Sequential Electron Transfer Research Articles

Probing Phenazine Electron Transfer and Retention in Pseudomonas Aeruginosa Biofilms

The opportunistic pathogen Pseudomonas aeruginosa forms metabolically stratified biofilms. Redox-active phenazine metabolites shuttle metabolic electrons from the oxygen depleted biofilm interior to the oxygen saturated exterior, facilitating anoxic survival. We are testing the hypothesis that phenazine retention and/or charge transfer through the biofilm matrix are mediated by extracellular DNA (eDNA), which is the most abundant polymer in the P. aeruginosa matrix and is known to provide structural integrity to the biofilm. Specifically, we are growing biofilms on electrode arrays and performing in vitro studies of phenazine interactions with DNA. Biofilm experiments show that conductivity through the matrix is mediated by sequential electron transfer between phenazine molecules. Transferring electrode grown biofilms to fresh liquid medium demonstrates that some phenazines are retained tightly in the biofilm matrix, while others rapidly diffuse away. The retention of different phenazines corresponds to their affinity for DNA in vitro, consistent with the idea that some phenazines might bind the eDNA in the biofilm. We are testing whether phenazines bound to DNA can also mediate charge transfer. Collectively, these experiments are helping us better understand how P. aeruginosa utilizes its self-produced extracellular electron shuttles to survive biofilm oxidant limitation, and are opening up new perspectives on the role(s) that eDNA may play in the biofilm matrix.

Theoretical studies of the second step of the nitric oxide synthase reaction: Electron tunneling prevents uncoupling

Nitric oxide (NO·) is a messenger molecule with diverse physiological roles including host defense, neurotransmission and vascular function. The synthesis of NO· from l-arginine is catalyzed by NO-synthases and occurs in two steps through the intermediary Nω-hydroxy-l-arginine (NHA). In both steps the P450-like reaction cycle is coupled with the redox cycle of the cofactor tetrahydrobiopterin (H4B). The mechanism of the second step is studied by Density Functional Theory calculations to ascertain the canonical sequence of proton and electron transfer (PT and ET) events. The proposed mechanism is controlled by the interplay of two electron donors, H4B and NHA. Consistent with experimental data, the catalytic cycle proceeds through the ferric-hydroperoxide complex (Cpd 0) and the following aqua-ferriheme resting state, and involves interim partial oxidation of H4B. The mechanism starts with formation of Cpd 0 from the ferrous-dioxy reactant complex by PT from the C-ring heme propionate coupled with hole transfer to H4B through the highest occupied π-orbital of NHA as a bridge. This enables PT from NHA+· to the proximal oxygen leading to the shallow ferriheme-H2O2 oxidant. Subsequent Fenton-like peroxide bond cleavage triggered by ET from the NHA-derived iminoxy-radical leads to the protonated Cpd II diradicaloid singlet stabilized by spin delocalization in H4B, and the closed-shell coordination complex of HO− with iminoxy-cation. The complex is converted to the transient C-adduct, which releases intended products upon PT to the ferriheme-HO− complex coupled with ET to the H4B+·. Deferred ET from the substrate or undue ET from/to the cofactor leads to side products.

Theoretical studies on anti-oxidant potential of alpinetin

Alpinetin is a natural flavanone which is found abundantly in ginger family and is a well-known traditional Chinese medicinal ingredient. It exhibits broad spectrum of biological properties like anti-inflammatory effect, anti-hemostasis, anti-microbial effects etc. In this work we have attempted to perform a Density functional theory calculation to reveal the electronic and structural information of alpinetin. Several anti-oxidant descriptors like hardness, softness, electronegativity, electrophilicity index, chemical potential etc. were calculated as part of this. From the value of these quantum chemical descriptors, it could be concluded that alpinetin can have significant anti-oxidant activity. Mechanism of antioxidant action was also theoretically studied. It shows that the molecule prefers hydrogen atom transfer mechanism in vaccuo and in non-polar solution but in polar solution it follows sequential proton electron transfer mechanism. Thus the potentiality of alpinetin as an antioxidant molecule is revealed.

Peptidomic Identification of Cysteine-Rich Peptides from Plants.

Plant cysteine-rich peptides (CRPs) constitute a majority of plant-derived peptides with high molecular diversity. This protocol describes a rapid and efficient peptidomic approach to identify a whole spectrum of CRPs in a plant extract and decipher their molecular diversity and bioprocessing mechanism. Cyclotides from C. ternatea are used as the model CRPs to demonstrate our methodology. Cyclotides exist naturally in both cyclic and linear forms, although the linear forms (acyclotide) are generally present at much lower concentrations. Both cyclotides and acyclotides require linearization of their backbone prior to fragmentation and sequencing. A novel and practical three-step chemoenzymatic treatment was developed to linearize and distinguish both forms: (1) N-terminal acetylation that pre-labels the acyclotides; (2) conversion of Cys into pseudo-Lys through aziridine-mediated S-alkylation to reduce disulfide bonds and to increase the net charge of peptides; and (3) opening of cyclic backbones by the novel asparaginyl endopeptidase butelase 2 that cleaves at the native bioprocessing site. The treated peptides are subsequently analyzed by liquid chromatography coupled to mass spectrometry using electron transfer dissociation fragmentation and sequences are identified by matching the MS/MS spectra directly with the transcriptomic database.

Sequential electron transfer governs the UV-induced self-repair of DNA photolesions

Cyclobutane pyrimidine dimers (CpDs) are among the most common DNA lesions occurring due to the interaction with ultraviolet light. While photolyases have been well known as external factors repairing CpDs, the intrinsic self-repairing capabilities of the GAT[double bond, length as m-dash]T DNA sequence were discovered only recently and are still largely obscure. Here, we elucidate the mechanistic details of this self-repair process by means of MD simulations and QM/MM computations involving the algebraic diagrammatic construction to the second order [ADC(2)] method. We show that local UV-excitation of guanine may be followed by up to three subsequent electron transfers, which may eventually enable efficient CpD ring opening when the negative charge resides on the T[double bond, length as m-dash]T dimer. Consequently, the molecular mechanism of GAT[double bond, length as m-dash]T self-repair can be envisaged as sequential electron transfer (SET) occurring downhill along the slope of the S1 potential energy surface. Even though the general features of the SET mechanism are retained in both of the studied stacked conformers, our optimizations of different S1/S0 state crossings revealed minor differences which could influence their self-repair efficiencies. We expect that such assessment of the availability and efficiency of the SET process in other DNA oligomers could hint towards other sequences exhibiting similar photochemical properties. Such explorations will be particularly fascinating in the context of the origins of biomolecules on Earth, owing to the lack of external repairing factors in the Archean age.

Long-lived Single Excitons, Trions, and Biexcitons in CdSe/CdTe Type-II Colloidal Quantum Wells

Light-harvesters with long-lived excited states are desired for efficient solar energy conversion systems. Many solar-to-fuel conversion reactions, such as H2 evolution and CO2 reduction, require multiple sequential electron transfer processes, which leads to a complicated situation that excited states involves not only excitons (electron-hole pairs) but also multi-excitons and charged excitons. While long-lived excitons can be obtained in various systems (e.g., semiconductor nanocrystals), multi-excitons and charged excitons are typically shorted-lived due to nonradiative Auger recombination pathways whereby the recombination energy of an exciton is quickly transferred to the third carrier on a few to hundreds of picoseconds timescale. In this work, we report a study of excitons, trions (an exciton plus an additional charge), and biexcitons in CdSe/CdTe colloidal quantum wells or nanoplatelets. The type-II band alignment effectively separates electrons and holes in space, leading to a single exciton lifetime of 340 ns which is ∼2 order of magnitudes longer than that in plane CdSe nanoplatelets. More importantly, the electron-hole separation also dramatically slows down Auger decay, giving rise to a trion lifetime of 70 ns and a biexciton lifetime of 11 ns, among the longest values ever reported for colloidal nanocrystals. The long-lived exciton, trion, and biexciton states, combined with the intrinsically strong light-absorption capability of two-dimensional systems, enable the CdSe/CdTe type-II nanoplatelets as promising light harvesters for efficient solar-to-fuel conversion reactions.

Computational investigation of the structure and antioxidant activity of some pyrazole and pyrazolone derivatives

Pyrazoles and pyrazolones constitute a group of organic compounds that have various medical applications such as antimicrobial, antipyretic, anti-inflammatory, antitumor and antioxidants. Pyrazolones can exist in different isomeric forms (CH, NH, OH) due to keto-enol, lactam-lactim and imine-enamine tautomerism. Determination of the most stable tautomeric form is thus important for understanding their biological roles at the molecular level. We performed a theoretical investigation of the structural and antioxidant properties of three synthetic pyrazolones (1–3), one synthetic pyrazole (4), one natural pyrazole (5) and two engineered hydroxyl derivatives of 1 (7, 8) and of 5 (9, 10) using the density functional theory at the B3LYP/6-311++G(d,p) level of theory in gas phase and in methanol (using the polarizable continuum model). It is found that substituents and solvents may influence the relative stability of pyrazolone isomers and that the CH tautomer is typically the least stable. Vertical ionization potentials, vertical electron affinities and X–H bond dissociation energies (X = C, N, O, S) are calculated for the global minimum structures and compared with those of the standard antioxidant flavonoid quercetin (6). Calculations predict that compounds 1 and 5 have antioxidant activity similar to 6 and that their mono and dihydroxyl derivatives (7–10) are more efficient antioxidants. Results also indicate that compounds 1–10 preferably interact with free radicals adopting the H atom transfer rather than the sequential electron transfer proton transfer mechanism. The study gives insight into the structural requirements for the design of highly efficient antioxidants.

Conformationally Dynamic Radical Transfer within Ribonucleotide Reductase.

Ribonucleotide reductases (RNR) catalyze the reduction of nucleotides to deoxynucleotides through a mechanism involving an essential cysteine based thiyl radical. In the E.coli class 1a RNR the thiyl radical (C439•) is a transient species generated by radical transfer (RT) from a stable diferric-tyrosyl radical cofactor located >35 Å away across the α2:β2 subunit interface. RT is facilitated by sequential proton-coupled electron transfer (PCET) steps along a pathway of redox active amino acids (Y122β ↔ [W48β?] ↔ Y356β ↔ Y731α ↔ Y730α ↔ C439α). The mutant R411A(α) disrupts the H-bonding environment and conformation of Y731, ostensibly breaking the RT pathway in α2. However, the R411A protein retains significant enzymatic activity, suggesting Y731 is conformationally dynamic on the time scale of turnover. Installation of the radical trap 3-amino tyrosine (NH2Y) by amber codon suppression at positions Y731 or Y730 and investigation of the NH2Y• trapped state in the active α2:β2 complex by HYSCORE spectroscopy validate that the perturbed conformation of Y731 in R411A-α2 is dynamic, reforming the H-bond between Y731 and Y730 to allow RT to propagate to Y730. Kinetic studies facilitated by photochemical radical generation reveal that Y731 changes conformation on the ns-μs time scale, significantly faster than the enzymatic kcat. Furthermore, the kinetics of RT across the subunit interface were directly assessed for the first time, demonstrating conformationally dependent RT rates that increase from 0.6 to 1.6 × 104 s-1 when comparing wild type to R411A-α2, respectively. These results illustrate the role of conformational flexibility in modulating RT kinetics by targeting the PCET pathway of radical transport.

Molecular-Level Insights into Oxygen Reduction Catalysis by Graphite-Conjugated Active Sites

Using a combination of experimental and computational investigations, we assemble a consistent mechanistic model for the oxygen reduction reaction (ORR) at molecularly well-defined graphite-conjugated catalyst (GCC) active sites featuring aryl-pyridinium moieties (N+-GCC). ORR catalysis at glassy carbon surfaces modified with N+-GCC fragments displays near-first-order dependence in O2 partial pressure and near-zero-order dependence on electrolyte pH. Tafel analysis suggests an equilibrium one-electron transfer process followed by a rate-limiting chemical step at modest overpotentials that transitions to a rate-limiting electron transfer sequence at higher overpotentials. Finite-cluster computational modeling of the N+-GCC active site reveals preferential O2 adsorption at electrophilic carbons alpha to the pyridinium moiety. Together, the experimental and computational data indicate that ORR proceeds via a proton-decoupled O2 activation sequence involving either concerted or stepwise electron transfer and ...

Identification of flavanones from Boesenbergia rotunda as potential antioxidants and monoamine oxidase B inhibitors

Boesenbergia rotunda is a ginger species with great medicinal importance. In this study, two flavanones found in B. rotunda namely pinocembrin and pinostrobin are analyzed for their antioxidant activity and monoamine oxidase inhibition. Frontier molecular orbitals and molecular electrostatic potential were analyzed to understand the electronic properties of these molecules. Quantum chemical descriptors gave an indication that these are potential antioxidant molecules. Mechanism of antioxidant activity was explored using Density Functional Theory (DFT) and was found to be hydrogen atom transfer (HAT) in the gaseous state and in non-polar medium and sequential proton loss and electron transfer (SPLET) in polar medium. Monoamine oxidase B (MAOB) is an important target in the treatment of Parkinson’s disease. Molecular docking studies conducted on these molecules show that they can dock well into the active site monoamine oxidase B. These molecules playing dual role as antioxidants and monoamine oxidase B inhibitors can be considered as good candidates for the treatment of Parkinson’s disease.

Characterization of photo-induced electron and hole transfer in a porphyrin based ambipolar organic molecule with cascade energy levels

A porphyrin based ambipolar organic molecule consisting of styrene based triphenylamine derivative (MTPA) as electron donor, s-triazine group (TRC) as electron acceptor 1 and metal-free tetraphenyl porphyrin (HTPP) as electron acceptor 2 was synthesized and characterized using computational methods and electrochemical and spectroscopic measurements. The kinetics analysis indicates that the photo-induced charge-separated states, MTPA.+-TRC-HTPP.-, were generated by sequential electron transfers from MTPA to TRC then to HTPP and/or a direct hole transfer from HTPP to MTPA. In toluene, the charge-separated states were formed with 54% through electron transfer and 46% through hole transfer once MTPA moiety was excited. However, in dichloromethane, they were formed with 75% through electron transfer and 25% through hole transfer. Furthermore, more charge-separated species were generated in dichloromethane than in toluene.

Geometries and Dynamics of Photoinduced Electron-Hole Pairs in Polyhexylthiophene-Fullerene Systems

For developments of the organic light-energy conversion systems, it is essential to achieve efficient long-range charge-separations (CS). Light-induced electrons and holes generated by the electron-transfer (ET) reactions are thus required to escape from the electrostatic binding at the initial stage. In the natural photosynthetic reaction centers, the efficient distant CS is accomplished with ~100% efficiency by cascading the redox states by the sequential ET processes to a series of cofactors, leading to the light-to-chemical energy conversion. In contrast, the photoinduced CS at the interfaces of organic photovoltaic (OPV) cells generates electron-hole pairs, eventually achieving light-to-electricity conversion after conductions of the electron and the hole through the donor (D) and the acceptor (A) domains, respectively. However, the D/A interfaces in the OPV cells often suffer from the charge-recombination (CR) at the early stage, leading to losses of the input photon energies. Concerning the mechanism of the efficient photo-career generations in the OPV systems, role of the hot charge-transfer (CT) excitons is currently under intense debate. A recent ultrafast spectroscopic study of a polymer/fullerene (PCPDTBT/PC60BM) OPV films clearly showed that the vibrationally unrelaxed electron-hole pairs (PCPDTBT+/PC60BM-) are generated in < 50 fs before time scales of the internal conversions (IC) of the polymer excited states.[1] On the other hand, Vandewal et al.[2] have demonstrated for the several polymer:PCBM solar cells that the internal quantum efficiency (IQE) is essentially independent of the excitation energy of the lights; even if the low-energy CT band is excited, the IQE is higher than 90 % for some blend films, giving rise to very high OPV performances. This result strongly indicates that the hot CT states are not necessarily required to produce the highly separated photo-careers. To elucidate why and how the photo-carriers escape from the Coulomb binding, it is important to directly observe locations and orientations of the intermediate charges just after the CSs. Time-resolved electron paramagnetic resonance (TREPR) and pulsed EPR methods have been powerful to obtain the electron spin-spin dipolar interaction and the spin-spin exchange coupling in the photoinduced CS states and thus have been utilized to characterize the interspin distances and the electronic couplings of the transient CS states. The TREPR analyses have been useful to obtain the CS state geometries for several systems, since the electron spin polarization (ESP) as the microwave absorption (A) and the emission (E) is sensitive to the molecular conformations.[3] Recent spin polarization analyses have revealed representative geometries of the transient electron-hole pairs in the OPV blends.[4] However, the highly inhomogeneous molecular environments at the bulk-heteojunction (BHJ) interfaces induces the large special distribution in the solid-state OPV materials and would prevent us from understanding the mechanism of the charge conductions. To unveil fundamental molecular mechanisms of the light-induced charge conductions, it is highly desired to investigate the structure and dynamics of the initial photo-carriers generated in a conjugated polymer-backbone in which the intramolecular orientation and the electronic interaction are well-defined at the initial CS stage. Thus far, no study has characterized the geometry, electronic coupling and charge-conduction dynamics of the photo-carriers in a covalently linked polymer system, although the polymer-fullerene dyads were synthetized and investigated by transient spectroscopies. In this respect, we have synthesized a new series of donor-bridge-acceptor (P3HT-Phm-C60, m = 0, 1, 2, 3) linked polymers composed of regio-regular poly(3-hexylthiophene) (P3HT) as the electron donor, ful-leropyrrolidine (C60) as the acceptor and oligo-p-phenylene bridges as the spacers.[5] We have herein investigated the molecular geometries and the hole-conduction dynamics of the photoinduced intramolecular CS states by using the TREPR in tetrahydrofuran (THF) at room temperature. We clearly show that, without going through the hot CT, localized holes are generated in the conjugated polymer-backbones nearby the phenylene units by quenching triplet excitons and then undergo one-dimensional intramolecular dissocia-tions from the electrons situated at C60 in the dyads. Grancini, G.; Maiuri, M.; Fazzi, D.; Petrozza, A.; Egelhaaf, H. J.; Brida, D.; Cerullo, G.; Lanzani, G. Nat. Mater. 2013, 12, 29-33.Vandewal, K.; Albrecht, S.; Hoke, E. T.; Graham, K. R.; Widmer, J.; Douglas, J. D.; Schubert, M.; Mateker, W. R.; Bloking, J. T.; Burkhard, G. F.; Sellinger, A.; Frechet, J. M. J.; Amassian, A.; Riede, M. K.; McGehee, M. D.; Neher, D.; Salleo, A. Nat. Mater. 2014, 13, 63-68.Kobori, Y.; Miura, T. J. Phys. Chem. Lett. 2015, 6, 113-123.Kobori, Y.; Noji, R.; Tsuganezawa, S. J. Phys. Chem. C 2013, 117, 1589-1599.Miura, T.; Tao, R.; Shibata, S.; Umeyama, T.; Tachikawa, T.; Imahori, H.; Kobori, Y. J . Am. Chem. Soc. 2016, 138, 5879-5885.

(Invited) Axially Assembled Photosynthetic Antenna-Reaction Center Mimics

A series of porphyrin based donor-acceptor conjugates in which the arrangement of various components are in perpendicular orientation to the porphyrin plane have been constructed and used to study the pathways of electron and energy transfer processes. The employed aluminum(III) porphyrin in these studies is found to be synthetically more advantageous as it provides reaction site on both faces of the porphyrin plane. The axial hydroxyl group on one side of the aluminum(III) porphyrin plane is able to form a covalent ester or ether bond with an energy harvester or electron acceptor while Lewis bases such as pyridine or an imidazole group form a metal-ligand axial coordination on other side of the plane hosting an electron donor or acceptor. The advantage of this configuration of the conjugates is that the donor and acceptor sides of the complex are located on opposite faces of the porphyrin which minimizes the rate of charge recombination. Moreover, the excited state \U0001d70b* orbital of porphyrin and axially coordinated ligand lie in the same space which causes an increase in the electronic coupling thus facilitating the electron/energy transfer processes. The distance dependence of sequential electron transfer in vertical, linear supramolecular triads constructed using tetrathiafulvalene (TTF), aluminum(III) porphyrin (AlPor) and fullerene (C60) entities will be discussed. Further, results of photoinduced energy transfer followed by electron transfer in antenna-reaction center mimicking BODIPY-AlPor-C60 supramolecular triads will be highlighted. Figure 1

Intramolecular Light-Driven Accumulation of Reduction Equivalents by Proton-Coupled Electron Transfer.

The photochemistry of a molecular pentad composed of a central anthraquinone (AQ) acceptor flanked by two Ru(bpy)32+ photosensitizers and two peripheral triarylamine (TAA) donors was investigated by transient IR and UV-vis spectroscopies in the presence of 0.2 M p-toluenesulfonic acid (TsOH) in deaerated acetonitrile. In ∼15% of all excited pentad molecules, AQ is converted to its hydroquinone form (AQH2) via reversible intramolecular electron transfer from the two TAA units (τ = 65 ps), followed by intermolecular proton transfer from TsOH (τ ≈ 3 ns for the first step). Although the light-driven accumulation of reduction equivalents occurs through a sequence of electron and proton transfer steps, the resulting photoproduct decays via concerted PCET (τ = 4.7 μs) with an H/D kinetic isotope effect of 1.4 ± 0.2. Moreover, the reoxidation of AQH2 seems to take place via a double electron transfer step involving both TAA+ units rather than sequential single electron transfer events. Thus, the overall charge-recombination reaction seems to involve a concerted proton-coupled two-electron oxidation of AQH2. The comparison of experimental data obtained in neat acetonitrile with data from acidic solutions suggests that the inverted driving-force effect can play a crucial role for obtaining long-lived photoproducts resulting from multiphoton, multielectron processes. Our pentad provides the first example of light-driven accumulation of reduction equivalents stabilized by PCET in artificial molecular systems without sacrificial reagents. Our study provides fundamental insight into how light-driven multielectron redox chemistry, for example the reduction of CO2 or the oxidation of H2O, can potentially be performed without sacrificial reagents.

Sequential Proton-Coupled Electron Transfer Mediates Excited-State Deactivation of a Eumelanin Building Block.

Skin photoprotection is commonly believed to rely on the photochemistry of 5,6-dihydroxyindole (DHI)- and 5,6-dihydroxyindole-2-carboxylic acid (DHICA)-based eumelanin building blocks. Attempts to elucidate the underlying excited-state relaxation mechanisms have been partly unsuccessful due to the marked instability to oxidation. We report a study of the excited-state deactivation of DHI using steady-state and time-resolved fluorescence accompanied by high-level quantum-chemistry calculations including solvent effects. Spectroscopic data show that deactivation of the lowest excited state of DHI in aqueous buffer proceeds on the 100 ps time scale and is 20 times faster than in methanol. Quantum-chemical calculations reveal that the excited-state decay mechanism is a sequential proton-coupled electron transfer, which involves the initial formation of a solvated electron from DHI, followed by the transfer of a proton to the solvent. This unexpected finding would prompt a revision of current notions about eumelanin photophysics and photobiology.

Time-domain shape of electron spin echo signal of spin-correlated radical pairs in polymer/fullerene blends

Temporal shape of electron spin echo (ESE) signal of photoinduced spin-correlated radical pairs (SCRP) in composite of conductive polymer P3HT and substituted fullerene PCBM is studied in details. ESE signals of radical pairs (RP) P3HT+/PCBM− are calculated in realistic model, taking into account finite microwave pulse length. Inhomogeneous broadening of resonant lines and interradical distance distribution are included. Experimentally observed ESE time-domain shape was found to contradict predictions of conventional SCRP theory, which would be valid in the case of very fast electron transfer. Thus, instantaneous formation of singlet SCRP is not the case for P3HT+/PCBM− pair, and spin system has enough time to evolve coherently during sequential electron transfer. While it is impossible to reproduce experimental data within simple singlet SCRP model, assumption of presence of additional – with respect to what is predicted by singlet SCRP theory – AE (absorption/emission) spin polarization gives convincing accordance with the experiment. Density matrix of RP P3HT+/PCBM− is a superposition of two contributions, namely the parts reflecting (i) antiphase polarization of original singlet-born SCRP and (ii) additional AE-polarization which is generated during initial stage of charge separation. AE-polarization affects experimental ESEEM (electron spin echo envelope modulation) traces, as well as ESE shape, making impossible their interpretation via simple singlet SCRP model. However, this effect can be eliminated by averaging of ESEEM traces over EPR spectral positions. Finally, choosing the optimal gate for ESE time-domain integration and proper microwave detection phase tuning are considered.

The role of acid-base equilibria in formal hydrogen transfer reactions: tryptophan radical repair by uric acid as a paradigmatic case.

The results presented in this work demonstrate the high complexity of chemical reactions involving species with multiple acid-base equilibria. For the case study investigated here, it was necessary to consider two radical species for tryptophan (Trp(-H)˙ and Trp˙+) and three fractions for uric acid (H3Ur, H2Ur- and HUr2-) in order to properly reproduce the experimental results. At pH = 7.4, two main reaction mechanisms were identified: proton-electron sequential transfer (PEST) and sequential proton gain-electron transfer (SPGET). Combined, they account for more than 99% of the overall reaction, despite the fact that they involve minor species, i.e., H3Ur and Trp˙+, respectively. The excellent agreement between the calculated overall rate constant and the experimental value seems to support this proposal. In addition, if only the dominant species at pH = 7.4 (H2Ur- and Trp(-H)˙) were considered, there would be a large discrepancy with the experimental value (about 4 orders of magnitude), which also supports the finding that the key species in this case are not the most abundant ones. The influence of the pH on the kinetics of the investigated reaction was explored. It was found that the maximum repairing ability of uric acid does not occur at physiological pH, but at a more acidic pH (pH = 5.0).

Kinetics of the outer-sphere oxidation of thiourea by heteropoly-α2-17-tungsto-1-vanadodiphosphate anion

Kinetics of the oxidation of thiourea (tu) by heteropoly-α2-17-tungsto-1-vanadodiphosphate anion, α2-[P2VVW17O62]7−, have been studied spectrophotometrically in aqueous acidic medium at 25 °C. At low pH (2.4–3.0), the neutral form of tu is the only reactive species. At higher pH (4.2–4.9), both neutral and deprotonated forms of tu participate in the reaction. The observed mixed-order kinetics suggest two parallel reactions: one in which the order in [tu] is unity, and a second in which it is two. In both cases, the order in [α2-[P2VVW17O62]7−] is unity. Based on the kinetic studies, a mechanism is proposed, in which a second-order proton-coupled electron transfer involving NH2CSNH2 and α2-[P2VVW17O62]7− proceeds through a sequential electron transfer, followed by proton transfer such that the reaction is an “activation-controlled” outer-sphere electron transfer process. By applying the Marcus equation, the self-exchange rate constants for the couples $${\text{NH}}_{2} {\text{CSNH}}_{2}^{ \cdot + }$$ /NH2CSNH2 and α2-[P2VVW17O62]7−/α2-[P2VIVW17O62]8− were evaluated.

Sequential and Coupled Proton and Electron Transfer Events in the S2 → S3 Transition of Photosynthetic Water Oxidation Revealed by Time-Resolved X-ray Absorption Spectroscopy.

The choreography of electron transfer (ET) and proton transfer (PT) in the S-state cycle at the manganese-calcium (Mn4Ca) complex of photosystem II (PSII) is pivotal for the mechanism of photosynthetic water oxidation. Time-resolved room-temperature X-ray absorption spectroscopy (XAS) at the Mn K-edge was employed to determine the kinetic isotope effect (KIE = τD2O/τH2O) of the four S transitions in a PSII membrane particle preparation in H2O and D2O buffers. We found a small KIE (1.2-1.4) for manganese oxidation by ET from Mn4Ca to the tyrosine radical (YZ•+) in the S0n → S1+ and S1n → S2+ transitions and for manganese reduction by ET from substrate water to manganese ions in the O2-evolving S3n → S0n step, but a larger KIE (∼1.8) for manganese oxidation in the S2n → S3+ step (subscript, number of accumulated oxidizing equivalents; superscript, charge of Mn4Ca). Kinetic lag phases detected in the XAS transients prior to the respective ET steps were assigned to S3+ → S3n (∼150 μs, H2O; ∼380 μs, D2O) and S2+ → S2n (∼25 μs, H2O; ∼120 μs, D2O) steps and attributed to PT events according to their comparatively large KIE (∼2.4, ∼4.5). Our results suggest that proton movements and molecular rearrangements within the hydrogen-bonded network involving Mn4Ca and its bound (substrate) water ligands and the surrounding amino acid/water matrix govern to different extents the rates of all ET steps but affect particularly strongly the S2n → S3+ transition, assigned as proton-coupled electron transfer. Observation of a lag phase in the classical S2 → S3 transition verifies that the associated PT is a prerequisite for subsequent ET, which completes Mn4Ca oxidation to the all-Mn(IV) level.

Visible Light Sensitized Production of Hydroxyl Radicals Using Fullerol as an Electron-Transfer Mediator.

Fullerenes and their derivatives are known to photosensitize the production of singlet oxygen (1O2), but their role in generating hydroxyl radical (•OH) under visible light has not been reported. Here, we demonstrate that fullerol can mediate the electron transfer from Rhodamine B dye to O2 under visible light irradiation, achieving simultaneous dye decolorization and •OH-induced degradation of 4-chlorophenol. The hydroxyl radical is proposed to be produced via a consecutive reduction of molecular oxygen by fullerol anion radical, which is formed through the electron transfer from the dye to the triplet state of fullerol. Mechanistic investigations using various probe reagents such as superoxide dismutase (superoxide quencher), t-butanol (•OH quencher), and coumarin (•OH probe) provided indirect evidence for the generation of •OH under visible light. Furthermore, spin trapping technique directly detected the oxidizing species such as •OH, HO2•, and 1O2 in the visible light irradiated solution of RhB/fullerol mixture. It was proposed that the photochemical oxidation mechanism depends on pH: •OH production is favored at acidic pH through fullerol-mediated sequential electron transfer while 1O2 is generated as a main oxidant at neutral and alkaline condition through the energy-transfer process. Therefore, the photochemical oxidation can be switchable between •OH-driven and 1O2-driven mechanism by a simple pH adjustment.