Modeling the Weak pH Dependence of Proton-Coupled Electron Transfer for Tryptophan Derivatives

Cui, Kai; Soudackov, Alexander V.; Hammes-Schiffer, Sharon

doi:10.1021/acs.jpclett.3c02282

Cited by 4 publications

(3 citation statements)

References 27 publications

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“…It bears clear resemblance in its structure and potential chemical reactivity to natural auxins, the plant growth-enhancing hormones, 34 and has been associated with strong bioactivity, 35 making CeO 2 and other potentially oxidative oxide NPs attractive candidates for the in situ nanozyme production of auxins from TRP. It is interesting to note that the formation of PIC as a single primary oxidation product of TRP, via the reaction mechanism resembling that presented by this work, was actually observed in rather small quantities on the photochemical oxidation of TRP in the polarized magnetic field 36 , 37 or simply by the singlet molecular oxygen generated photochemically in the presence of specific dyes. 38 In the reaction of TRP with oxidative NPs in this work, the PIC species are generated under ambient conditions, without special physical excitation and in darkness.…”

Section: Resultssupporting

confidence: 66%

Molecular Mechanisms in Metal Oxide Nanoparticle–Tryptophan Interactions

Nefedova,

Svensson,

Vanetsev

et al. 2024

Inorg. Chem.

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One of the crucial metabolic processes for both plant and animal kingdoms is the oxidation of the amino acid tryptophan (TRP) that regulates plant growth and controls hunger and sleeping patterns in animals. Here, we report revolutionary insights into how this process can be crucially affected by interactions with metal oxide nanoparticles (NPs), creating a toolbox for a plethora of important biomedical and agricultural applications. Molecular mechanisms in TRP− NP interactions were revealed by NMR and optical spectroscopy for ceria and titania and by X-ray single-crystal study and a computational study of model TRP−polyoxometalate complexes, which permitted the visualization of the oxidation mechanism at an atomic level. Nanozyme activity, involving concerted proton and electron transfer to the NP surface for oxides with a high oxidative potential, like CeO 2 or WO 3 , converted TRP in the first step into a tricyclic organic acid belonging to the family of natural plant hormones, auxins. TiO 2 , a much poorer oxidant, was strongly binding TRP without concurrent oxidation in the dark but oxidized it nonspecifically via the release of reactive oxygen species (ROS) in daylight.

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Section: Resultssupporting

confidence: 66%

Molecular Mechanisms in Metal Oxide Nanoparticle–Tryptophan Interactions

Nefedova,

Svensson,

Vanetsev

et al. 2024

Inorg. Chem.

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“…However, the zone diagram was generated with a kinetic model corresponding to the system with the insulating layers, where PT is significantly faster than ET and therefore may be assumed to be at equilibrium. The kinetics of the system without the insulating layers can be analyzed by numerically integrating the rate equations . Such an analysis is also consistent with a sequential ET–PT mechanism for the range of potentials and pH corresponding to the black triangles in Figure .…”

supporting

confidence: 57%

Direct Evidence for a Sequential Electron Transfer–Proton Transfer Mechanism in the PCET Reduction of a Metal Hydroxide Catalyst

Kessinger,

Xu,

Cui

et al. 2024

J. Am. Chem. Soc.

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The proton-coupled electron transfer (PCET) mechanism for the reaction M ox −OH + e − + H + → M red −OH 2 was determined through the kinetic resolution of the independent electron transfer (ET) and proton transfer (PT) steps. The reaction of interest was triggered by visible light excitation of [Ru II (tpy)(bpy′)H 2 O] 2+ , Ru II −OH 2 , where tpy is 2,2′:6′,2″-terpyridine and bpy′ is 4,4′-diaminopropylsilatrane-2,2′-bipyridine, anchored to In 2 O 3 :Sn (ITO) thin films in aqueous solutions. Interfacial kinetics for the PCET reduction reaction were quantified by nanosecond transient absorption spectroscopy as a function of solution pH and applied potential. Data acquired at pH = 5−10 revealed a stepwise electron transfer−proton transfer (ET−PT) mechanism, while kinetic measurements made below pK a (Ru III −OH/OH 2 ) = 1.3 were used to study the analogous interfacial reaction, where electron transfer was the only mechanistic step. Analysis of this data with a recently reported multichannel kinetic model was used to construct a PCET zone diagram and supported the assignment of an ET−PT mechanism at pH = 5−10. Ultimately, this study represents a unique example among M ox −OH/M red −OH 2 reactivity where the protonation and oxidation states of the intermediate were kinetically and spectrally resolved to firmly establish the PCET mechanism.

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“…Buffer species have been excluded by showing that the PCET rate constants for Y32, 2MP-C32, and 4MP-C32 oxidation are independent of the concentration of the phosphate buffer used in the TA experiments. , Regarding OH – as a possible acceptor, kinetic modeling of photoinduced PCET from a solvated tryptophan derivative has indicated that the significantly higher p K a (H 2 O/OH – ) of 14 compared to p K a (H 3 O + /H 2 O) of 0 could make OH – a viable proton acceptor for the tryptophan derivative, which has a p K a (indole/indolate) of ∼17, , even at neutral pH. Despite the relatively low concentration of OH – at neutral pH, the significantly greater equilibrium constant for forming a hydrogen bond with tryptophan and the higher PCET rate constant for OH – compared to water could create a competition between OH – and H 2 O as proton acceptors upon tryptophan oxidation . In contrast to tryptophan, however, the p K a (H 2 O/OH – ) of 14 is significantly greater than the p K a (phenol/phenolate) values of Y32 (11.3 ± 0.1), 2MP-C32 (9.7 ± 0.2), and 4MP-C32 (9.5 ± 0.1).…”

Section: Introductionmentioning

confidence: 99%

Proton-Coupled Electron Transfer upon Oxidation of Tyrosine in a De Novo Protein: Analysis of Proton Acceptor Candidates

Zhu,

Soudackov,

Tommos

et al. 2024

Biochemistry

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Redox-active residues, such as tyrosine and tryptophan, play important roles in a wide range of biological processes. The α 3 Y de novo protein, which is composed of three α helices and a tyrosine residue Y32, provides a platform for investigating the redox properties of tyrosine in a welldefined protein environment. Herein, the proton-coupled electron transfer (PCET) reaction that occurs upon oxidation of tyrosine in this model protein by a ruthenium photosensitizer is studied by using a vibronically nonadiabatic PCET theory that includes hydrogen tunneling and excited vibronic states. The input quantities to the analytical nonadiabatic rate constant expression, such as the diabatic proton potential energy curves and associated proton vibrational wave functions, reorganization energy, and proton donor−acceptor distribution functions, are obtained from density functional theory calculations on model systems and molecular dynamics simulations of the solvated α 3 Y protein. Two possible proton acceptors, namely, water or a glutamate residue in the protein scaffold, are explored. The PCET rate constant is greater when glutamate is the proton acceptor, mainly due to the more favorable driving force and shorter equilibrium proton donor−acceptor distance, although contributions from excited vibronic states mitigate these effects. Nevertheless, water could be the dominant proton acceptor if its equilibrium constant associated with hydrogen bond formation is significantly greater than that for glutamate. Although these calculations do not definitively identify the proton acceptor for this PCET reaction, they elucidate the conditions under which each proton acceptor can be favored. These insights have implications for tyrosine-based PCET in a wide variety of biochemical processes.

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Modeling the Weak pH Dependence of Proton-Coupled Electron Transfer for Tryptophan Derivatives

Cited by 4 publications

References 27 publications

Molecular Mechanisms in Metal Oxide Nanoparticle–Tryptophan Interactions

Molecular Mechanisms in Metal Oxide Nanoparticle–Tryptophan Interactions

Direct Evidence for a Sequential Electron Transfer–Proton Transfer Mechanism in the PCET Reduction of a Metal Hydroxide Catalyst

Proton-Coupled Electron Transfer upon Oxidation of Tyrosine in a De Novo Protein: Analysis of Proton Acceptor Candidates

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