Flash photolysis of aqueous phenol and cresols

Dobson, Gerard R.; Grossweiner, Leonard I.

doi:10.1039/tf9656100708

Cited by 84 publications

(38 citation statements)

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“…Recently, difference spectra of tyrosine radicals from pulse radiolysis experiments have been presented (57). These spectra were largely shifted (up to 40 nm) to red wavelengths when compared to other in vitro spectra in a large body of literature (58,61,69,70) and to Y Z in PSII (see Figure 3), and the extinction coefficients in the UV in ref 57 were smaller, about 50-75% of the values obtained for the oxidation of tyrosine in vitro (58,61,69,70) and in PSII (see ref 57 and references therein). Furthermore, the contribution of the reduced state to the oxidized-minus-reduced spectra seemed to be much higher than in other in Vitro and in vivo spectra (see above).…”

Section: Raw Absorption Difference Spectra and Their Deconvolutionmentioning

confidence: 99%

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Tyrosine-Z in Oxygen-Evolving Photosystem II: A Hydrogen-Bonded Tyrosinate

1998

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In oxygen-evolving photosystem II (PSII), a tyrosine residue, D1Tyr161 (Y Z ), serves as the intermediate electron carrier between the catalytic Mn cluster and the photochemically active chlorophyll moiety P 680 . A more direct catalytic role of Y Z , as a hydrogen abstractor from bound water, has been postulated. That Y Z ox appears as a neutral (i.e. deprotonated) radical, Y Z • , in EPR studies is compatible with this notion. Data based on electrochromic absorption transients, however, are conflicting because they indicate that the phenolic proton remains on or near to Y Z ox . In Mn-depleted PSII the electron transfer between Y Z and P 680 + can be almost as fast as in oxygen-evolving material, however, only at alkaline pH. With an apparent pK of about 7 the fast reaction is suppressed and converted into an about 100-fold slower one which dominates at acid pH. In the present work we investigated the optical difference spectra attributable to the transition Y Z f Y Z ox as function of the pH. We scanned the UV and VIS range and used Mn-depleted PSII core particles and also oxygen-evolving ones. Comparing these spectra with published in vitro and in vivo spectra of phenolic compounds, we arrived at the following conclusions: In oxygen-evolving PSII Y Z resembles a hydrogen-bonded tyrosinate, Y Z (-) ‚‚‚H (+) ‚B. The phenolic proton is shifted toward a base B already in the reduced state and even more so in the oxidized state. The retention of the phenolic proton in a hydrogen-bonded network gives rise to a positive net charge in the immediate vicinity of the neutral radical Y Z • . It may be favorable both for the very rapid reduction by Y Z of P 680 + and for electron (not hydrogen) abstraction by Y Z • from the Mn-water cluster.Photosystem II (PSII) 1 produces molecular oxygen from water. It is located in photosynthetic membranes of plants and cyanobacteria. The inner core of PSII is a heterodimer of the D1D2 subunits which show homology with the L and M subunits of the purple bacterial RC (1). The primary electron donor of PSII, P 680 , is located close to the lumenal side of the membrane. The absorption of a quantum of light drives the charge separation between P 680 and the quinone acceptor at the stromal side (2). The very high redox potential of P 680 + drives water oxidation. It is a four-stepped process with at least four increasingly oxidized, metastable states S 0 to S 3 , plus a transient state S 4 that decays in milliseconds into S 0 under release of O 2 . P 680 + is reduced (in nanoseconds) by a redox-active tyrosine residue, Y Z (D1Tyr161 (3, 4)). Y Z ox is, in turn, reduced in microseconds by the oxygenevolving complex (OEC). The structure and exact location of the OEC is still ill-defined. It is formed by four manganese atoms (5, 6), possibly another redox cofactor, X (7-9), plus at least one Cl -ion (10) and one Ca 2+ (11, 12) ion. These cofactors serve various functions during water oxidation: The Mn cluster stores at least two of the oxidizing equivalents (namely on transitions S ...

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Section: Raw Absorption Difference Spectra and Their Deconvolutionmentioning

confidence: 99%

“…The reasons for these discrepancies are still unknown. We preferred to use the more consistent set of spectra from the elder literature (58,61,69,70) for our simulations.…”

Section: Raw Absorption Difference Spectra and Their Deconvolutionmentioning

confidence: 99%

Tyrosine-Z in Oxygen-Evolving Photosystem II: A Hydrogen-Bonded Tyrosinate

1998

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“…This value is very similar to the value of 1.7 X (I/mole s)l/2 which can be calculated from our previous data (I) t o be the ratio of k8/kg1I2 for 4-methoxyphenol. If it is assumed that kg is similar to the recently determined value of -6 X 109 1 mole-l s-I for the dimerization of phenoxy and 4-methylphenoxy radicals in aqueous solution (9), then k8 will be about 15 1 mole-l s-l.…”

Section: T a B L E I1 T H E Inhibited Oxidation O F Tetralin At 65 "Cmentioning

confidence: 99%

“…I t was expected that this phenol would show the same kinetics as 4-methoxyphenol (eq. [111]), i.e., kinetics which result from the rapid coupling of phenoxy radicals (reaction [9]). This was in fact observed, i.e., a s the 2,G-dimethylphenol concentration was increased the rate decreased t o a constant limiting value which depended on [RH] X [Is]lt2 but was unaffected by the further addition of inhibitor.…”

Section: T a B L E I1 T H E Inhibited Oxidation O F Tetralin At 65 "Cmentioning

confidence: 99%

The Kinetics of the Inhibited Autoxidation of Tetralin. Ii

Howard¹,

Ingold²

1965

Can. J. Chem.

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The kinetics of the oxidation of tetralin inhibited by a number of phenols have been examined. The chain transfer reaction between phenoxy radicals and hydroperoxides is subject to both polar and steric effects. T h e rate of this reaction is increased by electron-attracting substituents on the phenol and is decreased both by ortho-alkyl groups on the phenol and by a n increase in the size of the substituents on the hydroperoxides.

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“…3 The photophysical properties of the phenol chromophore in aqueous solution have been extensively studied on both the ground 4 and excited potential energy surfaces (PESs). [5][6][7][8] Previous flash photolysis and transient absorption studies of phenol and tyrosine in aqueous solution 5,6,[8][9][10] reveal the formation of phenoxyl (or tyrosyl) radicals and solvated electrons, 11 but the precise details of the mechanism(s) leading to these photoproducts have remained elusive. The debate as to whether the process involves excited state proton transfer (ESPT), 7 autoionization, O-H bond fission (H-atom transfer), or excited-state proton-coupled electron transfer (PCET) pathways remains a matter of some controversy.…”

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confidence: 99%

Exploring Autoionization and Photoinduced Proton-Coupled Electron Transfer Pathways of Phenol in Aqueous Solution

Oliver

Zhang

Roy

et al. 2015

J. Phys. Chem. Lett.

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The excited state dynamics of phenol in water have been investigated using transient absorption spectroscopy. Solvated electrons and vibrationally cold phenoxyl radicals are observed upon 200 nm and 267 nm excitation, but with formation timescales that differ by more than four orders of magnitude. The impact of these findings is assessed in terms of the relative importance of autoionization versus proton-coupled electron transfer mechanisms in this computationally tractable model system. TOC GRAPHIC

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Flash photolysis of aqueous phenol and cresols

Cited by 84 publications

References 0 publications

Tyrosine-Z in Oxygen-Evolving Photosystem II: A Hydrogen-Bonded Tyrosinate

Tyrosine-Z in Oxygen-Evolving Photosystem II: A Hydrogen-Bonded Tyrosinate

The Kinetics of the Inhibited Autoxidation of Tetralin. Ii

Exploring Autoionization and Photoinduced Proton-Coupled Electron Transfer Pathways of Phenol in Aqueous Solution

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