THEORETICAL-STUDY OF THE ELECTRONIC-SPECTRUM OF PLASTOCYANIN

The electronic spectrum of the blue copper protein plastocyanin has be en studied by ab initio multiconfigurational second-order perturbation theory (the CASPT2 method). The six lowest electronic transitions hav e been calculated and assigned with an error of less than 2000 cm(-1). The singly occupied orbital in the ground state is Cu 3d-S-Cys 3p pi antibonding with some N-His 2p sigma character. The bright blue color originates from an electron transfer to this orbital from the correspo nding Cu 3d-S-Cys 3p pi bonding orbital. The influence of different li gand models on the spectrum has been thoroughly studied; Cu(imidazole) (2)(SCH3)(S(CH3)(2))(+) as a model of CuHis(2)CysMet is the smallest s ystem that gives converged results. The spectrum is surprisingly sensi tive to changes in the geometry, especially in the Cu-S bond distances ; a 5 pm change in the Cu-S-Cys bond length may change the excitation energies by as much as 2000 cm(-1). The effect of the surrounding prot ein and solvent on the transition energies has been modeled by point c harges and is found to be significant for some of the transitions (up to 2000 cm(-1)).