Spectroscopic and Electrochemical Studies on the Structure and Catalytic Activity of Oxygenase Model Complexes

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1. As model oxygenase reactions, the following cobalt Schiff base complex, Co (SB), catalyzed oxidations have been newly developed : epoxidation of olefins with NaOCl, monooxygenation of alkynes with oxygen, dehydrogenation of amines with t-butyl hydroperoxide (TBHP). These reactions were sensitive to the structure of Co (SB). With the aim of elucidating the effect of the strutture of Co (SB) on the reactivities, various Co (SB) were synthesized and redox potential E of Co (II) = Co (III) for these complexes were systematically determined. The electrochemical characteristics E was closely related to the strucural factors : a higher E was obtained with complexes having more electron-withdrawing or bulkier groups in the SB ligand, and also with complexes having nonplanar ligands. In the Co (SB) catalyzed epoxidation, the reactive species was found to be a water soluble anionic Co (SB) coordinating C10, and the higher E gives the higher concentration of the active complex. On the other ha … More nd, in the Co (SB) catalyzed monooxygenation of alkenes and alkynes with molecular oxygen, no relation between the reactivity and E was observed, but the shape of the complex plays a more important role. The catalysis of Co (SB) in the dehydrogenation of amines with TBHP is important because of a mimic of the P450 activity. The reactive species in this case is a Co (SB) (t-Bu00) complex. Under the reaction conditions, the peroxo complex decomposes to give ClO, which abstracts electron from the nitrogen atom in the substrate. Single crystal was obtained from a flavonolato-Co (salen), which is unsusceptible to the oxygenolysis in noncoordinative solvents. The X-ray analysis of this complex showed that the substrate anion is coordinated as a bidentate ligand. The NMR studes on this complex in DMF, where the complex undergoes readily oxygenolysis, showed that the substrate anion is coordinated as a monodentate ligand.2. Spectro-magnetic and electrochemical studies on the oxygenase like activity of Co (SB) and Co (PA) (PA=polyamines) towards catechols showed that starting with a Co (II) catalyst oxygen is first reduced to superoxo species followed by one-electron transfer from the substrate to the resulting Co (III) resulting in the formation of a ternary complex consisting of hydroperoxo-Co (III) -substrate semiquinone. On the other hand, starting from a Co (III) state, proton transfer from the substrate to an anion ligand of the complex followed by electron transfer from the resulting substrate anion to the Co (IIII) center to form a paramagnetic semiquinone complex intermediate, as confirmed by means of magnetic susceptibility method.3. Spectroscopic parameters related to the structure of heme-peroxide complex as models for the intermediate of oxygenase reactions are systematically accumlated during the research from 1989 to 1990. From the characteristic point of view thus obtained in the spectroscopy, coditions required for the formation of the peroxo heme complexes have been investigated. We find that the addition of sodium salt of ascorbic acid to a Fe (II)-oxygen complex at low temperature gives a 6 coordinate hydrogenperoxido-heme complex.As another model for a possible intermediate in the tryptophan dioxygenase reaction, the formation of a 6 coordinate heme-organic peroxid complex having a ternary complex structure including synthetic heme, oxygen, and the substrate are confirmed by ESR.These results are significant in order to give basic knowledge to understand the mechanism of the oxygenation reactions catalyzed by heme in nature. Less