ELECTRON EXCHANGE EFFECTS ON ELECTRON TRANSFER DYNAMICS

University Of California Berkeley, Berkeley CA

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Abstract

DESCRIPTION: (Verbatim from the Applicant's Abstract) There exist in nature a remarkably large number of metalloproteins and metalloenzymes whose active sites consist of more than one metal center. In some cases these metals exist independently, while in others the several metal centers comprise a polynuclear cluster. In systems such as these (e.g., non-heme iron-oxo proteins and iron-sulfur proteins), the electronic structures of the polynuclear clusters are largely determined by electron exchange interactions that exist amongst the various metal centers. This research project is aimed at elucidating whether or not the electron exchange present in such polynuclear metal clusters can itself play a functional role in biological processes: the specific target of interest is electron transfer. A model is described wherein variations in electron exchange coupling can be linked to changes in both the driving force and reorganization energies associated with electron transfer into and out of an exchange-coupled metal cluster. The objective of this research is to experimentally establish whether or not such a model is viable in systems in which metal clusters are involved in the electron transfer pathway. Due to the complex nature of metalloproteins, identification of electron exchange influences on biological function requires some a priori knowledge of the possible effects one might observe. This type of information is completely lacking in the literature. Studies described herein therefore focus on synthetically tailored molecules in which all aspects of electronic and molecular structure are under experimental control. The research represents a blend of both synthetic inorganic and physical-inorganic chemistries, in which synthesis is coupled with detailed spectroscopic studies ranging from static absorption measurements to femtosecond time-resolved probes of electron transfer. Both inter- and intra-molecular electron transfer is being studied in model complexes containing both the oxo-bridged diiron(III) core as well as two-center iron-sulfur clusters. Molecules are prepared in which variations in exchange coupling are systematically introduced (probed by variable-temperature magnetic measurements). Electron transfer kinetics are then monitored using a variety of static and time-resolved spectroscopies (e.g., absorption, emission, and vibrational). The intermolecular studies take the form of classic Stern-Volmer-type quenching studies, whereas the intramolecular dynamics are probed through synthetically-tailored systems employing covalently linked electron donors and acceptors. It is believed that establishing whether or not electron exchange can play a functional role in electron transfer processes will represent a significant conceptual step forward for understanding electron transfer reactions in general, as well as lending specific insights into the role of polynuclear metal clusters in transition metal-containing proteins.