Proton-coupled Electron Transport Research Articles

Molecular switching by proton-coupled electron transport drives giant negative differential resistance

To develop new types of dynamic molecular devices with atomic-scale control over electronic function, new types of molecular switches are needed with time-dependent switching probabilities. We report such a molecular switch based on proton-coupled electron transfer (PCET) reaction with giant hysteric negative differential resistance (NDR) with peak-to-valley ratios of 120 ± 6.6 and memory on/off ratios of (2.4 ± 0.6) × 103. The switching dynamics probabilities are modulated by bias voltage sweep rate and can also be controlled by pH and relative humidity, confirmed by kinetic isotope effect measurements. The demonstrated dynamical and environment-specific modulation of giant NDR and memory effects provide new opportunities for bioelectronics and artificial neural networks.

Rational design of hollow nitrogen-doped carbon supported nickel nanoparticles for efficient electrocatalytic CO2 reduction

Electrocatalytic carbon dioxide reduction reaction (CO2RR) is a promising approach to achieving a carbon-neutral cycle and producing valuable fuels and chemicals. Herein, we develop a nickel nanoparticle embedded in a hollow nitrogen-doped carbon (Ni@HNC) catalyst with the optimum carbon oxide (CO) Faradaic efficiency of 93.3% at − 0.79 V versus reversible hydrogen electrode (vs. RHE). A series of controlled experiments demonstrated that the PDA-assisted derivatized hollow nitrogen-doped carbon support facilitates the diffusion of CO2 molecules. As proton acceptors, the nickel active sites promote the proton-coupled electron transport process of electroreduction of CO2 to CO. Additionally, Ni@HNC was performed as a bifunctional catalyst, and the coupled anodic methanol oxidation reaction (MOR) can significantly reduce the voltage, about 424 mV at 10 mA cm-2 (versus oxygen evolution reaction, OER). For a two-electrode system, the Faradaic efficiency of CO was maintained above 85% at a wide voltage window (2.1 ∼ 3.1 V), with a maximum value of 98.7% at 2.7 V and a current density of 11.14 mA cm-2. This work provides an efficient strategy to obtain lower CO2RR voltage while acquiring high value-added formate from the anode.

Characterization of mutants deficient in N-terminal phosphorylation of the chloroplast ATP synthase subunit β.

Understanding the regulation of photosynthetic light harvesting and electron transfer is of great importance to efforts to improve the ability of the electron transport chain to supply downstream metabolism. A central regulator of the electron transport chain is ATP synthase, the molecular motor that harnesses the chemiosmotic potential generated from proton-coupled electron transport to synthesize ATP. ATP synthase is regulated both thermodynamically and post-translationally, with proposed phosphorylation sites on multiple subunits. In this study we focused on two N-terminal serines on the catalytic subunit β in tobacco (Nicotiana tabacum), previously proposed to be important for dark inactivation of the complex to avoid ATP hydrolysis at night. Here we show that there is no clear role for phosphorylation in the dark inactivation of ATP synthase. Instead, mutation of one of the two phosphorylated serine residues to aspartate to mimic constitutive phosphorylation strongly decreased ATP synthase abundance. We propose that the loss of N-terminal phosphorylation of ATPβ may be involved in proper ATP synthase accumulation during complex assembly.

Proton coupled electron transport of pH sensitive coumarin based ruthenium(II) complex: A functional mimic of photosystem II

In this work, we have explored a new pH sensitive coumarin based ruthenium(II) complex (R1QC) to mimic the role of P680 in donor side of photosystem II in natural photosynthesis. R1QC underwent protonation at higher H+ ion concentration to form a protonated complex (R1QCH+) which exhibited excited state intramolecular proton transport phenomenon. So as to mimic the proton coupled electron transport reactions those take place in the donor side of PS II after light excitation, the interaction between protonated Ru(II) complex and 2,3,5,6-tetramethyl-1,4-benzoquinone (duroquinone) in 1:1-acetonitrile:water solution at pH 10 was investigated by absorption, emission and electrochemical measurements. In the protonated complex (R1QCH+), Ru2+ core behaves as electron donor whereas coumarin based ligand behaves as proton donor to the quinone. This proton coupled electron transport interaction mimics the proposed reduction of the donor side of PS II.

Proton-Coupled Electron Transport in Two Distinct CYBASC Paralogs of Arabidopsis thaliana: A Comparative Characterization of Highly Conserved Tyrosine and Lysine Residues.

CYBASC proteins are ascorbate (AscH-) reducible, diheme b-containing integral membrane cytochrome b561 proteins (cytb561), which are proposed to be involved in AscH- recycling and facilitation of iron absorption. Two distinct CYBASC paralogs from the plant Arabidopsis thaliana, Atcytb561-A (A-paralog) and Atcytb561-B (B-paralog), have been found to differ in their visible-spectral characteristics and their interaction with AscH- and ferric iron chelates. A previously determined crystal structure of the B-paralog provides the first insights into the structural organization of a CYBASC member and implies hydrogen bonding between the substrate AscH- and the conserved lysine residues at positions 77 (B-K77) and 81 (B-K81). The function of the highly conserved tyrosine at position 70 (B-Y70) is not obvious in the crystal structure, but its localization indicates the possible involvement in proton-coupled electron transfer. Here we show that B-Y70 plays a major role in the modulation of the oxidation-reduction midpoint potential of the high-potential heme, EM(bH), as well as in AscH- oxidation. Our results support the involvement of the functionally conserved B-K77 in the stabilization of the dianion Asc2-. These findings are supported by the crystal structure of the B-paralog, but a comparative biochemical and biophysical characterization of the A- and B-paralogs implied distinct and more complex functions of the corresponding residues A-Y69 and A-K76 in the A-paralog. Our results emphasize the need for a high-resolution crystal structure of the A-paralog to illuminate the differences in functional organization between the two paralogs.

Beyond esterase-like activity of serum albumin. Histidine-(nitro)phenol radical formation in conversion cascade of p-nitrophenyl acetate and the role of infrared light.

Serum albumin, recognized mainly for its capacity to act as a carrier protein for many compounds, can also actively transform some organic molecules. As a starting point in this study, we consider esterase-like activity of bovine serum albumin (BSA) toward p-nitrophenyl acetate (p-NPA). Our results reveal that the reaction goes beyond ester hydrolysis step. In fact, the transformation product, p-nitrophenol (p-NP), becomes a substrate for further reaction with BSA in which its nitro group in subtracted and released in the form of HNO2 . Spectral data indicate that this cascade of events proceeds through formation of phenoxyl radical via proton-coupled electron transport (PCET) between OH group of p-NP and imidazole ring of histidine from the protein. Furthermore, the effect of application of electromagnetic radiation in the infrared range suggests that this remote physical trigger can support interactions based on PCET mechanism by acting on polarization and mutual alignment of water dipoles serving as effective water wires.

Structural insights into the electron/proton transfer pathways in the quinol:fumarate reductase from Desulfovibrio gigas

The membrane-embedded quinol:fumarate reductase (QFR) in anaerobic bacteria catalyzes the reduction of fumarate to succinate by quinol in the anaerobic respiratory chain. The electron/proton-transfer pathways in QFRs remain controversial. Here we report the crystal structure of QFR from the anaerobic sulphate-reducing bacterium Desulfovibrio gigas (D. gigas) at 3.6 Å resolution. The structure of the D. gigas QFR is a homo-dimer, each protomer comprising two hydrophilic subunits, A and B, and one transmembrane subunit C, together with six redox cofactors including two b-hemes. One menaquinone molecule is bound near heme bL in the hydrophobic subunit C. This location of the menaquinone-binding site differs from the menaquinol-binding cavity proposed previously for QFR from Wolinella succinogenes. The observed bound menaquinone might serve as an additional redox cofactor to mediate the proton-coupled electron transport across the membrane. Armed with these structural insights, we propose electron/proton-transfer pathways in the quinol reduction of fumarate to succinate in the D. gigas QFR.

Proton-Coupled Reduction of the Catalytic [4Fe-4S] Cluster in [FeFe]-Hydrogenases.

In nature, [FeFe]-hydrogenases catalyze the uptake and release of molecular hydrogen (H2 ) at a unique iron-sulfur cofactor. The absence of an electrochemical overpotential in the H2 release reaction makes [FeFe]-hydrogenases a prime example of efficient biocatalysis. However, the molecular details of hydrogen turnover are not yet fully understood. Herein, we characterize the initial one-electron reduction of [FeFe]-hydrogenases by infrared spectroscopy and electrochemistry and present evidence for proton-coupled electron transport during the formation of the reduced state Hred'. Charge compensation stabilizes the excess electron at the [4Fe-4S] cluster and maintains a conservative configuration of the diiron site. The role of Hred' in hydrogen turnover and possible implications on the catalytic mechanism are discussed. We propose that regulation of the electronic properties in the periphery of metal cofactors is key to orchestrating multielectron processes.

Protonengekoppelte Reduktion des katalytischen [4Fe‐4S]‐Zentrums in [FeFe]‐Hydrogenasen

AbstractIn der Natur katalysieren [FeFe]‐Hydrogenasen die Abgabe und Aufnahme von molekularem Wasserstoff (H2) an einem einzigartigen Eisen‐Schwefel‐Kofaktor. Das geringe elektrochemische Überpotential in der Wasserstoffabgabe‐Reaktion macht die [FeFe]‐Hydrogenasen zu einem hervorragenden Beispiel für effiziente Biokatalyse. Gegenwärtig sind die molekularen Details des Wasserstoffumsatzes jedoch noch nicht vollständig verstanden. Daher adressieren wir in dieser Untersuchung die initiale Reduktion des katalytischen Zentrums der [FeFe]‐Hydrogenasen mittels Infrarotspektroskopie und Elektrochemie und zeigen, dass der reduzierte Zustand Hred′ durch protonengekoppelten Elektronentransport gebildet wird. Ladungskompensation bindet das überschüssige Elektron am [4Fe‐4S]‐Zentrum und führt zu einer Stabilisierung der konservativen Konfiguration des [FeFe]‐Kofaktors. Die Rolle von Hred′ beim Wasserstoffumsatz und mögliche Auswirkungen auf den katalytischen Mechanismus werden diskutiert. Es liegt nahe, dass die Regulation elektronischer Eigenschaften in der Umgebung von metallischen Kofaktoren die Grundlage für Multielektronenprozesse bildet.

Tunneling Kinetics and Nonadiabatic Proton-Coupled Electron Transfer in Proteins: The Effect of Electric Fields and Anharmonic Donor-Acceptor Interactions.

A proper description of proton donor-acceptor (D-A) distance fluctuations is crucial for understanding tunneling in proton-coupled electron transport (PCET). The typical harmonic approximation for the D-A potential results in a Gaussian probability distribution, which does not appropriately reflect the electronic repulsion forces that increase the energetic cost of sampling shorter D-A distances. Because these shorter distances are the primary channel for thermally activated tunneling, the analysis of tunneling kinetics depends sensitively on the inherently anharmonic nature of the D-A interaction. Thus, we have used quantum chemical calculations to account for the D-A interaction and developed an improved model for the analysis of experimental tunneling kinetics. Strong internal electric fields are also considered and found to contribute significantly to the compressive forces when the D-A distance distribution is positioned below the van der Waals contact distance. This model is applied to recent experiments on the wild type (WT) and a double mutant (DM) of soybean lipoxygenase-1 (SLO). The compressive force necessary to prepare the tunneling-active distribution in WT SLO is found to fall in the ∼ nN range, which greatly exceeds the measured values of molecular motor and protein unfolding forces. This indicates that ∼60-100 MV/cm electric fields, aligned along the D-A bond axis, must be generated by an enzyme conformational interconversion that facilitates the PCET tunneling reaction. Based on the absolute value of the measured tunneling rate, and using previously calculated values of the electronic matrix element, the population of this tunneling-active conformation is found to lie in the range 10-5-10-7, indicating this is a rare structural fluctuation that falls well below the detection threshold of recent ENDOR experiments. Additional analysis of the DM tunneling kinetics leads to a proposal that a disordered (high entropy) conformation could be tunneling-active due to its broad range of sampled D-A distances.

Binding Study of Cis-Atovaquone with Cytochrome bc1 of Yeast

Tans-Atovaquone is widely used as an effective drug to treat uncomplicated malaria. But its cis-isomer is not a drug. In the present study, we report energy minimized binding pattern of trans-Atovaquone and its cisisomer with cytochrome bc1 (cytbc1) of yeast. The new feature of this molecular docking computation is that structural parameters of the drug molecules have been determined from their crystal structures. The energy minimized structures of protein-drug complexes show that H-bond distant between His-181 of cytochrome bc1 and C=O of Atovaquone for trans-Atovaquone is 2.85 A and 5.3 A with the cis-isomer. The role of this H-bonding interaction in dictating drug potency is in conformity with proton-coupled electron transport mechanism of drug action.

Influencing factors on heterogeneous water oxidation catalysis by di-μ-oxo dimanganese complex on mica as a synthetic model of photosystem II

The influencing factors on heterogeneous water oxidation catalysis (WOC) were investigated in a synthetic photosystem II model developed by adsorbing [(OH2)(terpy)MnIII(μ-O)2MnIV(terpy)(OH2)]3+ (terpy = 2,2′:6′,2″-terpyridine) (1) as an oxygen evolving center onto mica. For chemical WOC using a Ce4+ oxidant, the catalytic activity of 1 on mica increased by a factor of 2.3 or 1.4 by co-adsorption (0.015 mmol g−1) of redox-inactive trications of Al3+ or Ce3+ with 1 (0.15 mmol g−1), respectively, whereas it decreased by co-adsorption (0.25 mmol g−1) of excess Al3+ or Ce3+. The cooperative catalysis by two equivalents of the adsorbed 1 for water oxidation could be facilitated by enrichment of 1 by trications at their low co-adsorption conditions. The decreased catalytic activity at high trications co-adsorption conditions could be explained by impeded penetration of Ce4+ oxidant ions into a mica interlayer. For photochemical WOC containing a [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) photoexcitation center in mica, the drying treatment at 65 °C under the vacuum after 1 adsorption was required in adsorbate preparation, possibly to maintain favorable arrangement of 1 and [Ru(bpy)3]2+ in a mica interlayer. The drying treatment at 65 °C under the vacuum after [Ru(bpy)3]2+ adsorption inactivated the photochemical WOC. The proton-coupled electron transport from interior [Ru(bpy)3]2+ centers to ones near the surface in mica is considered to be suppressed by the drying treatment, which could be responsible for the inactivated photochemical WOC.

Biological effects of deuteronation: ATP synthase as an example

BackgroundIn nature, deuterium/hydrogen ratio is ~1/6600, therefore one of ~3300 water (H2O) molecules is deuterated (HOD + D2O). In body fluids the ratio of deuterons to protons is ~1/15000 because of the lower ionization constant of heavy water. The probability of deuteronation rather than protonation of Asp 61 on the subunit c of F0 part of ATP synthase is also ~1/15000. The contribution of deuteronation to the pKa of Asp 61 is 0.35.Theory and DiscussionIn mitochondria, the release of a deuteron into the matrix side half-channel of F0 is likely to be slower than that of a proton. As another example, deuteronation may slow down electron transfer in the electron transport chain (ETC) by interfering with proton coupled electron transport reactions (PCET), and increase free radical production through the leakage of temporarily accumulated electrons at the downstream complexes.ConclusionDeuteronation, as exemplified by ATP synthase and the ETC, may interfere with the conformations and functions of many macromolecules and contribute to some pathologies like heavy water toxicity and aging.

Thermal energy dissipation in reaction centres and in the antenna of photosystem II protects desiccated poikilohydric mosses against photo-oxidation

Seasonal differences have been observed in the ability of desiccated mosses to dissipate absorbed light energy harmlessly into heat. During the dry summer season desiccation-tolerant mosses were more protected against photo-oxidative damage in the dry state than during the more humid winter season. Investigation of the differences revealed that phototolerance could be acquired or lost even under laboratory conditions. When a desiccated poikilohydric moss such as Rhytidiadelphus squarrosus is in the photosensitive state, the primary quinone, Q(A), in the reaction centre of photosystem II is readily reduced even by low intensity illumination as indicated by reversibly increased chlorophyll fluorescence. No such reduction is observed even under strong illumination in desiccated mosses after phototolerance has been acquired. In this state, reductive charge stabilization is replaced by energy dissipation. As a consequence, chlorophyll fluorescence is quenched. Different mechanisms are responsible for quenching. One is based on the presence of zeaxanthin provided drying occurs in the light. This mechanism is known to be controlled by a protonation reaction which is based on proton-coupled electron transport while the moss is still hydrated. Another mechanism which also requires light for activation, but no protonation, is activated during desiccation. While water is slowly lost, fluorescence is quenched. In this situation, an absorption band formed at 800 nm in the light is stabilized. It loses reversibility on darkening. Comparable kinetics of fluorescence quenching and 800 nm signals as well as the linear relationship between non-photochemical fluorescence quenching (NPQ) and loss of stable charge separation in photosystem II reaction centres suggested that desiccation-induced quenching is a property of photosystem II reaction centres. During desiccation, quenchers accumulate which are stable in the absence of water but revert to non-quenching molecular species on hydration. Together with zeaxanthin-dependent energy dissipation, desiccation-induced thermal energy dissipation protects desiccated poikilohydric mosses against photo-oxidation, ensuring survival during drought periods.

Secondary stabilization reactions and proton-coupled electron transport in photosystem II investigated by electroluminescence and fluorescence spectroscopy.

The oxidized primary electron donor in photosystem II, P(680)(+), is reduced in several phases, extending over 4 orders of magnitude in time. Especially the slower phases may reflect the back-pressure exerted by water oxidation and provide information on the reactions involved. The kinetics of secondary electron-transfer reactions in the microseconds time range after charge separation were investigated in oxygen-evolving thylakoids suspended in H2O or D2O. Flash-induced changes of chlorophyll fluorescence yield and electric field-induced recombination luminescence were decomposed into contributions from oxidation states S(0), S(1), S(2), and S(3) of the oxygen-evolving complex and interpreted in terms of stabilization kinetics of the initial charge-separated state S(j)Y(Z)P(680)(+)Q(A)(-)Q(B). In approximately 10% of the centers, only charge recombination took place. Otherwise, no static heterogeneity was involved in the microsecond reduction of P(680)(+) by Y(Z) (stabilization) or Q(A)(-) (recombination). The recombination component in active centers occurs mainly upon charge separation in S(3), and, in the presence of D2O, in S(2) as well and is tentatively attributed to the presence of Y(Z)(ox)S(j-1) in equilibrium with Y(Z)S(j). A 20-30 micros stabilization occurs in all S-states, but to different extents. Possible mechanisms for this component are discussed. D2O was found to decrease: (i) the rate of the reaction Y(Z)(ox)S(1) to Y(Z)S(2), (ii) the equilibrium constant between P680(+)Y(Z)S(2) and P(680)Y(Z)(ox)S(2), (iii) the rate of the slow phase of P(680)(+) reduction for the S(3) --> S(0) transition, and (iv) the rate of electron transfer from Q(A)(-) to Q(B) /Q(B)(-). The increased 'miss probability' in D2O is due to (iii).

Amino acid residues involved in the coordination and assembly of the manganese cluster of photosystem II. Proton-coupled electron transport of the redox-active tyrosines and its relationship to water oxidation

The combination of site-directed mutagenesis, isotopic labeling, new magnetic resonance techniques and optical spectroscopic methods have provided new insights into cofactor coordination and into the mechanism of electron transport and proton-coupled electron transport in photosystem II. Site-directed mutations in the D1 polypeptide of this photosystem have implicated a number of histidine and carboxylate residues in the coordination and assembly of the manganese cluster, responsible for photosynthetic water oxidation. Many of these are located in the carboxy-terminal region of this polypeptide close to the processing site involved in its maturation. This maturation is a required precondition for cluster assembly. Recent proposals for the mechanism of water oxidation have directly implicated redox-active tyrosine Y Z in this mechanism and have emphasized the importance of the coupling of proton and electron transfer in the reduction of Y Z by the Mn cluster. The interaction of both homologous redox-active tyrosines Y Z and Y D with their respective homologous proton acceptors is discussed in an effort to better understand the significance of such coupling.