Bioinorganic Chemistry on Structure and Function of Unprecented Low-Spin Typed Non-Heme Iron(III) Complexes

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1. Nitrile hydratase isolated from Brevibacterium R312 and Pseudomonas chlororaphis B23 is a new iron-containing enzyme. The ESR study reveals that (1) nitrile hydratase is the first non-heme iron enzyme with a typical low-spin iron(III)-coordination environment, (2) axial positions of the iron(III) site in the native enzyme may be occupied by thiolate and aquo groups, and (3) aliphatic nitrile substrates directly bind to the iron(III)-active center by water substrate replacement.2. On the basis of unique iron(III) coordination site of bleomycin, model non-heme iron(III) complexes with a typical low-spin iron(III) have been designed and synthesized. The N50 donor set ligand including 4-methoxypyridine and t-butyl groups clearly exhibited ESR and ^1H-NMR features characteristic of low-spin iron(III) site.3. A synthetic model compound(PYML-8) for the metal-binding site of bleomycin with a 4-dimethylaminopyridine nucleus and a tert-butyl ether group showed dioxygen activation up to 125 % of that of natural bleomycin.4. EXAFS and X-ray crystallographic studies of the iron(III)-active site of nitrile hydratase did not give valuable informations, because of decomposition of the sample by X-ray irradiation and of small single crystal of the enzyme.