Preparation and spectroscopic studies of cobalt(II) derivatives of blue copper proteins

Preparation of cobalt(II) derivatives of type 1 copper proteins has been extended to include bean plastocyanin and azurin from Pseudomonas aeruginosa. Fluorescence quenching data suggest that Co(II) binds to the type 1 site of both apoplastocyanin and apoazurin. Electronic absorption spectral measurements have been made for cobalt(II)-stellacyanin, cobalt(II)-plastocyanin, and cobalt(II)-azurin. In each case two visible bands with moderate intensities and two rather strong ultraviolet absorption peaks are observed. The intensity pattern and the separation of the two ultraviolet bands in the Co(II) derivatives correspond closely to the two intense, low-lying absorption peaks in the analogous, type 1 copper proteins. The evidence suggests that the intense, two-band systems represent ligand-to-metal charge transfer transitions in both Cu(II) and Co(II) derivatives. It is proposed that the transitions in each case originate in the 3ppi and 3psigma orbitals of a cysteine sulfur ligand at the type 1 site. The visible absorption bands in the Co(II) derivatives are assigned to d-d transitions. The large (4)P splitting observed indicates that the type 1 Co(II) site is of low symmetry. The d-d band pattern suggests that the high-spin Co(II) center is either distorted tetrahedral or five-coordinate. The observed band intensities are in much closer accord with the values for Co(II) complexes known to have distorted tetrahedral structures.