The origin of the intense absorption in azurin.

The intense visible absorption band of native azurin and the corresponding transitions of the Ni(II) and the Co(II) derivatives were analyzed. The bands were shown to be analogous ligand-to-metal charge transfer transitions and the optical electronegativity of the ligand involved is estimated to be 2.6. Comparisons with small molecule systems strongly indicate that the donor involved is cysteine sulfur.