Models of the active sites of zinc-containing enzymes: The structure of acetato[hydrotris(3-tert-butyl-5-methylpyrazol-1-yl)borato]zinc(II)

The zinc complex acetato[hydrotris(3-tert-butyl-5-methylpyrazol-1-yl)borato]zinc (II) has been prepared and structurally characterised as a model of the active sites of the zinc-containing enzymes carbonic anhydrase and dihydroorotase. Crystals are monoclinic, space group P2(1)/n, a 15.995(2), b 19.861(4), c 10.530(2) Angstrom, beta 90.00(2)degrees, Z 4 and the structure has been refined to a residual of 0.048 based on 4096 reflections. The complex deviates. from C-3 symmetry as a consequence of interactions between the tripod ligand and the fourth ligand bound to the Zn atom. The Zn-N (tripod) bond lengths are affected by the degree of steric crowding but bonds to the fourth ligand are not. Comparison with the I-, OH- and HCO3- forms of human carbonic anhydrase I confirms that the complexes with tripodal ligands are excellent structural:models for the active site of this enzyme.