  We investigate the transport characteristics of a redox system weakly coupled
to leads in the Coulomb blockade regime. The redox system comprises a donor and
acceptor separated by an insulating bridge in a solution. It is modeled by a
two-site extended Hubbard model which includes on-site and inter-site Coulomb
interactions and the coupling to a bosonic bath. The current voltage
characteristics is calculated at high temperatures using a rate equation
approach. For high voltages exceeding the Coulomb repulsion at the donor site
the calculated transport characteristics exhibit pronounced deviations from the
behavior expected from single-electron transport. Depending on the relative
sizes of the effective on-site and inter-site Coulomb interactions on one side
and the reorganization energy on the other side we find negative differential
resistance or current enhancement.
