Ligand perturbation effects on a pseudotetrahedral Co(II)(His)(3)-ligand site - A magnetic circular dichroism study of the Co(II)-substituted insulin hexamer

Magnetic circular dichroism (MCD) spectra of a series of adducts formed by the Co(II)-substituted R-state insulin hexamer are reported, The His-B10 residues in this hexamer form tris imidazole chelates in which pseudotetrahedral Co(II) centers are completed by an exogenous fourth ligand, This study investigates how the MCD signatures of the Co(II) center in this unit are influenced by the chemical and steric characteristics of the fourth ligand, The spectra obtained for the adducts formed with halides, pseudohalides, trichloroacetate, nitrate, imidazole, and 1-methylimidazole appear to be representative of near tetrahedral Co(II) geometries. With bulkier aromatic ligands, more structured spectra indicative of highly distorted Co(II) geometries are obtained, The MCD spectrum of the phenolate adduct is very similar to those of Co(II)-carbonic anhydrase (alkaline form) and Co(II)-beta-lactamase. The MCD spectrum of the Co(II)-R(6)-CN- adduct is very similar to the CN- adduct of Co(II)-carbonic anhydrase. The close similarity of the Co(II)-R(6)-pentafluorophenolate and Co(II)-R(6)-phenolate spectra demonstrates that the Co(II)-carbonic anhydrase like spectral profile is preserved despite a substantial perturbation in the electron withdrawing nature of the coordinated phenolate oxygen atom. We conclude that this type of spectrum must arise from a specific Co(II) coordination geometry common to each of the Co(II) sites in the Co(II)-R(6)-phenolate, Co(II)-R(6)-pentafluorophenolate, Co(II)-beta-lactamase, and the alkaline Co(II)-carbonic anhydrase species, These spectroscopic results are consistent with a trigonally distorted tetrahedral Co(II) geometry (C-3v), an interpretation supported by the pseudotetrahedral Zn(II)(His)(3)(phenolate) center identified in a Zn(II)-R(6) crystal structure (Smith, G. D., and Dodson, G. G. (1992) Biopolymers 32, 441-445).