Study on Metalloenzymes with use of Artificial Supramolecules

金属酶在生物系统中扮演着多种角色。其中大多数化合物的结构和反应机理还没有得到很好的表征。尤其是酶的不稳定性是阐明它们的主要障碍。在本研究项目中，我们研究了(1)细胞色素C_3中的电子传递系统和(2)高等植物和蓝藻PSII中与放氧过程有关的含锰酶：细菌锰过氧化氢酶(Mn-CAT)和放氧复合体(OEC)的模拟。二聚体和四聚体可以形成相应的“咪唑化”络合物，这表明Fe(III)离子之间有很强的反铁磁性耦合作用，还原电位负移。这些化合物被认为是细胞色素C_3的良好模型。其次，我们研究了含锰酶的模拟，其中包括叶绿体中的锰过氧化氢酶和放氧复合体。它们都具有放氧功能。为了构建它们的功能模型来模拟放氧，我们使用了锰卟啉，它们在较高的氧化态下相当稳定，并根据它们的氧化态显示出特征的UV-Vis光谱。在H_2O_2歧化反应中，获得较高过氧化氢酶活性的最佳锰-锰分离条件为4.0A。该锰卟啉二聚体对水四电子氧化放氧反应也具有催化活性。这是利用结构明确的锰络合物催化氧化水的第一个例子。

Metalloenzymes play versatile roles in biological systems. Most of them have not well-characterized on their structures and reaction mechanisms. Especially, the instability of the enzymes is the major obstacle to elucidate them. In this research project, we have studied modeling of (1) electron transporting system in cytochrome c_3, and (2) manganesecontaining enzymes involved in oxygen evolving processes : bacterial manganese catalase (Mn-CAT) and oxygen evolving complex (OEC) in PSII of higher plants and cyanobacteria. Athracene-linked porphyrin dimers and tetramer can form the corresponding 'imidazoratesqueezed' complexes, which indicated large antiferromagnetic coupling between the Fe (III) ions and negative shifts on their reduction potential. These compounds are considered to be good models of cytochrome c_3. Next, we studied on the modeling of manganese containing enzymes, which involve manganese catalases and oxygen evolving complex in chloroplast. Both of them have function to evolve O_2. To construct their functional model to mimic the oxygen evolution, we used manganese porphyrins, which were considerably stable at their higher oxidation states and showed characteristic UV-vis spectrum depending upon their oxidation states. In the disproportionation of H_2O_2, we found the optimum Mn-Mn separation to attain high catalase activity to be 4.0 A.Such manganese porphyrin dimers also have catalytic activity on the evolution of O_2 by four-electron oxidation of water. This is the first example of the catalytic oxidation of water with use of manganese complexes possessing structurally well-defined structure.