the mineral cores; however, the reaction was incomplete and rapidly reached equilibrium. The addition of excess AH2 shifted the reaction to completion with a M3+/AH2 stoichiometry of 1.9−2.1, consistent with a single electron per metal atom reduction. The rate of reaction between M(O)OH and excess AH2 was measured by monitoring the decrease in mineral core absorbance with time. The reaction was first order in each reactant with second-order rate constants of 0.53 and 4.74 M-1 min-1, respectively, for Co− and Mn−HoSF at pH 9.0. From the variation of absorbance with increasing AH2 concentration, equilibrium constants at pH 9.0 of 5.0 ± 1.9 for Co−HoSF and 2.9 ± 0.9 for Mn−HoSF were calculated for 2M(O)OH + AH2 = 2M(OH)2 + D, where AH2 and D are ascorbic acid and dehydroascorbic acid, respectively. Consistent with these equilibrium constants, the standard potential for the reduction of Co(III)−HoSF is 42 mV more positive than that of the ascorbic acid reaction, while the standard potential of Mn(III)−HoSF is 27 mV positive relative to AH2. Fe2+ in solution with Co− and Mn−HoSF under anaerobic conditions was oxidized to form Fe(O)OH within the HoSF interior, resulting in partial displacement of the Co or Mn by iron.