THEORETICAL-STUDY OF THE STRUCTURAL AND SPECTROSCOPIC PROPERTIES OF STELLACYANIN

The electronic spectrum of the azurin Met121Gln mutant, a model of the blue copper protein stellacyanin, has been studied by ab initio multi configurational 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 wi th an error of less than 2400 cm(-1). The ground-state singly occupied orbital is found to be a predominantly pi antibonding orbital involvi ng Cu3d and S(cys)3p(pi). However, it also contains a significant amou nt (18%) of Cu-S-cys sigma antibonding character. This a interaction i s responsible for the appearance in the absorption spectrum of a band at 460 nm, with a significantly higher intensity than observed for oth er, axial, type 1 copper proteins (i.e., plastocyanin or azurin). The pi-sigma mixing is caused by the axial glutamine ligand binding at a m uch shorter distance to copper than the corresponding methionine ligan d 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 st ructures where the glutamine model coordinates to the copper ion via t he deprotonated NE atom instead of the OF atom. However, the resulting transition energies do not resemble the experimental spectrum obtaine d at normal or elevated pH. Thus, the results do not confirm the sugge stion that the coordinating atom of glutamine changes at high PH.