The emerging picture in biological redox reactions is that electron transfers are often coupled to proton transfers, termed proton-coupled electron transfer (PCET). Often, biological PCET occurs at multiple sites (MS), involving a long distance ET event coupled to a simultaneous, short-range proton transfer (PT). These multiple-site concerted proton electron transfer (MS-CPET) reactions are common in a number of essential strong bond activation mechanisms in biology, including C-H oxidation in oxygenases and water oxidation in photosynthesis. The striking distances separating the electron donors and acceptors in biological MS-CPET has inspired investigation of how nature manages the movement of proton and electron equivalents over such different distances. Many studies have shown that ET kinetics in biological systems are sensitive to the distance between and the nature of the bridging medium separating the donor and acceptor. A similiar understanding of how nature manages the simulatanoues transfer of protons and electrons through proteins is still developing. We sought to investigate the effect of ET distance in a biomimetic MS-CPET model system where a hydrogen-bonded tyrosine phenol could be oxidized by a photogenerated ruthenium (III) tris-bipyridine oxidant at varying distance. Herein we describe the synthesis, characterization, and CPET reactivity of a series of ruthenium tris-bipyridine appended peptides where a photogenerated ruthenium (III) oxidant is separated from an oxidizable tyrosine phenol by oligoproline sequences of increasing number. MS-CPET rate constants for tyrosine oxidation were measured using nanosecond transient absorption spectroscopy in phosphate buffered water. A typical negative exponential distance dependence is observed for tyrosine oxidation rate constants. In contrast, we observe an exceptionally small distance decay constants for tyrosine oxidation via MS-CPET and ET when compared to bimetallic ET transfer systems through the same oligoproline bridges.These results are discussed in the context of previous distance dependence studies in MS-CPET and non-Condon effects.

Multiple-site, long-distance concerted proton electron transfer in biomimetic metallopeptides

255th National Meeting (2018)

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