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Review
. 2007 Nov;107(11):5004-64.
doi: 10.1021/cr0500030.

Proton-coupled electron transfer

My Hang V Huynh  1 , Thomas J Meyer
Affiliations
Review

Proton-coupled electron transfer

My Hang V Huynh et al. Chem Rev. 2007 Nov.
No abstract available

PubMed Disclaimer

Figures

Figure 1
Figure 1
Structure of the Reaction Center of Photosystem II illustrating terminal chlorophyll P680, pheophytinD1, quinone acceptor QA, YZ (TyrZ-His(190)), the Oxygen Evolving Complex (OEC), and the sequence of electron transfer events induced by light absorption and sensitization. The critical energetic role proposed for His(190) as EPT acceptor base is also shown. Reproduced in part with permission from Ref. []. © 2004, American Association for the Advancement of Science.
Figure 2
Figure 2
(a) Thermodynamics of possible two electron reduction products. (b) Transition states for formaldehyde reduction (Os ) = [OsIII(tpy)(Cl)] or [OsIII (tpy)(Cl)](NSPh)]).
Figure 3
Figure 3
Energy-coordinate curves, vibrational levels, and vibrational wavefunctions illustrating electron transfer as the sum of vibronic transitions from initial level v = 0 to final levels v′.
Figure 4
Figure 4
Illustrating reactant (I) and product (II) vibrational wave functions, H (solid curve) and D (dashed curve), for the μ = 0 → ν = 0 vibronic channel for the cis-[RuIV(bpy)2(py)(O)]2+/cis-[RuII(bpy)2(py)(OH2)]2+comproportionation reaction in Scheme 3. The plots for cis-[RuIV(bpy)2(py)(O)]2+/cis-[RuII(bpy)2(py)(OD2)]2+ illustrate the decrease in vibrational overlap for the -OD2 complex which is the origin of the H2O/D2O KIE of 12. Reproduced with permission from Ref. []. © 2002, American Chemical Society.
Figure 5
Figure 5
E1/2 vs pH diagram for the RuIV/III and RuIII/II couples of cis-RuII(bpy)2(py)(OH2)2+, (RuII-OH2) at 25 °C, I = 0.1 M, vs NHE). The vertical dotted lines correspond to pKa,1 for RuII-OH2 (pKa = 10.6) and pKa,1 for cis-[RuIII(bpy)2(py)(H2O)]3+(RuIII-OH23+, pKa = 0.85). The remaining abbreviations are: cis-[RuIV(bpy)2(py)(O)]2+(RuIV=O2+) and cis-[RuIII(bpy)2(py)(OH)]2+(RuIII-OH2+). The half-cell reactions for the individual couples in the various pH regions are indicated as are the sixth ligands and whether they are O2−, OH−, or H2O. The E1/2-pH curves were calculated from the Nernst equation by using the pKa values and E1/2(cis-[RuII(bpy)2(py)(H2O)]3+/+) = 1.02 V and E1/2(cis-[RuII(bpy)2(py)(OH)]2+/+) = 0.46 V.,, Reproduced with permission from Ref. []. © 2007, American Chemical Society.
Figure 6
Figure 6
Calculated E°-pH diagram for the 1e− oxidation of tyrosine at 25°C, I = 0.1 M. The structure of tyrosine as it would occur in a peptide is also shown, where R and R′ represent the polypeptide chain.
Figure 7
Figure 7
E1/2-pH diagram for Os(tpy)(H2O)32+ (vs. SSCE; + 0.236 V vs. NHE) illustrating the 3e− oxidation of Os(III) to Os(VI) over a wide pH range and the instability toward disproportionation of intermediate oxidation states Os(IV) and Os(V). Dominant forms of the complex in the various E-pH regions are indicated on the diagram, for example (OH)3 for OsIII(tpy)(OH)3.
Figure 8
Figure 8
Plot of free energy dependence versus ln(kH/kH°), with kH° the rate constant at ΔG° = 0, for ET, PT, and PCET. See Edwards et al. for parameter details of each model. Reproduced with permission from ref. [55], © 2009 American Chemical Society.
Figure 9
Figure 9
As in Figure 8, plots of ln(kH/kH°) vs ΔG° for electronically adiabatic a) PT and b) PCET models The red curves correspond to a fixed proton transfer barrier and the blue curves from a model based on an increase in barrier as −ΔG° increases. Reproduced with permission from ref. [55], © 2009 American Chemical Society.
Figure 10
Figure 10
Generalized 2D free energy surfaces with the transition state along the dash-dotted line. Reproduced with permission from ref. [57]. © 2008 American Chemical Society.
Figure 11
Figure 11
a–d) Show possible relationships between saddle points and Cl points involving simultaneous electron and proton transfer. e) More O’Ferrall-Jencks type diagram for a possible HAT-PCET continuum. Reproduced with permission from ref. [65], © 2008 American Chemical Society.
Figure 12
Figure 12
Iminoxyl/oxime self-exchange reaction. Reproduced in part with permission from ref. [66], © 2007 American Chemical Society.
Figure 13
Figure 13
Transition-structure molecular orbitals for iminoxyl/oxime self-exchange by EPT. Reproduced with permission from ref. [66], © 2005 American Chemical Society.
Figure 14
Figure 14
Transition state and HOMO orbital for tert-butylperoxyl/phenol hydrogen atom transfer. Reproduced with permission from ref. [66], © 2007 American Chemical Society.
Figure 15
Figure 15
C2 transition state and orbitals for benzyl/toluene hydrogen transfer. Reproduced with permission from ref. [66], © 2007 American Chemical Society.
Figure 16
Figure 16
Transition states and orbital interactions for formamide-formamide radical EPT and HAT pathways. Reproduced with permission from ref. [70], © 2007 American Chemical Society.
Figure 17
Figure 17
Proposed mechanism for H2O2 production from a model Cu(II)Aβ and 3O2. Reproduced with permission from ref. [81], © 2009 American Chemical Society.
Figure 18
Figure 18
Schematic representation of predicted mechanism of oxidation at the Qp site. Reproduced with permission from ref. [82], © 2008 American Chemical Society.
Figure 19
Figure 19
Change in SOMO along the NH bond forming reaction path for oxidation of hydroquinone by cytochrom bc1 complex in Complex III. The transition state corresponds to 1.4. Reproduced with permission from Ref. []. Copyright 2008, American Chemical Society.
Figure 20
Figure 20
Initial C-C bond formation in CPA catalyzed by P450 StaP (CYP245A1).
Figure 21
Figure 21
QM/MM reaction profile for EPT in CPA. Reproduced with permission from ref. [83b], © 2008 American Chemical Society.
Figure 22
Figure 22
Experimental and theoretical Pourbaix diagram for Ru(OH2)(Q)(tpy)2+ as E1/2 values relative to the SCE. The red dashed and solid blue lines correspond to the experimental pKa and redox potentials. The black line are theoretical predictions. Reproduced with permission from Ref. []. © 2009, American Chemical Society.
Figure 23
Figure 23
Structure of the substituted phenols in their carboxylate forms.
Figure 24
Figure 24
Structure of 4,6-di-tert-butyl phenol with a variety of pre-positioned bases. ,
Figure 25
Figure 25
Reaction pathway for the debromination of 8-bromopurines. (8BG is 8-bromoguanosine and 8BA is 8-bromoxanthosine) Adapted from Ref. [105].
Figure 26
Figure 26
Structure of model Mn clusters of the oxygen evolving complex. Adapted from Refs. [–111].
Figure 27
Figure 27
Structure of the mixed-valence μ-oxo-manganese complex [(bpy)2MnIII(μ-O)2MnIV(bpy)2]3+.
Figure 28
Figure 28
Structure of the Ru-Hbpp dimer water oxidation catalyst.
Figure 29
Figure 29
Polypyridyl ligands
Figure 30
Figure 30
Structure of seven coordinate [RuIV(L)(pic)2(OH)]+.
Figure 31
Figure 31
Structure of PY5.
Figure 32
Figure 32
Mn oxo monomers which participate in C-H bond cleavage. Adapted from Ref. [].
Figure 33
Figure 33
[RuIV(tpa)(H2O)(O)2]2+.
Figure 34
Figure 34
Structure of tpip ligand.
Figure 35
Figure 35
Structure of the Ru acetate cluster [Ru3O(H3CCO2)6(py)2(O)]+.
Figure 36
Figure 36
Structure of permethylated-amine-guanidine bis(μ-oxo)copper dimer.
Figure 37
Figure 37
Structure of the dinucleating ligand H-L (1,3-bis[bis(6-methyl-2-pyridylmethyl)-aminomethyl]benzene).
Figure 38
Figure 38
Proposed mechanism of hydrogen evolution (adapted from ref 147).
Figure 39
Figure 39
Structure of Ni phosphine complexes with positioned pendant bases for dihydrogen oxidation.
Figure 40
Figure 40
Structure of [N4Py2RFeII(OTf)]+.
Figure 41
Figure 41
Structures of H2bim, H2bip, py-imH and TEMPO.
Figure 42
Figure 42
Structures of [RuIII(dmp−)(TPA)]2+ and [RuIII(dmdmp−)(TPA)]2+.
Figure 43
Figure 43
Structure of RuIIICOO and RuIIIPhCOO.
Figure 44
Figure 44
Structures of hydrogen-bonded adducts: (1) 4-hydroxy-4′-nitro-biphenyl (para-nitrophenyl-phenol) and t-butylamine (TBA) in 1,2 dicholoro-ethane (DCE). (2) 7-hydroxy-4-(tri-fluoromethyl)-coumarin and 1-methylimidazole in toluene.
Figure 45
Figure 45
Illustration of charge redistribution between ground state and excited state for 2-naphthol. Reproduced with permission from ref. [], © 2008 American Chemical Society.
Figure 46
Figure 46
Structures of pyrene excited state photoacids which have been investigated by Stark spectroscopy. The 1La transition dipole is shown. It lies along the molecular axis for all three molecules. Reproduced with permission from ref. [], © 2008 American Chemical Society.
Figure 47
Figure 47
The arginine superbase mechanism for C-N bond cleavage. Reproduced with permission from ref. [], © 2006 American Chemical Society.
Figure 48
Figure 48
Excited state reductive quenching by MS-EPT in free-base meso-(pyridyl)porphyrins. Reproduced with permission from ref. [], © 2009 Royal Society of Chemistry.
Figure 49
Figure 49
Potential energy surface diagrams (S2, S1, S0) involved in radiationless deactivation of the lowest ππ* excited state of the H-bonded pyrrole-pyridine adduct. Illustrative classical paths (in yellow) illustrate Franck-Condon excitation (arrow), and relaxation on the S1 and S0 surfaces, restoring the initial ground-state configuration. Reproduced with permission from ref, [], © 2007 American Chemical Society
Figure 50
Figure 50
Illustration of concerted and stepwise Excited State Double Proton transfer in 7-azaindole dimers. Reproduced with permission from ref. [], © 2008 Elsevier.
Figure 51
Figure 51
Proposed reaction mechanism for photosensitized 2-methyl-1,4-naphthoquinone oxidation to dmC. Reproduced with permission from ref. [], © 2008 Wiley.
Figure 52
Figure 52
Mechanism for EPT reductive quenching of [(bpy)2RuIII(bpz·−)]2+· by H2Q. Reproduced with permission from ref. [], © 2007 American Chemical Society.
Figure 53
Figure 53
Illustration of reductive EPT quenching of the emitting MLCT excited state of [Ru(bpy)2(pbim)]+ by ubiquinol. Reproduced with permission from ref. [78], © 2009 American Chemical Society.
Figure 54
Figure 54
Photodimerization of [RuII(bpy)2(L-L)]2+ (L-L = trans-1,2-bis(4-(4′-methyl)-2,2′-bipyridyl) ethane) in aqueous solution. Reproduced with permission from ref. [], © 2008 American Chemical Society.
Figure 55
Figure 55
Structures of RuY, RuesterY, and Re(P-Y) complexes with appended tyrosinyl groups. Reproduced with permission from ref. [], © 2007 American Chemical Society.
Figure 56
Figure 56
Structures of Ru(bpy)2(bpy-C(O)NH-ArOH)2+ (RuY), a salicylic acid derivate (Ru-SA), and a 2-hydroxyphenylacetic acid derivate (Ru-PA). Reproduced with permission from ref. [], © 2008 American Chemical Society.
Figure 57
Figure 57
Illustrating the excited state EPT quenching mechanism in the H-bonded adduct between IrbiimH2+ and the benzoate anion, dnb−. Reproduced with permission from ref. [], © 2008 Royal Society of Chemistry.
Figure 58
Figure 58
Illustrating the H-bonded amidinium-carboxylate salt-bridge assembly (1-H:2) between a Zn(II) porphyrin photoreductant (1-H) and a naphthalene diimide electron transfer acceptor (2). Reproduced with permission from Ref. []. © 2006, American Chemical Society.
Figure 59
Figure 59
Photochemically induced net H-atom transfer from DMF to a surface M=O site at MO3 (M = Mo, W)( σ-bonds are shown as solid lines and π-bonds by dotted lines). Adapted from Ref. [].
Figure 60
Figure 60
Watson-Crick ground state guanine-cytosine base pair illustrating the direction of proton transfer following excitation (vertical arrow). Adapted from Ref. []
Figure 61
Figure 61
Illustration of interconversion between A and B forms of the green fluorescent protein (GFP) chromophore through intermediate state I*. Figure from Stoner-Ma, 2008 (adapted from Brejc et. al.). Figure adapted from Ref. [] and reproduced with permission from Ref. []. © 2008, American Chemical Society
Figure 62
Figure 62
Proton wire effect leading to excted state proton transfer (ESPT) following excitation in wild-type GFP. Excitation at 370–400 nm, leads to stepwise proton transfer through the H-bonded network to Glu222. Reproduced with permission from Ref. []. © 2006, Elsevier.
Figure 63
Figure 63
Pourbaix diagram for [Ru3O(Ac)6(py)2(OHx)]n in aqueous solution; pKa values (vertical dotted lines) and slopes (in mV) are also noted in the Figure. Reproduced with permission from Ref. []. © 2006, Wiley.
Figure 64
Figure 64
Schematic representation of the potential energy profiles for an EPT mechanism. Please note CPET in figure is described as EPT in the text. Reproduced with permission from Ref. []. © 2010, American Chemical Society
Figure 65
Figure 65
Long distance proton coupled electron transfer achieved by inserting a hydrogen-bonded relay. Reproduced with permission from Ref. []. Copyright 2010, Wiley.
Figure 66
Figure 66
Variation of the apparent standard rate constant for the OsIII(bpy)2(py)(OH2)3+/OsII(bpy)2(py)(OH2)2+ couple with pH. (A and B) Blue dots are experimental rate constants in 0.1 M Britton–Robinson buffers in water and solid lines represent predicted variation of rate constants based on the stepwise ET-PT and PT-ET mechanisms. (B) Red dots: 0.1 M Britton– Robinson buffers in D2O. Inset shows the dependence of the apparent standard rate constant with increasing buffer base concentration in an acetate buffer of pH 5 in water. Reproduced with permission from Ref. []. © 2009, National Acadamy of the Sciences (USA).
Figure 67
Figure 67
(a) CVs for an ortho substituted phenol, structure shown in (c), at a scan rate 0.2 V/s in a 0.1 M n-NBu4PF6 acetonitrile solution, 20°C. Dotted line is experimental and solid line is the simulated CV assuming an EPT pathway. (b) Red line is the experimental CV for c; the blue and green lines are simulations based on the square scheme mechanism (scheme 34), with E0AB = 1.59, 1.39 V and E0CD = 0.23, 0.43 V vs SCE respectively, λ = 0.7 eV, KAC = 3.5 × 10−7, 9.8 × 10−4, KBD = 9 × 1016, 3.2 × 1013 for both cases. λ is the reorganization energy. Note that PCET rate constant determinations for individual steps requires assumption of a λ value (e.g. eq 62) Reproduced with permission from Ref. []. © 2006, American Chemical Society.
Figure 68
Figure 68
Impedance measurements in a Nafion membrane containing equal amounts of K3[Fe(CN)6] and K4[Fe(CN)6] (3.4 ×10−3 M) obtained by applying a 10 mV rms AC signal in the frequency range 10 mHz to 100 kHz. (inset bottom right: equivalent circuit corresponding to the semicircle fitted with the high frequency part of the response). Reproduced with permission from Ref. []. © 2006, American Chemical Society.
Figure 69
Figure 69
Spectroelectrochemical changes in acetonitrile during the oxidation of [(bpy)2Ru(H2pzbzim)Ru(bpy)2]3+: RuIIRuII → RuIIRuIII (a) and RuIIRuIII → RuIIIRuIII (b) with loss of a proton at each stage. Inset shows the intervalence charge transfer (IVCT) band obtained by spectral deconvolution Reproduced with permission from Ref. []. © 2010, Wiley.
Figure 70
Figure 70
Experimental designs for polarizing liquid-liquid interfaces: A) by applying an external voltage, and B) by using a common ion to generate a Galvani potential difference. CEA = aqueous counter electrode; REO = reference electrode for the organic phase; BA+ = bis(triphenylphospharanylidene)ammonium cation; TB− = tetrakis(pentafluorophenyl)borate anion.
Figure 71
Figure 71
Proposed ITIES mechanism for O2 reduction by a cobalt porphyrin catalyst., Reproduced with permission from ref. [], © 2009, American Chemical Society.
Figure 72
Figure 72
ECL at the liquid-liquid interface between the [Ru(bpy)3]3+/2+ couple in an aqueous layer and droplets of trioctylamine in methylene chloride. A) In aqueous solutions with pH > 11 with ET occurring at the liquid-liquid interface. B) Below the biphasic pKa (~11), PCET occurs at the liquid-liquid interface.
Figure 73
Figure 73
A polystyrene supported molybdenum(VI) bis-oxo catalyst for oxidation of trimethylphosphine. The proposed catalytic mechanism, involving EPT, is shown on the right. Diacetyl ferrocenium is the stoichiometric oxidant in this system. Adapted from Refs. [,]
Figure 74
Figure 74
pH-dependent PCET oxidation of ruthenium(VI) oxide species in a composite ruthenium oxide/ruthenium Prussian blue (RuPB) surface.
Figure 75
Figure 75
Electrochemical oxidation of glucose to gluconic acid.
Figure 76
Figure 76
Two SAM structures on gold electrodes, that undergo PCET. The hydroquinones involved in PCET are separated from each other by dilution with octane-1-thiol. The increased conjugation of the tether to the hydroquinone in B) increases the rate of electron transfer to the surface, increasing the rate of 2e−/2H+ oxidation to the quinone.
Figure 77
Figure 77
An isolated 1,4-benzoquinone site tethered to an alkanethiol monolayer on a gold surface studied by Burgess and coworkers.
Figure 78
Figure 78
Surface-bound (Au) 1-aminoanthroquinone monomer SAM used to study PCET by Abhayawardhana and Sutherland.
Figure 79
Figure 79
Structure of the OsII(bpy)2(4-AMP)(H2O)]2+ derivative imbedded in SAMs of ~18 Au-S(CH2)15COOH: 1 Au-S(CH2)16OH.
Figure 80
Figure 80
Cyclic voltammograms (CV) of TyrOH (0.1 mM) in 0.1 M HClO4/0.8M in LiClO4 at 300 mV/s, at ITO (red), at ITO-RuII(Γ = 1.2×10−10 mol/cm2) and at ITO-RuII + TyrOH at 25±2°C (blue). Reproduced with permission from Ref. []. © 2011, American Chemical Society.
Figure 81
Figure 81
Illustrating surface MS-EPT with electron transfer to ITO-Ru3+ and proton transfer to an added buffer base, B, as proton acceptor. Reproduced with permission from ref. [], © 2011 American Chemical Society.
Figure 82
Figure 82
Band gap injection of holes into a methanol overlayer on TiO2. (A and B) Top and side views, respectively, of the ground state and the associated lowest energy unoccupied orbital of the methanol overlayer. (C and D) The top and side views, respectively, of the structure and electron density, following electronic excitation and the resulting PCET. Ti is blue; O is red; C is orange; and H is white. Reproduced with permission from ref. [], © 2006 American Association for the Advancement of Science.
Figure 83
Figure 83
Illustration of intramolecular hydrogen bonding in a 2-(phenol)-benzimidazole porphyrin.
Figure 84
Figure 84
Illustrating the intercalated array of hydroquinone molecules between layers in in [CuF(tptm)]. Reproduce with permission from ref. [], © 2008 Springer.
Figure 85
Figure 85
Schematic diagram of a system in which nitric oxide reductase (NOR) was incorporated into a liposome with the dye phenol red used to monitor pH changes outside the liposome during NOR-catalyzed oxygen reduction catalysis. Reprinted with permission from ref. [], © 2007 Elsevier.
Figure 86
Figure 86
Illustration of CcO tethered to a gold electrode and embedded in a lipid bilayer membrane. Reproduced with permission from ref. [], © 2008 Biophysical Society.
Figure 87
Figure 87
Variation of RTln(kred) vs −ΔG°′ in eV by varying both E°′ for the oxidant and pKa for the acceptor base, see text. The dashed line is a plot of RTln kred = RTln k(0) + ΔG°′/2 (1 + ΔG°′/2λ) with k(0) = 8.9×102 s−1 λ = 0.90 eV. Reproduced with permission from ref. , © 2007 American Chemical Society.
Figure 88
Figure 88
pH dependence of kobs for intramolecular oxidation of Ru-Y, RuIII-TyrOH → RuII-TyrO· + H+, illustrating the appearance of a pH dependence below 0.5 mM added buffers, H2PO4−/HPO42−. The inset shows the pH dependence of kobs over an extended pH range. Similar effects were observed for the complexes RuesterY and Re(P-Y). Reproduced with permission from ref. []. © 2007 American Chemical Society.
Figure 89
Figure 89
Proposed mechanisms for electron transfer from tyrosine to tryptophan radical cation (TrpNH·+) in a series of synthetic peptides. With n < 2, the mechanism involves prior proton loss from TrpNH·+ followed by concerted EPT from TyrOH to TrpN·. With n > 2, the mechanism involves an external base and MS-EPT with electron transfer to TrpNH·+ and proton transfer to the base. Reproduced with permission from ref. [], © 2009 American Chemical Society.
Figure 90
Figure 90
Proton coupled electron transfer (PCET) mechanisms proposed for heme protein maquettes. The structure of the protein maquette is reported in the figure on the right and consists of both beta sheets and an alpha helix. Reproduced with permission from ref. [], © 2007 American Chemical Society.
Figure 91
Figure 91
Mechanism of proton coupled electron transfer for two related protein maquettes. A) Mechanism for redox kinetics at the iron heme center for b-[H10A24] where PCET reactivity is associated with the glutamate residue. B) Mechanism for redox kinetics for b-[Δ7-His] where PCET is associated with a histidine. Reproduced with permission from ref. [], © 2007, American Chemical Society.
Figure 91
Figure 91
Mechanism of proton coupled electron transfer for two related protein maquettes. A) Mechanism for redox kinetics at the iron heme center for b-[H10A24] where PCET reactivity is associated with the glutamate residue. B) Mechanism for redox kinetics for b-[Δ7-His] where PCET is associated with a histidine. Reproduced with permission from ref. [], © 2007, American Chemical Society.
Figure 92
Figure 92
Structure of the Reaction Center in PSII illustrating the sequence of e−/H+ events that occur in the S0 → S1 transition following single photon absorption. Reproduced in part with permission from Ref. [18]. © 2004, American Association for the Advancement of Science.
Figure 93
Figure 93
Illustrating the proposed 2e−/2H+ EPT oxidation of Mn(4)-OH2 to Mn-OH in the first stage of the Kok cycle.
Figure 94
Figure 94
An illustration of the proposed intra-coordination sphere proton transfer step in the first step of the Kok cycle.
Figure 95
Figure 95
Frost diagram (nE vs relative oxidation number) showing the cumulative reduction potentials of four species: the OEC in red (pH = 6, referenced to S0); a single manganese ion in green (pH = 6, referenced to Mn0); a hypothetical manganese tetramer, without PCET, in magenta (see text for details, referenced to MnIII 3MnIV); oxygen in blue (pH) 6, referenced to 2H2O; the species shown on the solid blue line are 2H2O (0), H2O + OH• (1), H2O2 (2), O2•− (3), and O2 (4)). The blue long-dashed line represents the four-electron S4/S0 couple. The two blue short-dashed lines represent the two two-electron couples, S4/S2 and S2/S0. The green dashed line represents the MnVII/MnIII couple. All reduction potentials are given versus the standard hydrogen electrode (SHE). Reproduced with permission from ref. [21], © 2006 American Chemical Society.
Figure 96
Figure 96
Energetics in PS II based on midpoint redox potentials, Em(QA/QA−) and Em(Phe a/Phe a−), together with free energy differences estimated in the literature. The value of Em(Phe a/Phe a−) is from Ref.; for the values of Em(P680 +/1P680*), Em(P680 +/P680), ΔGCS and ΔGS, and the gray boxes indicating uncertainties, see Discussion in Ref.; the value of Em(P680/P680+) was calculated from a kinetic analysis (Ref.) on the basis of the Em(QA/QA−) value determined by Shibamoto and coworkers; the value of ΔGQA/QB is from the literature,. The solid arrows denote the forward electron transfer in oxygen-evolving PS II complexes, while the broken arrows denote reverse (charge recombination) electron transfer for photo-protection of inactivated PS II. Reproduced with permission from ref. []. © 2009 American Chemical Society.
Figure 97
Figure 97
Generalized reaction coordinate for four-step oxidative water cleavage in the photosynthetic apparatus. Reproduced with permission from Ref. [], © 2007, Elsevier.
Figure 98
Figure 98
Scheme for ET from YZ to P680*. Y and B are YZ and D1-His, respectively, and the calculated pKa values of Y and B for these residues at each step are listed in the scheme. Reproduced with permission from ref. © 2007, Elsevier.
Figure 99
Figure 99
Two possible proton-donor pathways for YD·. The straight arrows indicate electron transfer. The curved arrows indicate proton transfer. In (A), His189D2 (reaction k1, blue) is involved in a multiple proton pathway which must include three or more protons of which two are shown. An internal water molecule (reaction k2, red) acts as a single-proton donor. Alternatively, in (B), His189D2 (reaction k1, blue) acts as a single-proton donor, and a chain of internal water molecules (reaction k2, red), of which two are shown, is involved in a multiple-proton pathway. Reproduced with permission from ref. [], © 2009 American Chemical Society.
Figure 100
Figure 100
The proposed proton exit channel beginning at aspartate D1-61. See also references by Voth et al. Reproduced with permission from ref. [22b], © 2006 American Chemical Society
Figure 101
Figure 101
A schematic diagram of amino acid residues along the catalytic chain of class I RNR. The crystal structure suggests that the protein exists as a homodimer with radical transport from subunit R2 to subunit R1 Electron transfers occurs over a 35 Å distance originating at Y122 on the R2 subunit of RNR and terminating at C439 on the R1 subunit. Reproduced with permission from ref. [], © 2006 American Chemical Society.
Figure 102
Figure 102
Structure of non-natural amino acids incorporated into ribonucleotide reductase.
Figure 103
Figure 103
Structural arrangement of the FADH·-W382-W359-W306 chain in DNA photolyase following laser flash photolysis at 500 nm with timescales for subsequent electron transfer hopping indicated on the figure. The axes in the figure highlight the different orientations of the ring systems. Reproduced with permission from ref. [], © 2008 American Chemical Society.
Figure 104
Figure 104
Structure of the Re-W122-Cu(II) Azurin derivative from a 1.5 Å resolution XRD-structure. The Re complex is ReI(CO)3(dmp)(His124)+. Distances: Cu to W122 (aromatic centroid)-11.1Å, W122 to Re-8.9 Å, Cu to Re-19.4 Å. Reproduced with permission from Ref. [], © 2008 American Association for the Advancement of Scicence.
Figure 105
Figure 105
Kinetic events following laser flash excitation of the MLCT excited state of the ReI(dmp) chromophore (ReII(dmp·−)) in ReI(CO)3(dmp)(His124)+|(W122)|AzCuI. The excited electron (red)-hole (blue) pair in the MLCT excited state is illustrated. MLCT excitation is followed by W122 reductive quenching to give the Redox Separated state ReI(dmp·−)-TrpNH·+ in < 50ns. Back electron transfer occurs on the microsecond timescale. Reproduced with permission from ref. [], © 2008 American Association for the Advancement of Scicence.
Figure 106
Figure 106
Chemical reaction of soybean lipoxygenase catalyzed oxidation of linoleic acid. Reproduced with permission from ref. [], © 2002 American Chemical Society.
Figure 107
Figure 107
XRD structure of cytochrome c peroxidase. In the enzyme, electron transfer originates at high potential (HP) heme and terminates 20 Å away at a low potential heme (LP) with tryptophan W97 as an electron transfer mediator located 20 Å from both iron hemes. Reproduced with permission from ref. [], © 2006 American Society for Biochemistry and Molecular Biology.
Figure 108
Figure 108
Proposed mechanism of activation for CcP for the reduction of H2O2. Reproduced with permission from ref. [], © 2009 American Chemical Society.
Figure 109
Figure 109
pH dependence of Ecat determined for voltammograms of NeCcP in the presence of 100μM substrate. Reproduced with permission from ref. [], © 2004 American Society for Biochemistry and Molecular Biology.
Figure 110
Figure 110
Cyclic voltammogram on a pyrolytic graphite edge electrode derivitized with CcP at A) 2mV/s and B) 100mV/s. Reproduced with permission from ref. [], © 2009 American Chemical Society.
Figure 111
Figure 111
XRD structure of cytochrome c oxidase showing the net movement of electrons and protons during enzyme turnover. Reproduced with permission from ref. [], © 2006 Nature Publishing Group.
Figure 112
Figure 112
Proposed reaction cycle for CcO. The cycle begins by cleavage of the O-O bond. One proton is transferred from E242 to the proton-loading site while an electron is transferred to the binuclear site. Reproduced with permission from ref. [], © 2009 Elsevier.
Figure 113
Figure 113
The structure of the tyrosine histidine adduct formed in cytochrome c oxidase (CcO). The histidine acts to support radical formation at the tyrosine hydroxyl group.
Figure 114
Figure 114
Cytochrome c oxidase has a network of hydrogen bonded water molecules and structurally positioned basic amino acid groups. There is evidence that water based equilibria aid in determining the position and availability of proton acceptors associated with the catalytic core of CcO. Reproduced with permission from ref. [] © 2009 Elsevier.
Scheme 1
Scheme 1
Scheme 2
Scheme 2
Scheme 3
Scheme 3
Scheme 4
Scheme 4
Scheme 5
Scheme 5
(at pH = 7)
Scheme 6
Scheme 6
PCET pathways for the oxidation of tyrosine by M(bpy)33+ in water with added buffer, +HB/B.
Scheme 6
Scheme 6
PCET pathways for the oxidation of tyrosine by M(bpy)33+ in water with added buffer, +HB/B.
Scheme 6
Scheme 6
PCET pathways for the oxidation of tyrosine by M(bpy)33+ in water with added buffer, +HB/B.
Scheme 7
Scheme 7
Scheme 8
Scheme 8
MS-EPT with H2O as the H+ acceptor compared to ET-PT (25°C, STP).
Scheme 8
Scheme 8
MS-EPT with H2O as the H+ acceptor compared to ET-PT (25°C, STP).
Scheme 8
Scheme 8
Scheme 9
Scheme 9
E° values vs NHE for the 1e− couples interrelating formaldehyde and methanol.
Scheme 10
Scheme 10
Acid-base redox potential (pKa-E°′) diagram for the Ru(IV/III) and Ru(III/II) redox couples of cis-RuII(bpy)2(py)(OH2)2+ with values taken from Figure 5, showing the dπ electron configurations at the metal. Potentials are vs NHE at 25 °C, I = 0.1 M. The vertical lines give pKa values and the slanted lines give the E°′ values for the pH dependent couples at pH 7.,
Scheme 11
Scheme 11
As in Scheme 10 with potentials in V vs NHE, pH 7 at 25 °C, I = 0.1 for the quinone/semiquinone/hydroquinone (Q/HQ•/H2Q) couples.,
Scheme 12
Scheme 12
Scheme 13
Scheme 13
Possible mechanisms for quercitin oxidation by dpph radicals. Adapted from Ref. [97].
Scheme 14
Scheme 14
HAT oxidation of a phenol by dpph·. Adapted from Ref. [97].
Scheme 15
Scheme 15
Proposed mechanism for acid-catalyzed oxidation of phenols by peroxyl radicals. Rds is rate-determining step. Adapted from Ref. [98].
Scheme 16
Scheme 16
Mechanism of oxidation of an ascorbic acid derivative to ascorbyl radical by the nitroxyl radical TEMPO. (iAscH2 is a organic soluble analog of ascorbic acid) Adapted from Ref. [101].
Scheme 17
Scheme 17
Pseudo self-exchange reaction based on TEMPO derivatives. Adapted from Ref. [].
Scheme 18
Scheme 18
Structures of the heterocycles. Reproduced with permission from Ref. []. Copyright 2008, American Chemical Society.
Scheme 18
Scheme 18
Proposed single-site water oxidation mechanism. Reproduced with permission from ref. [29], © 2010 American Chemical Society.
Scheme 20
Scheme 20
Catalytic oxygenation of organic substrates with a mononuclear nonheme iron(IV) oxo complex. Adapted from Ref. [126].
Scheme 21
Scheme 21
Reactivity of [(Cz)MnV(O)(L)]− with dihydroanthracene with L = F− or CN−. Adapted from Ref. [].
Scheme 22
Scheme 22
C-H activation by a triazamacrocyclic Cu(II) complex. Adapted from Ref. [137]
Scheme 23
Scheme 23
[Fe-(O22−)-Cu]+ H-atom abstraction from a phenol derivative. Adapted from Ref. [138]
Scheme 24
Scheme 24
Oxidative dehydrogenation of alcohols by V(O)2−. Adapted from Ref [139].
Scheme 25
Scheme 25
Proposed mechanism for electrocatalytic oxidation of hydrogen. Relative free energies are shown in the squares in kcal/mol (1.0 atm of H2′ pH 8.5). Reproduced with permission from ref. [147a], © 2009 American Chemical Society.
Scheme 26
Scheme 26
Hydrogen Evolution Pathways (adapted from ref. 150).
Scheme 27
Scheme 27
pH dependent reduction of O2 by α-PW12O404− by stepwise or concerted pathways. Adapted from ref. []
Scheme 28
Scheme 28
Oxidation of RuII(py-imH)(acac)2 by TEMPO. Adapted from Ref. [].
Scheme 29
Scheme 29
Self-oxidation of the octaaza ligand LH4. Adapted from Ref. [].
Scheme 30
Scheme 30
HAT interconversion of TPPFeIIIIm and TPPFeIIImH. Adapted from Ref. [].
Scheme 31
Scheme 31
Illustrating the multi-square, multiple acid-base equilibria scheme for [α-SiW11O39]8−; m = 2.4, n = 2. Reproduced with permission from Ref. []. © 2006, American Chemical Society.
Scheme 32
Scheme 32
Adapted from Ref. []
Scheme 33
Scheme 33
Electrochemical EPT with proton transfer to added water.
Scheme 34
Scheme 34
Reproduced with permission from Ref. []. © 2006, American Chemical Society.
Scheme 35
Scheme 35
Scheme 35
Scheme 35
Scheme 36
Scheme 36
Parallel pathways for the base-catalyzed oxidation of tyrosine with prior association of tyrosine (TyrOH) with HPO42− as the proton acceptor base. Pre-association with the base form of the buffer (KA) is followed by competitive multiple site-electron proton transfer (MS-EPT) and deprotonation to TyrO− followed by electron transfer oxidation (PT-ET)., Adapted from Refs. [15, 255]
Scheme 37
Scheme 37
Modified version of the Kok Cycle for oxidative activation and O2 evolution at the oxygen-evolving complex (OEC) of photosystem II. In the cycle, light absorption and excited state electron transfer result in stepwise oxidation of the OEC (S0 → S1, etc.) with trans-membrane proton transfer occurring from the stroma to the lumen. QB is plastoquinone and H2QB is plastoquinol. Reproduced from ref. []. © 2011 The American Society for Biochemistry and Molecular Biology.
Scheme 38
Scheme 38
Scheme 39
Scheme 39
Possible schemes for proton uptake upon reduction of the SOD active site. In (A), the redox coupled H+ acceptor would be coordinated solvent whereas in (B), the redox coupled H+ acceptor would be nearby Tyr34 (Y34). The upper end of each equilibrium includes short-hand notation for the four amino acid ligands to the metal ion (M). These are still present in the reduced state of SOD but are omitted for implicity. Reproduced with permission from ref. [], © 2003 Elsevier.
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