Biological systems often couple proton transfers with electron transfers to avoid the high energy intermediates associated with the stepwise pathways. When the electron and proton are delivered to separate reagents, this is referred to as multiple site-concerted proton electron transfer (MS-CPET). MS-CPET is found in a variety of natural systems, such as in the Tyrosone-Histidine site of Photosystem II. The proton transfer of MS-CPET occurs over short distances while the electron transfer can occur over significantly longer distances. A series of termolecular MS-CPET reactions of the form DH + B + Ox+ -&gt; D + HB+ + Ox have been examined to individually probe the effects of the proton transfer and electron transfer on the rate of MS-CPET. Second order rate constants were determined using stopped flow spectroscopy and span more than 4 orders of magnitude. The electron transfer and proton transfer components of the driving force for MS-CPET were varied with the redox potential of the oxidant as well as the pKa of the base, respectively. The second order rate constants for MS-CPET are affected equally by both variations in driving force. In addition, the proton transfer coordinate was systematically lengthened by increasing the sterics of the base, which resulted in a decrease in the rate of MS-CPET. These results will be discussed in the context of Marcus Theory as a model for MS-CPET reactivity.

Termolecular proton-coupled electron transfer reactions: Separating proton and electron transfer effects

251st National Meeting (2016)

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