SUPEREXCHANGE COUPLING AND ELECTRON TRANSFER IN LARGE MOLECULES : THROUGH-SPACE AND THROUGH-BOND INTERACTIONS

The superexchange coupling (TDA) between distant donor−acceptor (D−A) redox centers in large electron-transfer (ET) molecules, such as proteins, involves through-space (TS) as well as through-bond (TB) interactions. For these complex systems, the analysis of TDA either in terms of an electron tunneling through a potential energy barrier or in terms of an electronic-coupling pathway model may be inappropriate or, at best, requires some justification. From an analytical and numerical study of TDA in a folded random chain system, we clarify the role of both TS and TB interactions. For this molecular model system, we find that a proper description of ET processes requires both a renormalization of the medium orbital energies by TB interactions and, beyond any perturbative approach, a full account of TS couplings. Except under extreme energetic conditions, neither TB nor TS interactions alone are able to describe the complete behavior of the superexchange coupling. Depending on the relative signs of some param...