Theoretical study of the electronic spectrum of plastocyanin

Kristine Pierloot, Jan O. A. De Kerpel, Ulf Ryde & Björn Roos
J. Am. Chem. Soc. 119 (1997) 218-226

The electronic spectrum of the blue copper protein plastocyanin has been studied by ab initio multiconfigurational second-order perturbation theory (the CASPT2 method). The six lowest electronic transitions have been calculated and assigned with an error of less than 2000 cm-1. The singly occupied orbital in the ground state is  Cu 3d-SCys 3ppi antibonding with some NHis 2psigma character. The bright blue color originate from an electron transfer to this orbital from the corresponding Cu 3d-SCys 3ppi bonding orbital. The influence of different ligand models on the spectrum has been thoroughly studied; Cu(imidazole)2(SCH3)(S(CH3)2)+ as a model of CuHis2CysMet is the smallest system that gives converged results. The spectrum is surprisingly sensitive to changes in the geometry, especially in the Cu-S bond distances; a 5 pm change in the Cu-SCys bond length may change the excitation energies by as much as 2000cm-1. The effect of the surrounding protein and solvent on the transition energies has been modeled by point charges and is found to be significant for some of the transitions (up to 2500 cm-1).