Electronic properties of sulfhydryl- and imidazole-containing peptide-cobalt(II) complexes: their relationship to cobalt(II)-substituted “blue” copper proteins

The electronic properties of 2:1 sulfhydryl- and imidazole-containing peptide-Co(II) complexes have been investigated and compared with those of Co(II)-substituted “blue” copper proteins. The Co(II) complexes of N-mercaptoacetyl-L-histidine and 3-mercaptopropionyl-L-histidine gave the ligand field parameters of Δ_t = 4110 and B = 756 cm^-1, and of Δ_t = 4120 and B = 724 cm^-1, respectively. These values correspond well to those (Δ_t = 4900 and B = 730 cm^-1) of Co(II)-substituted “blue” copper proteins. The energy differences between S → M(II) charge transfer bands of Co(II)-Cu(II) couples were about 14,000 cm^-1 in both the proteins and the model complexes. The spectral results suggest that “blue” copper site has a pseudotetrahedral geometry and a deep absorption near 600 nm attributes to S → Cu(II) charge transfer.