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).