Proton-coupled electron transfer: a unifying mechanism for biological charge transport, amino acid radical initiation and propagation, and bond making/breaking reactions of water and oxygen

Chang, Christopher J.; Chang, Michelle C Y; Damrauer, Niels H.; Nocera, Daniel G.

doi:10.1016/j.bbabio.2003.08.010

Cited by 184 publications

(205 citation statements)

References 104 publications

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“…Moreover, to ensure charge neutrality in the system, proton transfer must accompany electron transfer (i.e., protoncoupled electron transfer; ref. 18); hence, electron and proton inventories both need to be managed (19). Water splitting additionally presents sizable thermodynamic and kinetics barriers to making and breaking the bonds required to facilitate the desired chemical reactions.…”

Section: The Basic Science Needs For Pvsmentioning

confidence: 99%

Powering the planet: Chemical challenges in solar energy utilization

Lewis

Nocera

2006

Proc. Natl. Acad. Sci. U.S.A.

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7,506

5,540

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Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, at least 2-fold by midcentury relative to the present because of population and economic growth. This demand could be met, in principle, from fossil energy resources, particularly coal. However, the cumulative nature of CO 2 emissions in the atmosphere demands that holding atmospheric CO 2 levels to even twice their preanthropogenic values by midcentury will require invention, development, and deployment of schemes for carbon-neutral energy production on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providing more energy in 1 hour to the earth than all of the energy consumed by humans in an entire year. In view of the intermittency of insolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. An especially attractive approach is to store solar-converted energy in the form of chemical bonds, i.e., in a photosynthetic process at a year-round average efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific challenges involved with this process include schemes to capture and convert solar energy and then store the energy in the form of chemical bonds, producing oxygen from water and a reduced fuel such as hydrogen, methane, methanol, or other hydrocarbon species.

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Section: The Basic Science Needs For Pvsmentioning

confidence: 99%

Powering the planet: Chemical challenges in solar energy utilization

Lewis

Nocera

2006

Proc. Natl. Acad. Sci. U.S.A.

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5,540

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“…Amino acid radical generation and transport are synonymous with protoncoupled electron transfer (PCET; Stubbe et al 2003) as is the activation of most substrate bonds at enzyme active sites (Chang et al 2004a). PCET is especially prevalent at metallo-cofactors that activate substrates at carbon, oxygen, nitrogen and sulphur atoms.…”

Section: Introductionmentioning

confidence: 99%

Proton-coupled electron transfer: the mechanistic underpinning for radical transport and catalysis in biology

Reece

Hodgkiss

Stubbe

et al. 2006

Phil. Trans. R. Soc. B

264

340

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Charge transport and catalysis in enzymes often rely on amino acid radicals as intermediates. The generation and transport of these radicals are synonymous with proton-coupled electron transfer (PCET), which intrinsically is a quantum mechanical effect as both the electron and proton tunnel. The caveat to PCET is that proton transfer (PT) is fundamentally limited to short distances relative to electron transfer (ET). This predicament is resolved in biology by the evolution of enzymes to control PT and ET coordinates on highly different length scales. In doing so, the enzyme imparts exquisite thermodynamic and kinetic controls over radical transport and radical-based catalysis at cofactor active sites. This discussion will present model systems containing orthogonal ET and PT pathways, thereby allowing the proton and electron tunnelling events to be disentangled. Against this mechanistic backdrop, PCET catalysis of oxygen-oxygen bond activation by mono-oxygenases is captured at biomimetic porphyrin redox platforms. The discussion concludes with the case study of radical-based quantum catalysis in a natural biological enzyme, class I Escherichia coli ribonucleotide reductase. Studies are presented that show the enzyme utilizes both collinear and orthogonal PCET to transport charge from an assembled diiron-tyrosyl radical cofactor to the active site over 35 Å away via an amino acid radical-hopping pathway spanning two protein subunits.

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“…electron transfer | proton-coupled electron transfer P roton-coupled electron transfer (PCET), in which electrons and protons are both transferred, is at the heart of many energy conversion processes in chemistry and biology (1)(2)(3)(4)(5)(6). PCET reactions can occur by sequential two-step transfers (e.g., electron transfer followed by proton transfer, ET-PT, or proton transfer followed by electron transfer, PT-ET) or by concerted electron-proton transfer (EPT) (1,2).…”

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Concerted electron-proton transfer in the optical excitation of hydrogen-bonded dyes

Westlake

Brennaman²,

Concepcion³

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

154

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The simultaneous, concerted transfer of electrons and protonselectron-proton transfer (EPT)-is an important mechanism utilized in chemistry and biology to avoid high energy intermediates. There are many examples of thermally activated EPT in ground-state reactions and in excited states following photoexcitation and thermal relaxation. Here we report application of ultrafast excitation with absorption and Raman monitoring to detect a photochemically driven EPT process (photo-EPT). In this process, both electrons and protons are transferred during the absorption of a photon. Photo-EPT is induced by intramolecular charge-transfer (ICT) excitation of hydrogen-bonded-base adducts with either a coumarin dye or 4-nitro-4′-biphenylphenol. Femtosecond transient absorption spectral measurements following ICT excitation reveal the appearance of two spectroscopically distinct states having different dynamical signatures. One of these states corresponds to a conventional ICT excited state in which the transferring H þ is initially associated with the proton donor. Proton transfer to the base (B) then occurs on the picosecond time scale. The other state is an ICT-EPT photoproduct. Upon excitation it forms initially in the nuclear configuration of the ground state by application of the Franck-Condon principle. However, due to the change in electronic configuration induced by the transition, excitation is accompanied by proton transfer with the protonated base formed with a highly elongated þ H─B bond. Coherent Raman spectroscopy confirms the presence of a vibrational mode corresponding to the protonated base in the optically prepared state.electron transfer | proton-coupled electron transfer P roton-coupled electron transfer (PCET), in which electrons and protons are both transferred, is at the heart of many energy conversion processes in chemistry and biology (1-6). PCET reactions can occur by sequential two-step transfers (e.g., electron transfer followed by proton transfer, ET-PT, or proton transfer followed by electron transfer, PT-ET) or by concerted electron-proton transfer (EPT) (1, 2). EPT pathways are important in avoiding high-energy intermediates, playing an integral role in photosynthesis and respiration, for example.Photo-driven EPT (photo-EPT), with electron and proton transfers occurring simultaneously during the optical excitation process, would appear to be ruled out on fundamental grounds, because electronic excitation occurs rapidly on the time scale for nuclear motions, including proton transfer. Using a combination of femtosecond pump-probe and coherent Raman techniques, we have observed simultaneous electron-proton transfer induced by intramolecular charge transfer (ICT) excitation in two different hydrogen-bonded adducts formed between an organic dye (A─O─H) and an external base (:B). One is formed between a para-nitrophenyl-phenol and an amine base, and the other between a coumarin derivative and an imidazole base (Fig. 1).The shift in electron density away from the hydroxyl group to the intramolecular ...

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Proton-coupled electron transfer: a unifying mechanism for biological charge transport, amino acid radical initiation and propagation, and bond making/breaking reactions of water and oxygen

Cited by 184 publications

References 104 publications

Powering the planet: Chemical challenges in solar energy utilization

Powering the planet: Chemical challenges in solar energy utilization

Proton-coupled electron transfer: the mechanistic underpinning for radical transport and catalysis in biology

Concerted electron-proton transfer in the optical excitation of hydrogen-bonded dyes

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