Concerted Two-electron Transfer Research Articles

Zn-doped tubular graphene nitride for visible light and sacrificial-agent-free H2O2 photosynthesis in water

Graphitic carbon nitride (g-C3N4) has become a favored universal photocatalyst. Despite its numerous advantages, the photocatalytic efficiency of g-C3N4 is hindered by the substantial recombination of photoexcited charge carriers and holes. In this work, we demonstrate that Zn-doped tubular carbon nitride photocatalyst (Zn-tCN) can serve as highly efficient catalysts for H2O2 photosynthesis. Mechanism studies confirm that the presence of Zn in g-C3N4 prolongs the lifetimes of photogenerated carriers and inhibits their recombination, which triggers the reduction of O2 to reaction intermediates (O2−), as supported by in situ electron paramagnetic resonance (EPR) spectroscopy. More importantly, sacrificial agent experiments coupled with in situ EPR results confirmed that the reaction mechanism involves a concerted two-electron transfer process. The optimal catalyst displays a H2O2 productivity of 162.4μmol g–1h−1 under visible-light irradiation without a sacrificial agent, which is 11.7 times higher than that of pristine g-C3N4 (13.8μmol g–1h−1). This work proposes a synthetic strategy for the preparation of high-performance Zn-doped g-C3N4, which offers insights and perspectives for developing highly active photocatalysts and deepening the understanding of photocatalytic mechanisms.

Mixed Conducting Polymers Alter Electron Transfer Thermodynamics to Boost Current Generation from Electroactive Microbes.

Electroactive microbes that can release or take up electrons are essential components of nearly every ecological niche and are powerful tools for the development of alternative energy technologies. Small-molecule mediators are critical for this electron transfer but remain difficult to study and engineer because they perform concerted two-electron transfer in native systems but only individual, one-electron transfers in electrochemical studies. Here, we report that electrode modification with ion- and electron-conductive polymers yields biosimilar, concerted two-electron transfer from Shewanella oneidensis via flavin mediators. S. oneidensis biofilms on these polymers show significantly improved per-microbe current generation and morphologies that more closely resemble native systems, setting a new paradigm for the study and optimization of these electron transfer processes. The unprecedented concerted electron transfer was found to be due to altered mediator electron transfer thermodynamics, enabling biologically relevant studies of electroactive biofilms in the lab for the first time. These important findings pave the way for a complete understanding of the ecological role of electroactive microbes and their broad application in sustainable technologies.

Imidazolium-based ionic liquids support biosimilar flavin electron transfer.

Understanding electron transport with electroactive microbes is key to engineering effective and scalable bio-electrochemical technologies. Much of this electron transfer occurs through small-molecule flavin mediators that perform one-electron transfers in abiotic systems but concerted two-electron transfer in biological systems, rendering abiotic systems less efficient. To boost efficiency, the principles guiding flavin electron transfer must be elucidated, necessitating a tunable system. Ionic liquids (ILs) offer such a platform due to their chemical diversity. In particular, imidazolium-containing ILs that resemble the amino acid histidine are bio-similar electrolytes that enable the study of flavin electron transfer. Using the model IL 1-ethyl-3-methylimidazolium ([Emim][BF4]), we observe concerted two-electron transfer between flavin mononucleotide and an unmodified glassy carbon electrode surface, while a one-electron transfer occurs in standard inorganic electrolytes. This work demonstrates the power of ILs to enable the mechanistic study of biological electron transfer, providing critical guidelines for improving electrochemical technologies based on these biological properties.

Electrochemical Characterization of Biomolecular Electron Transfer at Conductive Polymer Interfaces

Bio-electrochemical systems (BESs) are promising for renewable energy generation but remain hindered by inefficient electron transfer at electrode surfaces. As the toolbox of bio-anode materials increases, rigorous electrochemical characterization of emerging materials is needed. Here, we holistically characterize the electrochemical interaction of flavin mononucleotide (FMN), an electron shuttle in biological systems and a cofactor for oxidoreductase enzymes, with the bio-inspired mixed conducting polymer poly{3-[6′-(N-methylimidazolium)hexyl]thiophene} (P3HT-Im+). The behavior of this polymer is compared to the equivalent polymer without the histidine-like imidazolium. We find improved conductivity and charge storage in imidazolium-containing polymers beyond what is explained by differences in the electroactive area. The P3HT-Im+ further shows internal charge storage but with negligible faradaic contribution, indicating that charge storage capacity may translate to improved biocatalysis non-intuitive ways. Finally, one-electron transfer is observed between FMN and glassy carbon, while a bio-similar two-electron transfer is observed for the P3HT-Im+. To our knowledge, this is the first example of a concerted two-electron transfer between FMN and an electrode interface, which we attribute to the bio-inspired, histidine-like imidazolium functional groups in the polymer. These studies demonstrate the importance of bio-relevant materials characterization when such materials are deployed in BESs.

Formation of Dianions in Helium Nanodroplets

The formation of dianions in helium nanodroplets is reported for the first time. The fullerene cluster dianions (C60)n(2-) and (C70)n(2-) were observed by mass spectrometry for n≥5 when helium droplets containing the appropriate fullerene were subjected to electron impact at approximately 22 eV. A new mechanism for dianion formation is described, which involves a two-electron transfer from the metastable He(-) ion. As well as the prospect of studying other dianions at low temperature using helium nanodroplets, this work opens up the possibility of a wider investigation of the chemistry of He(-), a new electron-donating reagent.

De Novo Generation of Singlet Oxygen and Ammine Ligands by Photoactivation of a Platinum Anticancer Complex

De Novo Generation of Singlet Oxygen and Ammine Ligands by Photoactivation of a Platinum Anticancer Complex

The solution electrochemistry of tetrahydrobiopterin revisited

Re-investigation of the electrochemical behavior of the nitric oxide synthase (NOS) cofactor tetrahydrobiopterin on graphite electrodes has revealed drastic differences in reversibility of electron transfer (ET) depending on the type of electrode surface employed. In particular, slow electron transfer kinetics and quasireversibility on an unpolished glassy carbon electrode can mask underlying concerted two-electron transfer chemistry and cause the appearance of an apparent one-electron couple. Nonetheless, the thermodynamic instability of the radical intermediate prevents any detectable build-up of this intermediate under any conditions tested. Scan rate and pH-dependencies of the concerted two-electron couple indicate a kinetic barrier to formation of the radical that depends on proton availability. These observations resolve previous conflicting interpretations of tetrahydrobiopterin solution electrochemistry and comment on how NOS may stabilize the one-electron oxidized radical state that participates in enzymatic production of nitric oxide.

Proton-assisted Two-electron Transfer in Natural Variants of Tetraheme Cytochromes from Desulfomicrobium Sp.

The tetraheme cytochrome c3 isolated from Desulfomicrobium baculatum (DSM 1743)(Dsmb) was cloned, and the sequence analysis showed that this cytochrome differs in just three amino acid residues from the cytochrome c3 isolated from Desulfomicrobium norvegicum (Dsmn): (DsmnXXDsmb) Thr-37 --> Ser, Val-45 --> Ala, and Phe-88 --> Tyr. X-ray crystallography was used to determine the structure of cytochrome c3 from Dsmb, showing that it is very similar to the published structure of cytochrome c3 from Dsmn. A detailed thermodynamic and kinetic characterization of these two tetraheme cytochromes c3 was performed by using NMR and visible spectroscopy. The results obtained show that the network of cooperativities between the redox and protonic centers is consistent with a synergetic process to stimulate the hydrogen uptake activity of hydrogenase. This is achieved by increasing the affinity of the cytochrome for protons through binding electrons and, reciprocally, by favoring a concerted two-electron transfer assisted by the binding of proton(s). The data were analyzed within the framework of the differences in the primary and tertiary structures of the two proteins, showing that residue 88, close to heme I, is the main cause for the differences in the microscopic thermodynamic parameters obtained for these two cytochromes c3. This comparison reveals how replacement of a single amino acid can tune the functional properties of energy-transducing proteins, so that they can be optimized to suit the bioenergetic constraints of specific habitats.

Energetics of Concerted Two-Electron Transfer and Metal−Metal Bond Cleavage in Phosphido-Bridged Molybdenum and Tungsten Carbonyl Complexes

The kinetics and thermodynamics of concerted two-electron transfer and metal−metal bond cleavage in the binuclear phosphido-bridged complexes [M2(μ-PPh2)2(CO)8]0/2- [M = Mo (10/2-), W(20/2-)] have been determined by variable scan-rate cyclic voltammetry in 0.3 M TBAPF6/acetone. The reductions of 1° and 2° are accompanied by an increase of 1.08 Å in M−M distance and expansion and contraction by 29° of the M−P−M and P−M−P angles, respectively, within an intact M2(μ-PPh2)2 unit. The one-electron electrode potentials of these systems are highly inverted: ΔE°‘ = − = +0.17 V for 10/2- and +0.18 V for 20/2-, and the rate constant of the second heterogeneous electron-transfer reaction is smaller than the first: ksh,2/ksh,1 = 0.1 for Mo and 0.018 for W. Results are consistent with progressive cleavage of the metal−metal bond in two one-electron steps, of which the second is rate-limiting, because it is accompanied by a larger part of the structural change. EHMO calculations reveal that the redox-active orbital is a metal−metal antibonding (σ*) orbital with substantial bridging-ligand character that decreases markedly in energy on passing from the metal−metal bonded M(I)2 state to nonbonded M(0)2. Despite this feature, electron-transfer thermodynamics and kinetics are not significantly metal-dependent. Rather, comparisons with structurally similar sulfido-bridged complexes reveal that electron-transfer energetics are influenced more extensively by the bridging ligand, with more positive potentials and larger electron-transfer rates observed for RS- versus R2P- bridged species.

Multidimensional Electron Transfer Pathways in a Tetrahedral Tetrakis{4-[N,N-di(4-methoxyphenyl)amino]phenyl}Phosphonium Salt: One-Step vs Two-Step Mechanism

Different electron-transfer pathways have been investigated in a three-dimensional redox system: tetrakis{4-[N,N-di(4-methoxyphenyl)amino]phenyl}phosphonium tetrafluoroborate 1+BF4- which comprises of four triarylamine redox centers arranged in a pseudo-tetrahedral geometry. Using a UV/Vis/NIR spectroelectrochemical setup we generated the mixed-valence species 12+, 13+, and 14+ and measured the UV/Vis/NIR spectra. These spectra were analyzed and interpreted according to a multi-dimensional Marcus−Hush approach. The analysis revealed that both the photoexcited as well as the thermal ET in 13+ are forbidden as a concerted two-electron transfer but allowed as two consecutive one-electron transfer steps.

Two-electron transfer reactions in polar solvents

Chemical, biological, and electrode based electron transfer (ET) processes involve multielectron events. However, an adequate framework in which to describe these complex reactions does not yet exist. A theory for two-electron transfer reactions in Debye solvents is developed. The theory is formulated by generalizing Zusman’s model of ET reactions [L. D. Zusman, Chem. Phys. 49, 295 (1980)] to those involving three parabolic wells: One for the doubly reduced donor, one for the singly reduced donor/singly reduced acceptor, and one for the doubly reduced acceptor. The ET processes are described in terms of diffusional motion along a one-dimensional reaction coordinate with tunneling transitions at the intersection points of the parabolas. Two competing mechanisms of two-electron transfer arise. One is a process with two sequential single electron steps D=A→D−A−→DA=. The other involves ET in one concerted two-electron step (D=A→DA=). The general rate expressions for two-electron transfer are obtained. When the stepwise mechanism dominates, the free energy of activation is predicted to depend upon the driving forces of the two sequential steps but is independent of the overall driving force of the reaction. When concerted two-electron transfer dominates, the Marcus relation is obtained with a reorganization energy associated with the shift of two electrons. The dynamical solvent effect in two-electron ET processes is predicted to be unusual. Two distinct regimes exist, each with essentially linear 1/τL dependence (with τL the solvent longitudinal relaxation time): one for slow solvents and one for fast solvents. A combination of solvent and free energy studies could be used to elucidate the mechanism of multielectron processes in chemical and biological systems.

Effects of metals, ligands and antioxidants on the reaction of oxygen with 1,2,4-benzenetriol

1,2,4-Benzenetriol is an active metabolite of the human leukemogen benzene that reacts rapidly with molecular oxygen (O 2). The mechanism of autoxidation of benzenetriol is scantily characterized, and little is known of the effects of metals, metal chelators, radical scavengers, and antioxidants on the rate of reduction of O 2. Here, we report that catalytic amounts of Cu 2+ and Fe 3+ accelerated the oxidation of benzenetriol (250 μM) in a dose-dependent manner. Fe 3+ (50 μM) increased the rate of autoxidation by 91%, and Cu 2+ (10 μM) increased it 11-fold. In the absence of added metals, superoxide dismutase inhibited and desferrioxamine stimulated the autoxidation. In the Cu 2+-catalyzed reaction, superoxide dismutase neither inhibited nor stimulated, while desferrioxamine abolished the catalysis by Cu 2+. In the presence of Fe 3+, superoxide dismutase slowed the reaction, but desferrioxamine, surprisingly, did not. The presence of both superoxide dismutase and desferrioxamine blocked the autoxidation, either in the presence or absence of metals. We conclude: (1) superoxide is a propagator of sequential one-electron transfer reactions in the absence of added metals; (2) addition of Cu 2+, unlike Fe 3+, removes the dependence of the reaction on propagation by superoxide, presumably changing the radical-propagated chain reaction to a concerted two-electron transfer; (3) the further addition of desferrioxamine restores superoxide-dependent propagation. Taken with our previous data on the genotoxicity of benzenetriol, These findings have implications regarding a role for transition metals in the carcinogenicity of benzene.

Low-temperature magnetic circular dichroism studies of the photoreaction of horseradish peroxidase compound I.

Horseradish peroxidase (HRP) compound I is photolabile at all temperatures between room temperature and 4 K. The photoredox reaction has been studied in frozen glassy solutions by using optical absorption and magnetic circular dichroism spectra following photolysis of HRP compound I with visible-wavelength light at 4.2 and 77 K. The photochemical process is characterized as a concerted two-electron transfer reaction which results in the conversion of the Fe(IV) heme pi-cation radical species of HRP compound I into a low-spin Fe(III) heme species. This reaction occurs even when photolysis is carried out at 4.2 K. Spectra recorded between 4.2 and 80 K for the low-spin ferric hydroxide complex of HRP closely resemble the data measured for the photochemical product. The proposed mechanism for the photoreaction is (formula; see text) No evidence is found for the formation of an Fe(II) heme at these temperatures.