ANALYSIS OF EXCHANGE INTERACTION AND ELECTRON DELOCALIZATION AS INTRAMOLECULAR DETERMINANTS OF INTERMOLECULAR ELECTRON-TRANSFER KINETICS

During the past decades, spectroscopic characterization of exchange in teractions and electron delocalization has developed into a powerful t ool for the recognition of metal clusters in metalloproteins. By contr ast, the biological relevance of these interactions has received littl e attention thus far. This paper presents a theoretical study in which this problem is addressed. The rate constant for intermolecular elect ron-transfer reactions which are essential in many biological processe s is investigated. An expression is derived for the dependence of the rate constant for self-exchange on the delocalization degree of the mi xed-valence species. This result allows us to rationalize published ki netic data. In the simplest case of electron transfer from an exchange -coupled binuclear mixed-valence donor to a diamagnetic acceptor, the rate constant is evaluated, taking into account spin factors and excha nge energies in the initial and final state. The theoretical analysis indicates that intramolecular spin-dependent electron delocalization ( double exchange) and Heisenberg-Dirac-van Vleck (HDvV) exchange have a n important impact on the rate constant for intermolecular electron tr ansfer. This correlation reveals a novel relationship between magnetoc hemistry and electrochemistry. Contributions to the electron transfer from the ground and excited states of the exchange-coupled dimer have been evaluated. For clusters in which these states have different degr ees of delocalization, the excited-state contributions to electron tra nsfer may become dominant at potentials which are less reductive than the potential at which the rate constant for the transfer from the gro und state is maximum. The rate constant shows a steep dependence on HD vV exchange, which suggests that an exchange-coupled cluster can act a s a molecular switch for exchange-controlled electron gating. The rele vance of this result is discussed in the context of substrate specific ity of electron-transfer reactions in biology. Our theoretical analysi s points toward a possible biological role of the spin-state variabili ty in iron-sulfur clusters depending on cluster environment.