Structural and spectroscopic properties of blue copper proteins

Ulf Ryde, Mats H. M. Olsson, Björn O. Roos, Kristin Pierloot, Jan O. A. De Kerpel
Encyclopaedia of Computational Chemistry,  (1997) invited article, in press, July, 1998.

A series quantum chemical studies of the structural and spectroscopic properties of a class of copper proteins - the blue copper proteins - is reviewed. The project is an example of how modern quantum mechanical methods can be used to study both ground and excited state properties in rather large and complex molecules. The density functional B3LYP method was used to optimize the coordination structure around the copper ion. The result was a very plastic structure, which can shift from trigonal to distorted tetragonal depending on the details of the surrounding protein. The results give no support to earlier suggestions that the protein strains the Cu(II) coordination sphere into a Cu(I) like structure. The trigonal structure is characterized by a pi bond between the copper ion and the cysteine ligand, while in the more tetragonal structures this bond is replaced by a sigma bond.

The close relationship between the electronic structure and the spectroscopic properties was studied by calculations of the electronic spectra for a number of model compounds.  The protein and the surrounding solvent was modeled by point charges, one  for each atom. These calculations have led to a detailed understanding of the spectroscopic characteristics of the different types of cysteine-containing copper proteins. They give an example of how multiconfigurational SCF theory  combined with multiconfigurational second-order perturbation theory can be used to study excited states of rather complex molecular systems.