Department of Chemistry and Pharmacy, University Erlangen-Nuremberg , 91058 Erlangen, Germany.
Department of Chemistry and Biochemistry, Auburn University , Auburn, Alabama 36849, United States.
J Am Chem Soc. 2017 Feb 1;139(4):1472-1484. doi: 10.1021/jacs.6b08394. Epub 2017 Jan 23.
Readily exchangeable water molecules are commonly found in the active sites of oxidoreductases, yet the overwhelming majority of studies on small-molecule mimics of these enzymes entirely ignores the contribution of water to the reactivity. Studies of how these enzymes can continue to function in spite of the presence of highly oxidizing species are likewise limited. The mononuclear Mn complex with the potentially hexadentate ligand N-(2-hydroxy-5-methylbenzyl)-N,N',N'-tris(2-pyridinylmethyl)-1,2-ethanediamine (L) was previously found to act as both a HO-responsive MRI contrast agent and a mimic of superoxide dismutase (SOD). Here, we studied this complex in aqueous solutions at different pH values in order to determine its (i) acid-base equilibria, (ii) coordination equilibria, (iii) substitution lability and operative mechanisms for water exchange, (iv) redox behavior and ability to participate in proton-coupled electron transfer (PCET) reactions, (v) SOD activity and reductive activity toward both oxygen and superoxide, and (vi) mechanism for its transformation into the binuclear Mn complex with L-L and its hydroxylated derivatives. The conclusions drawn from potentiometric titrations, low-temperature mass spectrometry, temperature- and pressure-dependent O NMR spectroscopy, electrochemistry, stopped-flow kinetic analyses, and EPR measurements were supported by the structural characterization and quantum chemical analysis of proposed intermediate species. These comprehensive studies enabled us to determine how transiently bound water molecules impact the rate and mechanism of SOD catalysis. Metal-bound water molecules facilitate the PCET necessary for outer-sphere SOD activity. The absence of the water ligand, conversely, enables the inner-sphere reduction of both superoxide and dioxygen. The L complex maintains its SOD activity in the presence of OH and Mn-oxo species by channeling these oxidants toward the synthesis of a functionally equivalent binuclear Mn species.
可快速交换的水分子通常存在于氧化还原酶的活性部位，但绝大多数关于这些酶的小分子模拟物的研究完全忽略了水对反应性的贡献。关于这些酶如何在存在高氧化物种的情况下继续发挥作用的研究同样有限。具有潜在六配位配体 N-(2-羟基-5-甲基苄基)-N,N',N'-三(2-吡啶基甲基)-1,2-乙二胺（L）的单核 Mn 配合物先前被发现既可以作为 HO 响应性 MRI 对比剂，又可以模拟超氧化物歧化酶（SOD）。在这里，我们在不同 pH 值的水溶液中研究了这个配合物，以确定它的（i）酸碱平衡，（ii）配位平衡，（iii）取代不稳定性和水交换的作用机制，（iv）氧化还原行为和参与质子耦合电子转移（PCET）反应的能力，（v）SOD 活性和对氧气和超氧化物的还原活性，以及（vi）其转化为具有 L-L 和其羟基化衍生物的双核 Mn 配合物的机制。通过电位滴定、低温质谱、温度和压力依赖的 O NMR 光谱、电化学、停流动力学分析和 EPR 测量得出的结论得到了所提出的中间物种的结构表征和量子化学分析的支持。这些综合研究使我们能够确定瞬态结合的水分子如何影响 SOD 催化的速率和机制。金属结合的水分子促进了外球 SOD 活性所需的 PCET。相反，水配体的缺失使超氧化物和二氧化氧都能够进行内球还原。在存在 OH 和 Mn-氧物种的情况下，L 配合物通过将这些氧化剂引导到功能等效的双核 Mn 物种的合成中来保持其 SOD 活性。
