Department of Chemistry, The University of Michigan, 48109-1055, Ann Arbor, MI, USA.
Photosynth Res. 1993 Jan;38(3):303-8. doi: 10.1007/BF00046754.
The series of complexes [Mn(IV)(X-SALPN)(μ2-O)]2, 1: X=5-OCH3; 2: X=H; 3: X=5-Cl; 4: X=3,5-diCl; 5: X=5-NO2, contain Mn2O2 cores with Mn-Mn separations of 2.7 Å. These molecules can be protonated to form [Mn(IV)(X-SALPN)(μ2-O,OH)]2 (+) in which a bridging oxide is protonated. The pKa values for the series of [Mn(IV)(X-SALPN)(μ2-O,OH)]2 (+) track linearly versus the shift in redox potential with a slope of 84 mV/pKa. This observation suggests that the Mn2O2 core can be considered as a unit in which the free energy of protonation is directly related to the ability to reduce the Mn(IV) ion. The marked sensitivity of the reduction potential to the presence of protons presents a mechanism in which an enzyme can control the oxidizing capacity of an oxo manganese cluster by the degree and timing of oxo bridge protonation.
该系列配合物[Mn(IV)(X-SALPN)(μ2-O)]2，1：X=5-OCH3；2：X=H；3：X=5-Cl；4：X=3,5-diCl；5：X=5-NO2，包含具有 2.7 Å 的 Mn-Mn 间隔的Mn2O2核。这些分子可以质子化形成[Mn(IV)(X-SALPN)(μ2-O,OH)]2 (+)，其中桥连氧化物被质子化。[Mn(IV)(X-SALPN)(μ2-O,OH)]2 (+)系列的 pKa 值与氧化还原电势的偏移呈线性关系，斜率为 84 mV/pKa。这一观察结果表明，Mn2O2核可以被视为一个单元，其中质子化的自由能与还原 Mn(IV)离子的能力直接相关。还原电势对质子存在的显著敏感性提出了一种机制，其中酶可以通过氧桥质子化的程度和时间来控制过锰氧簇的氧化能力。
