Cobalt spectroscopy and the structure of the active site of carbonic anhydrase

Theoretical models of divalent cobalt bound to the active site of the enzyme, carbonic anhydrase, are constructed to study the effect of the first-shell geometry and coordination number on the experimentally observed visible spectra. Favorable comparisons of the ab initio calculations of the hexa-aquo complex of Co(II) with experimental results provides a test of the method. The d–d spectra variation with pH in the enzyme is found to be determined by the first-shell complex geometry as obtained from energy optimization. The low pH complex is predicted to be predominantly five-coordinate with two waters in the first-shell, whereas the high pH complex is predominantly four-coordinate. Equilibria between the four- and five-coordinate structures at low pH is not indicated by the calculation.