Boulatov Roman, Collman James P, Shiryaeva Irina M, Sunderland Christopher J
Department of Chemistry, Stanford University, Stanford, CA 94309, USA.
J Am Chem Soc. 2002 Oct 9;124(40):11923-35. doi: 10.1021/ja026179q.
Catalytic reduction of O(2) and H(2)O(2) by new synthetic analogues of the heme/Cu site in cytochrome c and ubiquinol oxidases has been studied in aqueous buffers. Among the synthetic porphyrins yet reported, those employed in this study most faithfully mimic the immediate coordination environment of the Fe/Cu core. Under physiologically relevant conditions, these biomimetic catalysts reproduce key aspects of the O(2) and H(2)O(2) chemistry of the enzyme. When deposited on an electrode surface, they catalyze the selective reduction of O(2) to H(2)O at potentials comparable to the midpoint potential of cytochrome c. The pH dependence of the half-wave potentials and other data are consistent with O-O bond activation at these centers proceeding via a slow generation of a formally ferric-hydroperoxo intermediate, followed by its rapid reduction to the level of water. This kinetics is analogous to that proposed for the O-O reduction step at the heme/Cu site. It minimizes the steady-state concentration of the catalytic intermediate whose decomposition would release free H(2)O(2). The maximum catalytic rate constants of O(2) reduction by the ferrous catalyst and of H(2)O(2) reduction by both ferric and ferrous catalysts are comparable to those reported for cytochrome oxidase. The oxidized catalyst also displays catalase activity. Comparison of the catalytic properties of the biomimetic complexes in the FeCu and Cu-free forms indicates that, in the regime of rapid electron flux, Cu does not significantly affect the turnover frequency or the stability of the catalysts, but it suppresses superoxide-releasing autoxidation of an O(2)-catalyst adduct. The distal Cu also accelerates O(2) binding and minimizes O-O bond homolysis in the reduction of H(2)O(2).
在水性缓冲液中研究了细胞色素c和泛醇氧化酶中血红素/Cu位点的新型合成类似物对O₂和H₂O₂的催化还原作用。在已报道的合成卟啉中，本研究中使用的那些最忠实地模拟了Fe/Cu核心的直接配位环境。在生理相关条件下，这些仿生催化剂重现了该酶O₂和H₂O₂化学的关键方面。当沉积在电极表面时，它们在与细胞色素c的中点电位相当的电位下催化O₂选择性还原为H₂O。半波电位的pH依赖性和其他数据与这些中心处的O - O键活化过程一致，即通过缓慢生成一个形式上的铁 - 氢过氧中间体，然后将其快速还原到水的水平。这种动力学类似于在血红素/Cu位点提出的O - O还原步骤的动力学。它使催化中间体的稳态浓度最小化，该中间体的分解会释放游离的H₂O₂。亚铁催化剂还原O₂的最大催化速率常数以及铁和亚铁催化剂还原H₂O₂的最大催化速率常数与细胞色素氧化酶报道的相当。氧化态的催化剂还表现出过氧化氢酶活性。对FeCu和无Cu形式的仿生配合物的催化性能进行比较表明，在快速电子通量的情况下，Cu不会显著影响催化剂的周转频率或稳定性，但它会抑制O₂ - 催化剂加合物释放超氧化物的自氧化反应。远端的Cu还加速了O₂的结合，并使H₂O₂还原过程中O - O键的均裂最小化。
