Forward Electron Transfer Research Articles

Phe265 of the D1 protein is required to stabilize plastoquinone binding in the QB-binding site of photosystem II in Synechocystis sp. PCC 6803

In Photosystem II electrons from water splitting pass through a primary quinone electron acceptor (QA) to the secondary plastoquinone (QB). The D2 protein forms the QA-binding site and the D1 protein forms the QB-binding site. A non-heme iron sits between QA and QB resulting in a quinone-Fe-acceptor complex that must be activated before assembly of the oxygen-evolving complex can occur. An extended loop (residues 223–266) between the fourth (helix D) and fifth (helix E) helices of the D1 protein activates forward electron transfer via a conformational change that stabilizes a bidentate bicarbonate ligand to the non-heme iron while simultaneously stabilizing the binding of QB. We show that positioning of D1:Phe265 to provide a hydrogen bond to the distal oxygen of QB is required for forward electron transfer. In addition, mutations targeting D1:Phe265, resulted in a 50 mV decrease in the QB/QB– midpoint potential.

Undulating Free Energy Landscapes Buffer Redox Chains from Environmental Fluctuations.

Roller-coaster or undulating free energy landscapes, with alternating high and low potential cofactors, occur frequently in biological redox chains. Yet, there is little understanding of the possible advantages created by these landscapes. We examined the tetraheme subunit associated with Blastochloris viridis reaction centers, comparing the dynamics of the native protein and of hypothetical (in silico) mutants. We computed the variation in the total number of electrons in wild type (WT) and mutant tetrahemes connected to an electron reservoir in the presence of a time-varying potential, as a model for a fluctuating redox environment. We found that roller-coaster free energy landscapes buffer the redox cofactor populations from these fluctuations. The WT roller-coaster landscape slows forward and backward electron transfer in the face of fluctuations, and may offer the advantage of sustaining the reduction of essential cofactors, such as the chlorophyll special pair in photosynthesis, even though an undulating landscape introduces thermodynamically uphill steps in multistep redox chains.

N2O Decomposition on Singly and Doubly (K and Li)-Doped Co3O4 Nanocubes─Establishing Key Factors Governing Redox Behavior of Catalysts.

The intimate mechanism of N2O decomposition on bare and redox-tuned Co3O4 nanocubes (achieved by single (Li or K) and double (Li and K) doping) was elucidated. The catalysts synthesized by the hydrothermal method were characterized by X-ray electron absorption fine structure measurements, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and Kelvin Probe techniques. TPSR and steady-state isothermal catalytic tests reveal that the N2O turnover frequencies are critically sensitive to the work function of the catalysts, adjusted purposely by doping. For the catalysts obtained by one-pot hydrothermal synthesis, lithiation of the Co3O4 nanocubes leads to the formation of {Li'8a, Co·16d} species, decreasing steadily the work function and the activity, while for the catalysts prepared by postsynthesis impregnation, formation of {Li'8a, Co'16d, Co··16c} species leads to a volcano-type dependence of the catalytic activity and the work function in parallel. The beneficial effect of potassium was discussed in terms of mitigation of surface potential buildup due to the accumulation of ionosorbed oxygen intermediates (surface electrostatics), which hinders the interfacial electron transfer. Analysis of the catalytic activity response to the redox tuning of Co3O4, substantiated by DFT calculations, allowed for a straightforward conceptualization of the redox nature of the N2O decomposition in terms of the lineup of frontier orbitals of the N2O/N2O- and O2-/O2 reactants with the surface DOS structure and the resultant molecular orbital interactions. The positions of the virtual bonding 3πg0(N2O)-α-3dz2 and the occupied 2πg1(O2-)-α-3dz2 states relative to the Fermi energy level play a crucial role in the regulation of the forward and backward interfacial electron transfer events, which drive the redox process.

Effect of Metal Work Function on Electron Transfer Kinetics in Electrochemical Reactions

The longstanding problem of explaining the observed dependence of the kinetic rates of electrochemical reactions on the work function of the electrode material (metal) is revisited, based on a recent paper.1 It was proposed that this dependence could be explained in terms of the effect of the work function of the metal on the free energies of activation of the forward and backward electron transfer reactions. In this presentation, we will explore details of the proposed mechanism.In essence, the mechanism is based on the effect of work function on the Galvani potential of the metal at any given applied potential. Even though the total electron energy (Fermi level) of an electron in the electrode must be the same, regardless of the metal, when it is at equilibrium with a given electrolyte, the manner in which that energy is partitioned between the effect of work function and the effect of potential is significantly different for different electrode materials. A higher work function leads to more stabilization of an electron in the metal (lower free energy) and, since the total free energy must remain the same, this lower free energy due to work function must be compensated by a more negative Galvani potential.The classical analysis of the effect of a more negative potential is based on the theory that a change in potential has a greater effect on the initial (reactant) free energy than it does on the free energy of the transition state. Consequently, the activation energy of the forward (cathodic) reaction is decreased by some fraction α of the corresponding increase in electron energy in the metal. This type of analysis is generally used to quantify the effect of changes in applied voltage and derive current-voltage relationships. In this paper we extend the analysis to take account of changes in the (Galvani) potential ϕm of the metal due to changes in work function Φm.It is reasonable to assume that a change in ϕm will have a similar effect whether it occurs due to a change in applied voltage or due to a change in the work function of the metal. This implies that an increase in work function with a corresponding negative shift in Galvani potential can lead to a lowering of activation energy and so to faster kinetics. However, we must also consider a possible change in activation energy arising from the decreased electron energy (increased stabilization) due to higher work function. If this caused a change in activation energy it would be expected to be in the opposite direction to the change due to Galvani potential, since it would be expected to lower the initial free energy by a greater amount than that of the transition state.The presentation will consider in detail the interplay of these effects and the consequent implications for modelling electrode kinetics. K. S. R. Dadallagei, D. L. Parr IV, J. R. Coduto, A. Lazicki, S. DeBie, C. D. Haas, and J. Leddy, J. Electrochem. Soc. 170, 086508 (2023)

Ultrafast Nonequilibrium Dynamics of Vibrationally Hot Electron Transfer in Flavodoxin.

The understanding of ultrafast short-range electron transfer (ET) in proteins remains challenging, and thorough studies on well-defined biological systems are demanding. Here, we utilized two types of flavodoxins and designed a series of mutants on two positions to systematically characterize the complete photoinduced redox cycles. We identified one position with a favorable orientation and distance for ultrafast ET in a few femtoseconds and the other position is relatively flexible with a longer ET time scale. We found that all forward and back ET dynamics are ultrafast nonequilibrium processes, occurring through highly vibronic states and ending in vibrationally hot ground states with subsequent cooling relaxation to efficiently dissipate photon energy into the protein environment.

Effect of cationic antiseptics on fluorescent characteristics and electron transfer in cyanobacterial photosystem I complexes.

In this study, the effects of cationic antiseptics such as chlorhexidine, picloxidine, miramistin, and octenidine at concentrations up to 150µM on fluorescence spectra and its lifetimes, as well as on light-induced electron transfer in protein-pigment complexes of photosystem I (PSI) isolated from cyanobacterium Synechocystis sp. PCC 6803 have been studied. In doing so, octenidine turned out to be the most "effective" in terms of its influence on the spectral and functional characteristics of PSI complexes. It has been shown that the rate of energy migration from short-wavelength forms of light-harvesting chlorophyll to long-wavelength ones slows down upon addition of octenidine to the PSI suspension. After photo-separation of charges between the primary electron donor P700 and the terminal iron-sulfur center(s) FA/FB, the rate of forward electron transfer from (FA/FB)- to the external medium slows down while the rate of charge recombination between reduced FA/FB- and photooxidized P700+ increases. The paper considers the possible causes of the observed action of the antiseptic.

Nanosecond time-resolved infrared spectroscopy for the study of electron transfer in photosystem I.

Microsecond time-resolved step-scan FTIR difference spectroscopy was used to study photosystem I (PSI) from Thermosynechococcus vestitus BP-1 (T. vestitus, formerly known as T. elongatus) at 77K. In addition,photoaccumulated (P700+-P700) FTIR difference spectra were obtained at both 77 and 293K. The FTIR difference spectra are presented here for the first time. To extend upon these FTIR studies nanosecond time-resolved infrared difference spectroscopy was also used to study PSI from T. vestitus at 296K. Nanosecond infrared spectroscopy has never been used to study PSI samples at physiological temperatures, and here it is shown that such an approach has great value as it allows a direct probe of electron transfer down both branches in PSI. In PSI at 296K, the infrared flash-induced absorption changes indicate electron transfer down the B- and A-branches is characterized by time constants of 33 and 364ns, respectively, in good agreement with visible spectroscopy studies. These time constants are associated with forward electron transfer from A1- to FX on the B- and A-branches, respectively. At several infrared wavelengths flash-induced absorption changes at 296K recover in tens to hundreds of milliseconds. The dominant decay phase is characterized by a lifetime of 128ms. These millisecond changes are assigned to radical pair recombination reactions, with the changes being associated primarily with P700+ rereduction. This conclusion follows from the observation that the millisecond infrared spectrum is very similar to the photoaccumulated (P700+-P700) FTIR difference spectrum.

Bicarbonate activation of the monomeric photosystem II-PsbS/Psb27 complex.

In thylakoid membranes, photosystem II (PSII) monomers from the stromal lamellae contain the subunits PsbS and Psb27 (PSIIm-S/27), while PSII monomers (PSIIm) from granal regions lack these subunits. Here, we have isolated and characterized these 2 types of PSII complexes in tobacco (Nicotiana tabacum). PSIIm-S/27 showed enhanced fluorescence, the near absence of oxygen evolution, and limited and slow electron transfer from QA to QB compared to the near-normal activities in the granal PSIIm. However, when bicarbonate was added to PSIIm-S/27, water splitting and QA to QB electron transfer rates were comparable to those in granal PSIIm. The findings suggest that the binding of PsbS and/or Psb27 inhibits forward electron transfer and lowers the binding affinity for bicarbonate. This can be rationalized in terms of the recently discovered photoprotection role played by bicarbonate binding via the redox tuning of the QA/QA•- couple, which controls the charge recombination route, and this limits chlorophyll triplet-mediated 1O2 formation. These findings suggest that PSIIm-S/27 is an intermediate in the assembly of PSII in which PsbS and/or Psb27 restrict PSII activity while in transit using a bicarbonate-mediated switch and protective mechanism.

InP quantum dots: Stoichiometry regulates carrier dynamics.

The optical properties of non-toxic indium phosphide (InP) quantum dots (QDs) are impinged by the existence of characteristic deep trap states. Several surface engineering strategies have been adopted to improve their optical quality, which has promoted the use of InP QDs for various technological applications. An antithetical approach involves the effective utilization of the deep trap states in InP QDs to modulate back electron transfer rates. Here, we explore the influence of the core-size of InP on their In-to-P stoichiometry and charge transfer dynamics when bound to an acceptor molecule, decyl viologen (DV2+). The mechanism of interaction of InP and DV2+ based on the quenching sphere model established the presence of (i) a 1:1 complex of DV2+ bound on InP and (ii) immobile quenchers in the quenching sphere, depending on the concentration of DV2+. While the forward electron transfer rates from photoexcited InP to bound DV2+ does not substantially vary with an increase in core size, the back electron transfer rates are found to be retarded. Findings from inductively coupled plasma-optical emission spectroscopy (ICP-OES) and X-ray photoelectron spectroscopy (XPS) reveal that the In to P ratio is higher for QDs with larger core size, which further brings about increased carrier trapping and a decreased rate of charge recombination. Furthermore, long-lived charge-separated states in DV2+ bound to InP, extending to hundreds of milliseconds, are obtained by varying the number of DV2+ in the quenching sphere of the QDs.

Modelling of Tungsten (C59W), Osmium (C59Os), and Platinum (C59Pt) Doped Fullerenes for Drug Delivery of Biguanides (BNG) and Metformin (MET): DFT Perspective

AbstractOwing to the vast range of exciting ideas and chances for the diagnosis and treatment of diabetes, the number of people with the disease has increased significantly in both developed and developing nations, with the greatest burden typically falling on indigenous people and socially disadvantaged groups. Nanostructures, however, have drawn a lot of interest because of their capacity to specifically target diabetes cells without damaging healthy cells. Herein, the potential of fullerene (C60) surface doped with C59Os, C59W, and C59Pt is explored as drug delivery systems for biguanides and metformin using density functional theory (DFT) computation at the M06‐2X/Gen/LanL2DZ/6‐311++G(d,p) level of theory. Interestingly, results obtained in this investigation specify that metformin interacts with both the doped and updoped surfaces more strongly than biguanides. Fascinatingly, the energy gap obtained from frontier molecular orbital (FMO) accounted for the significant reactivity and stability strength in the following order: MET@C59W>MET@C59Pt>MET@C59Os>MET@C60, whereas BGN@C59 W>BGN@C59Pt>BGN@C59Os>BGN@C60. Substantially, hydrogen bonding was found to be crucial in the interactions between the drug molecules and all doped and undoped surfaces through non‐covalent interaction (NCI) and quantum theory of atoms in molecules (QTAIM). Also worth mentioning is the complexes′ propensity for forward electron transfer, which is aided by their severe electronegative values, thus explicating that the doped surfaces are a preferable carrier for both metformin and biguanides. Although, theoretically, it follows that doped surfaces are promising drug delivery targets most notably for metformin.

Triplet states in the reaction center of Photosystem II.

In oxygenic photosynthesis sunlight is harvested and funneled as excitation energy into the reaction center (RC) of Photosystem II (PSII), the site of primary charge separation that initiates the photosynthetic electron transfer chain. The chlorophyll ChlD1 pigment of the RC is the primary electron donor, forming a charge-separated radical pair with the vicinal pheophytin PheoD1 (ChlD1+PheoD1-). To avert charge recombination, the electron is further transferred to plastoquinone QA, whereas the hole relaxes to a central pair of chlorophylls (PD1PD2), subsequently driving water oxidation. Spin-triplet states can form within the RC when forward electron transfer is inhibited or back reactions are favored. This can lead to formation of singlet dioxygen, with potential deleterious effects. Here we investigate the nature and properties of triplet states within the PSII RC using a multiscale quantum-mechanics/molecular-mechanics (QM/MM) approach. The low-energy spectrum of excited singlet and triplet states, of both local and charge-transfer nature, is compared using range-separated time-dependent density functional theory (TD-DFT). We further compute electron paramagnetic resonance properties (zero-field splitting parameters and hyperfine coupling constants) of relaxed triplet states and compare them with available experimental data. Moreover, the electrostatic modulation of excited state energetics and redox properties of RC pigments by the semiquinone QA- is described. The results provide a detailed electronic-level understanding of triplet states within the PSII RC and form a refined basis for discussing primary and secondary electron transfer, charge recombination pathways, and possible photoprotection mechanisms in PSII.

Through-Bond-Driven Through-Space Interactions in a Fullerene C60 Noncovalent Dyad: An Unusual Strong Binding between Spherical and Planar π Electron Clouds and Culmination of Dyadic Fractals.

Hydrogen-bond-induced π-depletion as a criterion for π-stacking, a configurationally unique noncovalent strategy enabled an unconventional strong binding between the spherical N-fulleropyrrolidine (NFP) and the planar distributions of π electron clouds of three substituted pybates to form noncovalent fulleropyrrolidino-4-(pyrenyl) butanoate dyads of large computed interaction energies, varying between 37.49 and 44.93 kcal/mol. The geometrical distortion/bending of the alkyl tail of pybate in the noncovalent dyad was experimentally corroborated via UV-vis absorption spectroscopy, which translated into spectral broadening along with pronounced shifts in the n-π* transitions of the oxy-substituted pyrene in different solvents, ensuring through-bond interactions. Facile electron transfer through H-bond influenced the dynamic dispersive forces to be active, revealing the supremacy of through-bond over through-space interactions. The analyses of intermolecular forces using an absolutely localized molecular orbital-based energy decomposition analysis (ALMO-EDA) scheme revealed intricate insights into the intermolecular interactions and characteristic charge transfer; the dominance of forward electron transfer (pybate to NFP) over the reverse in offering stabilization was noted. Charge transfer was investigated further from natural bond orbital (NBO) and absolutely localized molecular orbital-based charge-transfer analysis (ALMO-CTA) methods, establishing the supremacy of donor-to-acceptor electron transfer over the reverse (acceptor-to-donor) one. The characteristic self-assembly of the noncovalent dyad in suitable solvents led to the formation of fractal networks via reaction-limited cluster aggregation with a fractal dimension of 2.37. Adoption of constrained molecular dynamics simulations indicated probable wrapping of pybates around NFP, leading to fractal-like assembly.

Reverse Electron Transfer is a More Dominant Source of Mitochondrial ROS Production in the Heart and Kidney Outer Medulla than in the Kidney Cortex

RationaleReactive oxygen species (ROS; e.g. O2·– and H2O2) play important roles in both physiological and pathophysiological processes. ROS in low concentrations contribute to physiological processes, such as cellular redox signaling and phagocytosis, whereas ROS in high concentrations are toxic to the cell causing tissue injury contributing to the pathogenesis of cardiovascular and chronic renal diseases, including salt‐sensitive hypertension. Mitochondria, which produce ROS as byproducts of aerobic respiration via both forward electron transfer (FET) and reverse electron transfer (RET) are known to be one of the major cellular sources of ROS. Although it is recognized that the RET mechanism in which electrons flow back from complex II to complex I contribute significantly to ROS production in cardiac mitochondria, the mechanisms of ROS production and the role of RET in kidney mitochondria has remained poorly understood.MethodWe evaluated the relative contributions of FET and RET towards overall ROS production in mitochondria isolated from the heart and kidney cortex and outer medulla (OM) of adult Sprague‐Dawley rats. H2O2 emission was measured by a spectrofluorometer in isolated mitochondria in the presence of either Succinate (Suc) simulating RET or succinate+rotenone (Suc+Rot) simulating FET. Furthermore, we measured mitochondrial rates of H2O2 production along with respiration and membrane potential under three respiratory states namely (i) leak state (state 2; after substrate addition), (ii) oxidative phosphorylation (OxPhos) state (state 3; after ADP addition), and (iii) maximum respiratory state (state 5; after the addition of the uncoupler FCCP).ResultsIt was found that mitochondria isolated from the heart and kidney cortex produced the least and the most ROS, respectively. The rate of ROS production in the presence of Suc+Rot compared to Suc alone decreased significantly in the heart and to a lesser extent in OM, indicating significant contribution of RET to overall ROS production in the heart and slightly in the OM. In contrast, there was not significant difference in ROS production rates in the presence of Suc and Suc +Rot in mitochondria from kidney cortex, showing that RET is not predominant in the kidney cortex. Also, we observed significant reduction in the ROS production rate in state 4 compared to state 2 in the heart mitochondria compared to kidney cortex and OM. A possible explanation for these differential results is that oxaloacetate (OAA), produced by the tricarboxylic acid cycle, accumulates resulting in succinate dehydrogenase (SDH) inhibition more rapidly in the heart than in the kidney affecting mitochondrial ROS production, respiration, and bioenergetics.ConclusionRET mechanism contributes to mitochondrial ROS production significantly in the heart and slightly in the kidney OM, but not in the kidney cortex. OAA accumulation contributes to SDH inhibition significantly in the heart than in the kidney.

Ultrafast Dynamics of Nonequilibrium Short-Range Electron Transfer in Semiquinone Flavodoxin.

Short-range protein electron transfer (ET) is crucially important in light-induced biological processes such as in photoenzymes and photoreceptors and often occurs on time scales similar to those of environment fluctuations, leading to a coupled dynamic process. Herein, we use semiquinone Anabaena flavodoxin to characterize the ultrafast photoinduced redox cycle of the wild type and seven mutants by ultrafast spectroscopy. We have found that the forward and backward ET dynamics show stretched behaviors in a few picoseconds (1-5 ps), indicating a coupling with the local protein fluctuations. By comparison with the results from semiquinone D. vulgaris flavodoxin, we find that the electronic coupling is crucial to the ET rates. With our new nonergodic model, we obtain smaller values of the outer reorganization energy (λoγ) of environment fluctuations and the reaction free energy force (ΔGγ), a signature of nonequilibrium ET dynamics.

Comparison of ROS production by forward and reverse electron transfer mechanisms in the heart and kidney cortex and outer medulla

Reactive oxygen species (ROS) at low concentrations play a key role in physiological processes, including phagocytosis and redox signaling in phagocytic and nonphagocytic cells, respectively. However, when produced in excess, they are toxic to the cell and can contribute to pathogenesis of cardiovascular and renal diseases. One of the major sources of ROS is mitochondria, produced as a byproduct of aerobic respiration. There is evidence that reverse electron transfer (RET) mechanism in which electrons flow back from complex II to complex I contributes significantly to ROS production in cardiac mitochondria.

EPR of Type I photosynthetic reaction centers.

Light-induced reactions in photosynthetic reaction centers are initiated by the absorption of a photon, which results in the transfer of a single electron and the generation of radical ions in the donor and acceptor molecules involved in the charge-separated state. Electron paramagnetic resonance (EPR) spectroscopy is the ideal method for the study of such reactions. In addition to measuring spectra of the electron transfer cofactors in continuous light, reactions can be initiated by brief flashes of light, thereby allowing the kinetics of forward electron transfer as well as recombination reactions to be obtained. Because the donor and acceptor pairs are so closely spaced and because light induced charge separation is so rapid, the donor and early acceptors are in a quantum mechanically spin entangled state, which confers properties such as increased sensitivity, the ability to measure reactions on the nanosecond timescale, and the determination of bond angles between cofactors. Additionally, distances between pairs of cofactors can be measured by detecting the modulation of a phase shifted "out-of-phase" electron spin echo signal. In this methods article, we will describe how continuous wave EPR, time resolved EPR, and pulsed EPR can be used to measure these properties in Type I photosynthetic reaction centers. Methods of analysis are described for the bound electron transfer cofactors in the heterodimeric Photosystem I reaction center of plants and cyanobacteria and in the homodimeric reaction centers found in phototrophic members of the phyla Bacillota, Chlorobiota, and Acidobacteriota.

Solvatofluorochromic Contrast with Supramolecular Stereoisomers Using Linked Rotaxane Structures to Investigate Local Solvation in Excited Donor-Bridge-Acceptor Systems

Abstract To optimize the photophysical properties of donor-bridge-acceptor (D-B-A) systems, the elucidation of the excited state of each component (i.e., D, B, and A) plays a crucial role in controlling the forward and back electron transfer rates. However, the direct evaluation of solvation spheres and their rearrangement on the excited bridge is still challenging. In this work, we report a method that exploits a linked rotaxane structure to uncover the solvation effects on an excited bridge by leveraging solvatofluorochromic contrast. We synthesize a D-insB-A system, in which a diphenylacetylene-based bridging moiety is insulated by a covalently linked permethylated α-cyclodextrin (PM α-CD), as a structural isomer of the uninsulated counterpart (D-uninsB-A). In the D-insB-A system, PM α-CD selectively covers the bridge while the donor and acceptor remain exposed. In this way, a comparison of the fluorescence wavelengths of D-insB-A and D-uninsB-A can reveal the local solvation effects on the excited bridge moiety. The investigation of the fluorescence properties in various solvents highlights a weak solvent interaction with the excited bridge in protic solvents and strong one in chlorinated polar solvents. These results provide crucial insight into the properties of excited D-B-A systems from the perspective of external environments, including solvent interactions.

A Standalone, Battery‐Free Light Dosimeter for Ultraviolet to Infrared Light

AbstractLight sensors are widely used to monitor light intensities, for instance in medical applications, in agriculture or for conservation of art. Most of these sensors are electronic devices that record continuously but applications that only require information of integrated intensities, measured over a long time, could greatly benefit from an integrating dosimeter that does not require a power supply. In this work a wireless and quantitative light dosimeter is presented based on SrAl2O4:Eu2+,Sm3+, a phosphor that exhibits stable energy storage upon exposure to blue and ultraviolet light. It is shown that a forward electron transfer from europium to samarium can be induced under illumination with blue or ultraviolet light while the reverse electron transfer can be achieved by illuminating the phosphors with green to infrared light. This reverse transfer is accomplished through excitation of the divalent samarium and results in bright, green optically stimulated luminescence. The stable energy storage, in combination with the possibility for optical read‐out, makes SrAl2O4:Eu2+,Sm3+ ideally suited to be used as an integrating light dosimeter for monochromatic to broadband light, from the ultraviolet to the near infrared. To demonstrate this, a proof of concept dosimeter was developed in which this phosphor was successfully used to measure average daylight intensities.

The Concept of Photozymes: Short Peptides with Photoredox Catalytic Activity for Nucleophilic Additions to α‐Phenyl Styrenes

AbstractConventional photoredox catalytic additions of alcohols to olefins require additives, like thiophenol, to promote back electron transfer. The concept of “photozymes” assumes that forward and backward electron transfer steps in a photoredox catalytic cycle are controllable by substrate binding to photocatalytically active peptides. Accordingly, we synthesized a short tripeptide modified with 1,7‐dicyano‐perylene‐3,4 : 9,10‐tetracarboxylic acid bisimide as photoredox catalyst. This peptide undergoes an unconventional photoredox catalytic cycle with the radical anion and dianion of the perylene bisimide‐peptide as intermediates. The photoredox catalytic reactions with α‐phenyl styrenes as substrates require remarkably low catalyst loadings (0.5 mol%) and give the methoxylation products in high yields. The concept of “photozymes” for photoredox catalysis has significant potential for other photocatalytic reactions, in particular with respect to enantioselective photocatalysis.

Femtosecond Time-Resolved Absorption Study of Signaling State of a BLUF Protein PixD from the Cyanobacterium Synechocystis: Hydrogen-Bond Rearrangement Completes during Forward Proton-Coupled Electron Transfer.

Femtosecond time-resolved absorption measurements were carried out for the dark and signaling states of a BLUF (Blue Light Using FAD) protein, PixD, from the cyanobacterium Synechocystis. When the dark state was excited, FAD semiquinone radical (FADH•) was produced from the S1 state, and FADH• led to the signaling state. On the other hand, photoexcitation of the signaling state generated FADH• and FAD anion radical (FAD•-), and they decayed back to the original signaling state. In both cases, FADH• was formed and decayed with a proton-coupled electron transfer (PCET) via the hydrogen-bond network that involves FAD, Gln50, and Tyr8, and hence the kinetics of FADH• directly reflects the hydrogen-bond structure in the FAD-binding sites. It was found that the formation rate of FADH• was significantly different between the dark and signaling states, whereas the decay rate was the same. This indicates that the hydrogen-bond network of FAD-Gln50-Tyr8 in the dark and signaling states is initially different but it becomes indistinguishable after FADH• is formed, implying that the FAD-Gln50-Tyr8 hydrogen-bond network is rearranged during the PCET to generate FADH•. The present results best agree with the model in which the Gln tautomerizes without rotation in the signaling-state formation.