The electronic spectrum of the azurin Met121Gln mutant, a model of the blue copper protein stellacyanin, has been studied by ab initio multiconfigurational second-order perturbation theory (the CASPT2 method), including the effect of the protein and solvent by point charges. The six lowest electronic transitions have been calculated and assigned with an error of less than 2 400 cm-1. The ground-state singly occupied orbital is found to be a predominantly pi-antibonding orbital involving Cu3d and SCys 3ppi. However, it also contains a significant amount (18 %) of Cu-SCys sigma-antibonding character. This sigma-interaction is responsible for the appearance in the absorption spectrum of a band at 460 nm, with a significantly higher intensity than observed for other, axial, type 1 copper proteins (i.e. plastocyanin or azurin). The pi-sigma mixing is caused by the axial glutamine ligand, binding at a much shorter distance to copper than the corresponding methionine ligand in the normal blue copper proteins. This explains why, based on its spectral properties, stellacyanin is classified among the rhombic type 1 copper proteins, although its structure is clearly trigonal, as it is for the axial proteins.
Calculations have also been performed on structures where the glutamine
model coordinates to the copper ion via the deprotonated NE
atom instead of the OE atom. However, the resulting transition energies
do not resemble the experimental spectrum obtained at normal or elevated
pH. Thus, the results do not confirm the suggestion
that the coordinating atom of glutamine changes at high pH.