Quantum chemical calculations of the reorganization energy of blue-copper proteins

The inner-sphere reorganization energy for several copper complexes related to the active site in blue-copper protein has been calculated with the den sity functional B3LYP method. The best model of the blue-copper proteins, C u(Im)(2)(SCH3)(S(CH3)(2))(0/+), has a self-exchange inner-sphere reorganiza tion energy of 62 kJ/mol, which is at least 120 kJ/mol lower than for Cu(H2 O)(4)(+/2+). This lowering of the reorganization energy is caused by the so ft ligands in the blue-copper site, especially the cysteine thiolate and th e methionine thioether groups. Soft ligands both make the potential surface s of the complexes flatter and give rise to oxidized structures that are qu ite close to a tetrahedron (rather than tetragonal). Approximately half of the reorganization energy originates from changes in the copper-ligand bond lengths and half of this contribution comes from the Cu-S-Cys bond. A tetr agonal site, which is present in the rhombic type 1 blue-copper proteins, h as a slightly higher (16 kT/mol) inner-sphere reorganization energy than a trigonal site, present in the axial type 1 copper proteins. A site with the methionine ligand replaced by an amide group, as in stellacyanin, has an e ven higher reorganization energy, about 90 kJ/mol.