Proton-Coupled Electron Transfer in Biology: Results from Synergistic Studies in Natural and Model Systems

Reece, Steven Y.; Nocera, Daniel G.

doi:10.1146/annurev.biochem.78.080207.092132

Cited by 414 publications

(451 citation statements)

References 130 publications

(155 reference statements)

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“…These experiments elucidate not only the role for hydrogen bonding in proton transfer, but also corroborate work by Thomas demonstrating increased stability of hydrogen-bound phenoxyl radicals [129]. Proton movement in PCET processes is limited to short (i.e., hydrogenbonding contact) distances [130]; however, hydrogen-bonding networks are frequently used as "proton wires" [131] to accomplish long-range proton transfer and are well documented in GFP [132], ferredoxin I of Azotobacter vinelandii [133], cytochrome c oxidase [134], and numerous other proteins [135]. In a notable example, a hydrogen-bonding network between Tyr122 and Cys439 in ribonucleotide reductase (RNR) allows for a net proton transfer over > 30 Å [136].…”

Section: Phenolssupporting

confidence: 83%

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

Miller

Tarantino

Knowles

2016

Topics in Current Chemistry Collections

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Proton-coupled electron transfers (PCETs) are unconventional redox processes in which both protons and electrons are exchanged, often in a concerted elementary step. While PCET is now recognized to play a central a role in biological redox catalysis and inorganic energy conversion technologies, its applications in organic synthesis are only beginning to be explored. In this chapter we aim to highlight the origins, development and evolution of PCET processes most relevant to applications in organic synthesis. Particular emphasis is given to the ability of PCET to serve as a non-classical mechanism for homolytic bond activation that is complimentary to more traditional hydrogen atom transfer processes, enabling the direct generation of valuable organic radical intermediates directly from their native functional group precursors under comparatively mild catalytic conditions. The synthetically advantageous features of PCET reactivity are described in detail, along with examples from the literature describing the PCET activation of common organic functional groups.

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Section: Phenolssupporting

confidence: 83%

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

Miller

Tarantino

Knowles

2016

Topics in Current Chemistry Collections

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“…These unique electron properties of quinones are exploited for syntheses, both, in laboratory and by nature in vivo. 1,2 Substituents on the quinoid ring modify electron density affecting oxidation potential and stability of the semiquinone radical (Scheme 1). Perhalogenated benzoquinones are easily reduced and their radicals are rather stable; four electronegative substituents make electron density in the ring significantly lower.…”

Section: Introductionmentioning

confidence: 99%

Stabilisation of tetrabromo- and tetrachlorosemiquinone (bromanil and chloranil) anion radicals in crystals

et al. 2011

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Crystal structures of alkali salts of tetrahaolgenosemiquinone anion radical acetone solvates and their solvent-free salts are determined. p-Semiquinone anion radical reveals enhanced aromaticity of the ring compared to the quinone. A pair of p-stacked radical anion (psemiquinone) rings occurs in crystal structures of potassium and rubidium salts of tetrachlorosemiquinone anion acetone solvates and their potassium tetrabromo analogue. The ring centroid separation distances are about 3.2 Å and carbon-carbon contacts between the contiguous rings are 0.3 Å shorter than the sum of van der Waals radii. The spin-coupling of the two unpaired electrons between the two anion radical rings (forming a stacked dimer) correlates with the diamagnetic property of the crystals. Magnetic properties of alkali salts of tetrahaolgenosemiquinone anion radical acetone solvates were examined by electron paramagnetic resonance spectroscopy. IntroductionQuinones and semiquinone radicals undergo easily reversible oxidation-reduction reactions and they are excellent electron carriers. Due to influence of the functional groups, quinones may have various values of the standard redox potential. The oxidation potential of the quinones with electron-donating groups such as -OH, is lower, and with electronwithdrawing groups such as -Cl and -NO2, the potential becomes higher. These unique electron properties of quinones are exploited for syntheses, both, in laboratory and by nature in vivo. 1,2 Substituents on the quinoid ring modify electron density affecting oxidation potential and stability of the semiquinone radical (Scheme 1). Perhalogenated benzoquinones are easily reduced and their radicals are rather stable; four electronegative substituents make electron density in the ring significantly lower. Sodium and potassium salts of tetrachlorosemiquinone anion radical were first prepared in 1912 by Torrey and Hunter 3 by reaction of alkali iodide and tetrachloroquinone in acetone. Green salts with formulae NaC 6 C l4 O 2 and KC 6 C l4 O 2 precipitated from cold acetone but quickly decomposed upon heating. During the last fifty years perhalogenosemiquinones were studied by various techniques: EPR, 4-6 UV/Vis 7 and IR/Raman spectroscopy [8][9][10][11] and computational methods. [12][13][14][15] Several charge-transfer systems involving tetrachloro-1,4-benzoquinone (Cl4Q) [16][17][18][19][20] and tetrabromo-1,4-benzoquinone (Br4Q), 21,22 where radical anions can be stabilised under appropriate conditions, have also been designed. A crystallographic study of tetrachlorosemiquinone radical anion-in its well-known potassium salt3-was attempted in a Rudjer Bo_skovi_c Institute, Bijeni_cka 54, HR-10000 Zagreb, Croatia. E-mail: kmolcano@irb.hr Molčanov, K., , "Stabilisation of tetrabromo-and tetrachlorosemiquinone (bromanil and chloranil) anion radicals in crystals", CrystEngComm, Vol.13, No.16, 1973 23 using a film method (Weissenberg camera). However, due to an extremely weak diffraction of poor crystals the authors could only determine unit cells of...

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“…The MD model of the GS T4P predicts an electrostatic environment around the aromatic residues that could favour the transient protonation of acidic residues to reduce the oxidation potential of neighbouring tyrosines to the levels needed to function as relay amino acids in vivo (Feliciano et al ., 2015). Such mechanism of proton‐coupled electron transfer tunes the redox activity of aromatic residues to enable fast rates of charge transport despite the low potentials that operate in biological systems (Stubbe et al ., 2003; Reece and Nocera, 2009). Indeed, whereas the oxidization of tyrosine (TyrOH) to its radical cation (TyrO • H + ) in water would require a strong oxidant ( E 0 = 1.44 V vs. Normal Hydrogen Electrode [NHE]), deprotonation of the tyrosine to tyrosinate (TyrO – ) reduces the oxidation potential in half ( E 0 = 0.71 V vs. NHE) (Hammarstrom and Styring, 2011).…”

Section: Geobacter T4p: a Paradigm In Structure And Functionmentioning

confidence: 99%

“…The spatial distance between the caged metal and the tyrosine is less than 2 nm, a distance optimal for tunnelling (Gray and Winkler, 2005). The proximity of the carboxyl ligands to the tyrosine also favours its deprotonation, which could reduce the oxidation potential of the aromatic side‐chain to promote fast rates of electron transfer to the pilus‐bound metal via a proton‐coupled hopping mechanism (Stubbe et al ., 2003; Reece and Nocera, 2009). …”

Section: Geobacter T4p: a Paradigm In Structure And Functionmentioning

confidence: 99%

Harnessing the power of microbial nanowires

Reguera

2018

Microbial Biotechnology

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SummaryThe reduction of iron oxide minerals and uranium in model metal reducers in the genus Geobacter is mediated by conductive pili composed primarily of a structurally divergent pilin peptide that is otherwise recognized, processed and assembled in the inner membrane by a conserved Type IVa pilus apparatus. Electronic coupling among the peptides is promoted upon assembly, allowing the discharge of respiratory electrons at rates that greatly exceed the rates of cellular respiration. Harnessing the unique properties of these conductive appendages and their peptide building blocks in metal bioremediation will require understanding of how the pilins assemble to form a protein nanowire with specialized sites for metal immobilization. Also important are insights into how cells assemble the pili to make an electroactive matrix and grow on electrodes as biofilms that harvest electrical currents from the oxidation of waste organic substrates. Genetic engineering shows promise to modulate the properties of the peptide building blocks, protein nanowires and current‐harvesting biofilms for various applications. This minireview discusses what is known about the pilus material properties and reactions they catalyse and how this information can be harnessed in nanotechnology, bioremediation and bioenergy applications.

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Proton-Coupled Electron Transfer in Biology: Results from Synergistic Studies in Natural and Model Systems

Cited by 414 publications

References 130 publications

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

Stabilisation of tetrabromo- and tetrachlorosemiquinone (bromanil and chloranil) anion radicals in crystals

Harnessing the power of microbial nanowires

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