Zhang Bo, Harb John N, Davis Robert C, Kim Jae-Woo, Chu Sang-Hyon, Choi Sang, Miller Tim, Watt Gerald D
Department of Chemistry, Chemical Engineering, Brigham Young University, Provo, Utah 84602, USA.
Inorg Chem. 2005 May 16;44(10):3738-45. doi: 10.1021/ic049085l.
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.
合成了在马脾铁蛋白（HoSF）内部含有800 - 1500个钴原子或250 - 1200个锰原子，以Co(O)OH和Mn(O)OH作为矿物核心的马脾铁蛋白（Co - HoSF和Mn - HoSF），并测定了其化学反应性、还原动力学和还原电位。含有M(O)OH矿物核心（M = Co或Mn）的HoSF的微库仑法和化学还原反应迅速且定量，还原化学计量比为1.05±0.10 e/M，形成稳定的M(OH)(2)矿物核心。在pH 9.0时，双电子还原剂抗坏血酸（AH(2)）能有效还原矿物核心；然而，反应不完全且迅速达到平衡。加入过量的AH(2)可使反应完全进行，M(3+)/AH(2)化学计量比为1.9 - 2.1，这与每个金属原子单电子还原一致。通过监测矿物核心吸光度随时间的降低来测量M(O)OH与过量AH(2)之间的反应速率。在pH 9.0时，Co - HoSF和Mn - HoSF的反应对每种反应物均为一级反应，二级反应速率常数分别为0.53和4.74 M⁻¹ min⁻¹。根据吸光度随AH(2)浓度增加的变化，计算出在pH 9.0时，对于2M(O)OH + AH(2) = 2M(OH)(2) + D（其中AH(2)和D分别为抗坏血酸和脱氢抗坏血酸），Co - HoSF的平衡常数为5.0±1.9，Mn - HoSF的平衡常数为2.9±0.9。与这些平衡常数一致，Co(III) - HoSF还原的标准电位比抗坏血酸反应的标准电位正42 mV，而Mn(III) - HoSF的标准电位相对于AH(2)正27 mV。在厌氧条件下，Co - HoSF和Mn - HoSF溶液中的Fe(2+)被氧化，在HoSF内部形成Fe(O)OH，导致Co或Mn被铁部分取代。
