title: Kinetic and thermodynamic characterization of the cobalt and manganese oxyhydroxide cores formed in horse spleen ferritin.
abstract: 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 (AH(2)), a two-electron reductant, effectively reduced the mineral cores; however, the reaction was incomplete and rapidly reached equilibrium. The addition of excess AH(2) shifted the reaction to completion with a M(3+)/AH(2) 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 AH(2) 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 AH(2) 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 + AH(2) = 2M(OH)(2) + D, where AH(2) 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 AH(2). Fe(2+) 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.
authors.affiliation: Department of Chemistry, Chemical Engineering, Brigham Young University, Provo, Utah 84602, USA.
authors.forename: Bo
authors.initials: B
authors.lastname: Zhang
authors.forename: John N
authors.initials: JN
authors.lastname: Harb
authors.forename: Robert C
authors.initials: RC
authors.lastname: Davis
authors.forename: Jae-Woo
authors.initials: JW
authors.lastname: Kim
authors.forename: Sang-Hyon
authors.initials: SH
authors.lastname: Chu
authors.forename: Sang
authors.initials: S
authors.lastname: Choi
authors.forename: Tim
authors.initials: T
authors.lastname: Miller
authors.forename: Gerald D
authors.initials: GD
authors.lastname: Watt
mesh_terms: D000818:Animals; D003035:Cobalt / Q000737:chemistry*; D005293:Ferritins / Q000737:chemistry* / Q000378:metabolism; D005296:Ferrous Compounds / Q000737:chemistry / Q000378:metabolism; D006736:Horses; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000737:chemistry*; D007700:Kinetics; D008345:Manganese / Q000737:chemistry*; D010084:Oxidation-Reduction; D013154:Spleen / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D005296:Ferrous Compounds; D003035:Cobalt; D008345:Manganese; D005293:Ferritins; D007501:Iron