Enhancing tris(pyrazolyl)borate-based models of cysteine/cysteamine dioxygenases via the sterics - increased reactivities, full product characterization and hints to initial superoxide formation.

The design of biomimetic model complexes for the cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO) are reported, where the 3-His coordination of the iron ion is simulated by three pyrazole donors of a trispyrazolyl borate ligand (Tp) and protected cysteine and cysteamine represent substrate ligands. It is found that the replacement of phenyl groups - attached at the 3-positions of the pyrazole units in a previous model - by mesityl residues has massive consequences, as the latter arrange to a more spacious reaction pocket. Thus, the reaction with O 2 proceeds much faster and afterwards the first structural characterization of an iron(II) h 2 - O , O -sulfinate product became possible. If one of the three Tp-mesityl groups is placed in the 5-position, an even larger reaction pocket results, which leads to yet faster rates and accumulation of a reaction intermediate at low temperatures, as shown by UV-Vis and Mössbauer spectroscopy. After comparison with the results of investigations on the cobalt analogues this intermediate is tentatively assigned to an iron(III) superoxide species.