DIELECTRIC CONTINUUM MODEL FOR CALCULATING REORGANIZATION FREE-ENERGIES OF ELECTRON-TRANSFER IN PROTEINS

A dielectric continuum model is developed for calculating polarization reorganization energies of electron transfer reactions that occur in proteins. The model is based on an earlier microscopic formulation of the Marcus electron transfer theory. The classical Marcus result, lamb da=Delta G(1-0)(op)-Delta G(1-0), for the free energy of polarization reorganization is derived from the microscopic theory. Both Delta G(1- 0)(op) and Delta G(1-0) denote the electrostatic free energy due to a positive unit charge (+e) distributed in the region representing the e lectron donor and a negative unit charge (-e) distributed in the regio n representing the electron acceptor. In calculating Delta G(1-0)(op), the donor and acceptor as well as the environment surrounding them ta ke the optical dielectric constant epsilon(op). In calculating Delta G (1-0), the donor and acceptor keep the optical dielectric constant but the environment takes the static dielectric constant epsilon. The env ironment consists of the protein matrix (where epsilon(op)=epsilon(op) and epsilon=epsilon(p)) and the solvent (where epsilon(op)=epsilon(s) (op) and epsilon=epsilon(s)). The polarization reorganization free ene rgy can be approximated as the sum of two components lambda(1) and lam bda(2). In calculating lambda(1), the protein region is extended outwa rd to infinity. For the case where the donor and acceptor are modeled as spheres (with both radii equal to a and center-center distance at r ) and the electron charge is put at either center, a Marcus result, /e psilon(p)(op))-(1/epsilon(p))][(1/a)-(1/r)]e(2), is found to be highly accurate (maximum error 4%). In calculating lambda(2), the protein re gion is extended inward to fill the donor and acceptor. The magnitude of lambda(2) is usually much smaller than lambda(1). A toy electron-tr ansfer protein is studied both by the dielectric continuum model and b y implementing the microscopic formulation through computer simulation s. Agreement of the results from the two approaches demonstrates the a ccuracy of the dielectric continuum model. (C) 1996 American Institute of Physics.