Multiple-site Proton-Coupled Electron Transfer in Phenols and Hemes

JAMES M. MAYER; JEFFREY J. WARREN; JOEL SCHRAUBEN; M. CATTANEO; VIRGINIA W. MANNER; JESICA WHITMAN

Vancouver

Congreso; 15th International Conference on BioInorganic Chemistry; 2011

Canadian Society for Chemistry

Multiple-site proton-coupled electron transfer in phenols and hemes J. Mayer 1 ; J. J. Warren 1 ; J. N. Schrauben 1 ; M. Cattaneo 1 ; V. W. Manner 1 ; J. Whitman 1 1. Chemistry, University of Washington, Seattle, WA, United States. Many biochemical redox reactions are proton-coupled. Often the electron and proton electron transfer to spatially separate sites, as in tyrosine-Z oxidation in photosystem II (electron to P680+, proton to a nearby histidine) and in proton pumping by cytochrome c oxidase. This presentation will focus on detailed studies of model systems for such multiple-site proton-coupled electron transfer (MS-PCET) processes in hemes and phenols. In ascorbate peroxidase, the substrate acts as a hydrogen bond donor to one of the heme propionates, and it is this hydrogen that is lost upon oxidation to the ascorbyl radical. A model for this process has been developed using the mono-methyl ester of protoporphyrin IX and N-methylimidazole. button. To return to the previous page, close this The analogous reaction with the hydroxylamine TEMPOH has been shown to proceed by concerted proton-electron transfer, and the ascorbate reaction likely proceeds similarly, despite the large separation between the redox active iron center and the basic propionate. To explore the effect of distance in more detail, a system has been developed with a more rigid porphyrin, 5-(4-carboxyphenyl)-10,15-20-triphenylporphyrin. The rates of these processes will be discussed in light of the Marcus theory of electron transfer (ET) and its extensions into proton-coupled electron transfer. In ET theory, the dependence on distance is a result of the reactions being non-adiabatic, and the coupling of the reactant and product surfaces depends on the distance. To probe the importance of non-adiabatic character in MS-CPET, oxidations of phenols with pendant bases are being examined as a function of driving force. Using time-correlated single-photon counting, rate constants are being measured as –ΔG° approaches the intrinsic barrier λ, thus mapping the Marcus rate/driving force parabola. All together, these studies are providing a new and detailed perspective on MS-PCET and the theoretical models used to describe it.