Abstract
The magnetic circular dichroism (MCD) induced in the visible absorption bands of Co(II) carbonic anhydrase by a 45 kilogauss magnetic field is described. Based on a comparison to the absorption and MCD spectra of Co(II) in tetrahedral, distorted tetrahedral, and octahedral ligand fields, band assignments are made for both the active alkaline form of the Co(II) enzyme and its anion complexes. The MCD shows that all anion complexes appear to have similar if not identical molecular geometry around the cobalt ion which generates a nearly tetrahedral ligand field. It is proposed that the active alkaline form of the Co(II) enzyme is a trigonally distorted tetrahedron. The relationship of these coordination geometries to the mechanism of action is discussed.
Highlights
CD and magnetic circular dichroism (MCD) are bot,h expressed in terms of molecular ellipticity, [e] = 2.303 (4500/7r) (EL - 6~) with units of deg cm2 per dmole
For the MCD the exact magnitude of the inducing field is indicated on each figure
Substitution of Co(I1) for the native Zn(II) ion of carbonic anhydrase has provided a valuable spectroscopic probe for the active center of this enzyme because of the intense visible absorption bands centered on the cobalt ion (l-3)
Summary
The magnetic circular dichroism (MCD) induced in the visible absorption bands of Co(I1) carbonic anhydrase by a 45 kilogauss magnetic field is described. Based on a comparison to the absorption and MCD spectra of Co(n) in tetrahedral, distorted tetrahedral, and octahedral ligand fields, band assignments are made for both the active alkaline form of the Co(I1) enzyme and its anion complexes. It is proposed that the active alkaline form of the Co(II) enzyme is a trigonally distorted tetrahedron. The relationship of these coordination geometries to the mechanism of action is discussed. Temperature dependence of the MCD* between 0” and 40” was measured by incubating the sample in a variable temperature bath and maintaining the temperature by supplying or extracting heat from the metal tube surrounding the cell. For the MCD the exact magnitude of the inducing field is indicated on each figure
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