Analysis of Exchange Interaction and Electron Delocalization as Intramolecular Determinants of Intermolecular Electron-Transfer Kinetics

During the past decades, spectroscopic characterization of exchange interactions and electron delocalization has developed into a powerful tool for the recognition of metal clusters in metalloproteins. By contrast, the biological relevance of these interactions has received little attention thus far. This paper presents a theoretical study in which this problem is addressed. The rate constant for intermolecular electron-transfer reactions which are essential in many biological processes is investigated. An expression is derived for the dependence of the rate constant for self-exchange on the delocalization degree of the mixed-valence species. This result allows us to rationalize published kinetic 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 exchange 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 an important impact on the rate constant for intermolecular electron transfer. This correlation reveals a novel relationship between magnetochemistry 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 degrees of delocalization, the excited-state contributions to electron transfer may become dominant at potentials which are less reductive than the potential at which the rate constant for the transfer from the ground state is maximum. The rate constant shows a steep dependence on HDvV exchange, which suggests that an exchange-coupled cluster can act as a molecular switch for exchange-controlled electron gating. The relevance of this result is discussed in the context of substrate specificity of electron-transfer reactions in biology. Our theoretical analysis points toward a possible biological role of the spin-state variability in iron-sulfur clusters depending on cluster environment.