The axial methionine ligand may control the redox reorganizations in the active site of blue copper proteins

Structural and energetic reorganizations in redox reaction of type 1 copper proteins are studied by density functional and ab initio molecular orbital calculations. Model complexes of the active site with varying number of ligands, from Cu(SCH(3))(0/+) to Cu(SCH(3))(Im)(2)(S(CH(3))(2))(0/+), where Im denotes imidazole, are investigated. Following the findings of structural instability in Cu(I)(SCH(3))(Im)(2) and its stabilization by the addition of the axial methionine (Met) ligand model, the structure and energetics are examined as functions of the Cu-S(Met) distance in the range of 2.1-3.3 A?. The reorganization energies in both redox states exhibit a minimum at the Cu-S(Met) distance of ～2.4?A?, whereas the ionization potential increases monotonically. The changes of reorganization energies correlate well with one of the Cu-N(His) distances rather than the Cu-S(Cys) distance. The estimated Arrhenius factor for oxidation of plastocyanin by P700(+) (in photosystem I) changes by an order of magnitude when the Cu-S(Met) distance fluctuates between 2.4 and 3.0 A?, whereas the factor for reduction of plastocyanin by cytochrome f is nearly constant. Together with the data from our previous classical molecular dynamics simulation of solvated protein, we argue that the electron transfer rate is affected, and thus may be controlled, by the fluctuation of a weakly bound axial Met ligand. We also present the assessment of various exchange-correlation functionals, including those with the long-range correction, against the CCSD(T) reference and on the basis of a perturbative adiabatic connection model. For Cu(SCH(3)) and Cu(SCH(3))(Im), simple correlations have been found between the reorganization energies and the amount of Hartree-Fock exchange.