The role of the cysteine thiolate ligand for the unusual copper coordination geometry in the blue copper proteinshas been studied by comparing the electronic structure, geometry, and energetics of a number of small Cu(II) complexes. The geometries have been optimised with the density functional B3LYP method and energies have been calculated with multiconfigurational second-order perturbation theory (the CASPT2 method).
Most small inorganic Cu(II) complexes assume a tetragonal geometry,
where four ligands make sigma bonds to a Cu d orbital. If a ligand lone-pair
orbital instead forms a pi bond to the copper ion, it formally occupies
two ligand positions in a square coordination, and the structure becomes
trigonal. Large, soft, and polarisable ligands, such as SH-
and SeH-, give rise to covalent copper-ligand bonds and structures
close to a tetrahedron, which might be trigonal or tetragonal with approximately
the same stability. On the other hand, small and hard ligands, such as
NH3 , H2O, and OH-, give ionic bonds and
flattened tetragonal structures.
It is shown that axial type 1 (blue) copper proteins have a trigonal structure with a pi bond to the cysteine sulphur atom, whereas rhombic type 1 and type 2 proteins have a tetragonal structure with mainly sigma bonds to all strong ligands. The soft cysteine ligand is essential for the stabilisation of a structure that is close to a tetrahedron (either trigonal or tetragonal), which ensures a low reorganisation energy during electron transfer.