Further comments on the redox potentials of tryptophan and tyrosine

Harriman, Anthony

doi:10.1021/j100308a011

Cited by 415 publications

(460 citation statements)

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“…The resulting oxidation of the Bi-PhOH unit dramatically enhances the acidity of the phenolic proton (Δ pKa ∼ 12) (25,26), which creates the chemical potential required for proton transfer to the hydrogen-bonded benzimidazole moiety (27)(28)(29)(30). Therefore, excitation of triad 1 is expected to lead to eventual formation of a charge-separated state characterized by cationic benzimidazole, a phenoxyl radical, a neutral PF 10 , and a reduced TCNP porphyrin (BiH þ -PhO • -PF 10 -TCNP •− ).…”

Section: Resultsmentioning

confidence: 99%

Mimicking the electron transfer chain in photosystem II with a molecular triad thermodynamically capable of water oxidation

Megiatto

Antoniuk-Pablant

Sherman

et al. 2012

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

110

111

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In the photosynthetic photosystem II, electrons are transferred from the manganese-containing oxygen evolving complex (OEC) to the oxidized primary electron-donor chlorophyll P680 •+ by a proton-coupled electron transfer process involving a tyrosine-histidine pair. Proton transfer from the tyrosine phenolic group to a histidine nitrogen positions the redox potential of the tyrosine between those of P680 •+ and the OEC. We report the synthesis and time-resolved spectroscopic study of a molecular triad that models this electron transfer. The triad consists of a high-potential porphyrin bearing two pentafluorophenyl groups (PF 10 ), a tetracyanoporphyrin electron acceptor (TCNP), and a benzimidazole-phenol secondary electron-donor (Bi-PhOH). Excitation of PF 10 in benzonitrile is followed by singlet energy transfer to TCNP ( τ = 41 ps), whose excited state decays by photoinduced electron transfer ( τ = 830 ps) to yield . A second electron transfer reaction follows ( τ < 12 ps), giving a final state postulated as BiH + -PhO • -PF 10 -TCNP •- , in which the phenolic proton now resides on benzimidazole. This final state decays with a time constant of 3.8 μs. The triad thus functionally mimics the electron transfers involving the tyrosine-histidine pair in PSII. The final charge-separated state is thermodynamically capable of water oxidation, and its long lifetime suggests the possibility of coupling systems such as this system to water oxidation catalysts for use in artificial photosynthetic fuel production.

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Section: Resultsmentioning

confidence: 99%

Mimicking the electron transfer chain in photosystem II with a molecular triad thermodynamically capable of water oxidation

Megiatto

Antoniuk-Pablant

Sherman

et al. 2012

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

110

111

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“…Tyr-8 (Tyr-21 in AppA) is a good candidate because it is the closest aromatic residue to the flavin and has been shown to be critical for photocycling activity in BLUF domains (12,15,16). The redox properties of flavin and Tyr gives a favorable driving force for the ET reaction: the midpoint potential for flavin͞flavin •Ϫ is approximately Ϫ0.8 V and for Tyr͞Tyr •ϩ is Ϸ0.93 V (25,27), providing a driving force for ET of ⌬G ϭ Ϫ0.62 eV given that the energy of the 0-0 transition of S 0 3 S 1 is Ϸ 2.35 eV (1 eV ϭ 1.602 ϫ 10 Ϫ19 J) (27).…”

Section: Discussionmentioning

confidence: 99%

Hydrogen-bond switching through a radical pair mechanism in a flavin-binding photoreceptor

Gauden

Stokkum²,

Key³

et al. 2006

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

215

432

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BLUF (blue light sensing using FAD) domains constitute a recently discovered class of photoreceptor proteins found in bacteria and eukaryotic algae, where they control a range of physiological responses including photosynthesis gene expression, photophobia, and negative phototaxis. Other than in well known photoreceptors such as the rhodopsins and phytochromes, BLUF domains are sensitive to light through an oxidized flavin rather than an isomerizable cofactor. To understand the physicochemical basis of BLUF domain photoactivation, we have applied femtosecond transient absorption spectroscopy to the Slr1694 BLUF domain of Synechocystis PCC6803. We show that photoactivation of BLUF domains proceeds by means of a radical-pair mechanism, driven by electron and proton transfer from the protein to the flavin, resulting in the transient formation of anionic and neutral flavin radical species that finally result in the long-lived signaling state on a 100-ps timescale. A pronounced deuteration effect is observed on the lifetimes of the intermediate radical species, indicating that proton movements underlie their molecular transformations. We propose a photoactivation mechanism that involves a successive rupture of hydrogen bonds between a conserved tyrosine and glutamine by light-induced electron transfer from tyrosine to flavin and between the glutamine and flavin by subsequent protonation at flavin N5. These events allow a reorientation of the conserved glutamine, resulting in a switching of the hydrogen-bond network connecting the chromophore to the protein, followed by radicalpair recombination, which locks the glutamine in place. It is suggested that the redox potential of flavin generally defines the light sensitivity of flavin-binding photoreceptors.flavoprotein ͉ light sensing ͉ photochemistry ͉ reaction mechanism ͉ spectroscopy

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“…Interestingly, reduction of N 3 (formed by pulse radiolysis) by the dipeptide TrpTyr has been reported to lead to the formation of Trp • radicals followed by oxidation of the Tyr residue by Trp • to yield Tyr • radicals. 20 In the present case, the nature of the attack sites may depend on initial recognition of Pt IV azido complex 1 and differ from those of azidyl radicals generated by other methods.…”

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confidence: 84%

“…16 19 The redox potentials of both L-Trp and L-Tyr show a marked pH dependence. 20 Surprisingly, the presence of L-Tyr (up to 1 mM) had little effect on the trapping of azidyl radicals by DMPO (see the SI). However, L-Trp was effective in suppressing radical trapping, even at low physiologically relevant concentrations.…”

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confidence: 98%

Tryptophan Switch for a Photoactivated Platinum Anticancer Complex

Butler¹,

Woods

Farrer³

et al. 2012

J. Am. Chem. Soc.

110

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Copies of full items can be used for personal research or study, educational, or not-forprofit purposes without prior permission or charge. Provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. (1) is potently cytotoxic to cancer cells when irradiated with visible (blue) light. We show that the acute photocytotoxicity can be switched off by low doses (500 μM) of the amino acid L-tryptophan. EPR and NMR spectroscopic experiments using spin traps show that L-Trp quenches the formation of azidyl radicals, probably by acting as an electron donor. L-Trp is wellknown as a mediator of electron transfer between distant electron acceptor/donor centers in proteins, and such properties may make the free amino acid clinically useful for controlling the activity of photochemotherapeutic azido Pt IV drugs. Since previous work has demonstrated the ability of photoactivated 1 to platinate DNA, this suggests that the high potency of such photoactive platinum complexes is related to their dual attack on cancer cells by radicals and Pt II photoproducts.

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Further comments on the redox potentials of tryptophan and tyrosine

Cited by 415 publications

References 0 publications

Mimicking the electron transfer chain in photosystem II with a molecular triad thermodynamically capable of water oxidation

Mimicking the electron transfer chain in photosystem II with a molecular triad thermodynamically capable of water oxidation

Hydrogen-bond switching through a radical pair mechanism in a flavin-binding photoreceptor

Tryptophan Switch for a Photoactivated Platinum Anticancer Complex

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