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Flavodoxins are enzymes that contain the redox-active flavin mononucleotide (FMN) cofactor and play a crucial role in numerous biological processes, including energy conversion and electron transfer. Since the redox characteristics of flavodoxins are significantly impacted by the molecular environment of the FMN cofactor, the evaluation of the interplay between the redox properties of the flavin cofactor and its molecular surroundings in flavoproteins is a critical area of investigation for both fundamental research and technological advancements, as the electrochemical tuning of flavoproteins is necessary for optimal interaction with redox acceptor or donor molecules. In order to facilitate the rational design of biomolecular devices, it is imperative to have access to computational tools that can accurately predict the redox potential of both natural and artificial flavoproteins. In this study, we have investigated the feasibility of using non-equilibrium thermodynamic integration protocols to reliably predict the redox potential of flavodoxins. Using as a test set the wild-type flavodoxin from Clostridium Beijerinckii and eight experimentally characterized single-point mutants, we have computed their redox potential. Our results show that 75% (6 out of 8) of the calculated reaction free energies are within 1 kcal/mol of the experimental values, and none exceed an error of 2 kcal/mol, confirming that non-equilibrium thermodynamic integration is a trustworthy tool for the quantitative estimation of the redox potential of this biologically and technologically significant class of enzymes. [ABSTRACT FROM AUTHOR]

Sediment microbial fuel cell (SMFC) is a type of MFC without a proton exchange membrane. However, SMFC have had problems with low-power production performance. In this paper, the effects of native bacteria (K1) in oily sludge and their electro-oil-induced domestication on the power generation and oil removal performance of SMFC were studied. The results showed that K1 belonged to Ochrobactrum intermedium. During the domestication process, an upward trend was shown in the OD600 and ORP values in the culture medium, and it grown best at 0.7 V. Ochrobactrum intermedium K1 significantly increased the average output voltage, electromotive force, and maximum power density of SMFC and reduced the apparent internal resistance of the battery. The maximum power density was 169.43 mW/m3, which was 8.59 times higher than that of the control group. Ochrobactrum intermedium K1 improved the degradation of crude oil by SMFC. Ochrobactrum intermedium K1 enhanced the degradation of high-carbon alkanes and even-carbon alkanes in n-alkanes. Cyclic voltammetry and chronoamperometry tests showed that after acclimation, Ochrobactrum intermedium K1 improved the extracellular electron transfer efficiency (EET) mediated by c-Cyts and flavin by increasing the surface protein redox potential. [ABSTRACT FROM AUTHOR]

Nitrate (NO3−-N) and nitrites (NO2−-N) are common pollutants in various water bodies causing serious threats not only to aquatic, but also to animals and human beings. In this study, we developed a strategy for efficiently reducing nitrates in microbial fuel cells (MFCs) powered by a granular activated carbon (GAC)-biocathode. GAC was developed by acclimatizing and enriching denitrifying bacteria under a redox potential (0.3 V) generated from MFCs. Thus, using the formed GAC-biocathode we continued to study their effect on denitrification with different cathode materials and circulation speeds in MFCs. The GAC-biocathode with its excellent capacitive property can actively reduce nitrate for over thirty days irrespective of the cathode material used. The stirring speed of GAC in the cathode showed a steady growth in potential generation from 0.25 V to 0.33 V. A rapid lag phase was observed when a new carbon cathode was used with enriched GAC. While a slow lag phase was seen when a stainless-steel cathode was replaced. These observations showed that effective storage and supply of electrons to the GAC plays a crucial role in the reduction process in MFCs. Electrochemical analysis of the GAC properties studied using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and zeta potential showed distinct properties with different abiotic and biocathode conditions. We found that the enrichment of electrotrophic bacteria on GAC facilitates the direct electron transfer in the cathode chamber for reducing NO3−-N in MFCs as observed by scanning electron microscopy. [ABSTRACT FROM AUTHOR]

Samarium diiodide is a versatile single electron transfer (SET) agent with various applications in organic chemistry. Lewis structures regularly insinuate the existence of a ketyl radical when samarium diiodide binds a carbonyl group. The study presented here investigates this electron transfer by the means of computational chemistry. All electron CASPT2 calculations with the inclusion of scalar relativistic effects predict an endotherm electron transfer from samarium diiodide to acetone. Energies calculated with the PBE0-D3(BJ) functional and a small core pseudopotential are in good agreement with CASPT2. The calculations confirm the experimentally measured increase of the samarium diiodide reduction potential through the addition of hexamethylphosphoramide also known as HMPA. [ABSTRACT FROM AUTHOR]

Simple Summary: Many biological systems contain iron–sulfur clusters, which are typically found as components of electron transport proteins. Continuum electrostatic calculations were used to investigate the effect of protein environment on the redox properties of the three iron–sulfur clusters in the cyanobacterial photosystem I. Our results show a good correlation between the estimated and the measured reduction potential. Moreover, the results indicate that the low potential of FX is shown to be due to the interactions with the surrounding residues and ligating sulfurs. Our results will help in understanding the electron transfer reaction in photosystem I. Photosystem I is a light-driven electron transfer device. Available X-ray crystal structure from Thermosynechococcus elongatus showed that electron transfer pathways consist of two nearly symmetric branches of cofactors converging at the first iron–sulfur cluster FX, which is followed by two terminal iron–sulfur clusters FA and FB. Experiments have shown that FX has lower oxidation potential than FA and FB, which facilitates the electron transfer reaction. Here, we use density functional theory and Multi-Conformer Continuum Electrostatics to explain the differences in the midpoint E m potentials of the FX, FA and FB clusters. Our calculations show that FX has the lowest oxidation potential compared to FA and FB due to strong pairwise electrostatic interactions with surrounding residues. These interactions are shown to be dominated by the bridging sulfurs and cysteine ligands, which may be attributed to the shorter average bond distances between the oxidized Fe ion and ligating sulfurs for FX compared to FA and FB. Moreover, the electrostatic repulsion between the 4Fe-4S clusters and the positive potential of the backbone atoms is lowest for FX compared to both FA and FB. These results agree with the experimental measurements from the redox titrations of low-temperature EPR signals and of room temperature recombination kinetics. [ABSTRACT FROM AUTHOR]

Cytochrome P450 reductase (CYPOR) provides electrons to all human microsomal cytochrome P450s (cyt P450s). The length and sequence of the "140s" FMN binding loop of CYPOR has been shown to be a key determinant of its redox potential and activity with cyt P450s. Shortening the "140s loop" by deleting glycine-141(ΔGly141) and by engineering a second mutant that mimics flavo-cytochrome P450 BM3 (ΔGly141/Glu142Asn) resulted in mutants that formed an unstable anionic semiquinone. In an attempt to understand the molecular basis of the inability of these mutants to support activity with cyt P450, we expressed, purified, and determined their ability to reduce ferric P450. Our results showed that the ΔGly141 mutant with a very mobile loop only reduced ~7% of cyt P450 with a rate similar to that of the wild type. On the other hand, the more stable loop in the ΔGly141/Glu142Asn mutant allowed for ~55% of the cyt P450 to be reduced ~60% faster than the wild type. Our results reveal that the poor activity of the ΔGly141 mutant is primarily accounted for by its markedly diminished ability to reduce ferric cyt P450. In contrast, the poor activity of the ΔGly141/Glu142Asn mutant is presumably a consequence of the altered structure and mobility of the "140s loop". [ABSTRACT FROM AUTHOR]

Herein, we tune the redox potential of 3,6-diphenyl-1,2,4,5-tetrazine (DPT) by introducing various electron-donating/withdrawing groups (methoxy, t-butyl, H, F, and trifluoromethyl) into its two peripheral benzene rings for use as electrode material in a Li-ion cell. By both the theoretical DFT calculations and the practical cyclic voltammetry (CV) measurements, it is shown that the redox potentials (E1/2) of the 1,2,4,5-tetrazines (s-tetrazines) have a strong correlation with the Hammett constant of the substituents. In Li-ion coin cells, the discharge voltages of the s-tetrazine electrodes are successfully tuned depending on the electron-donating/withdrawing capabilities of the substituents. Furthermore, it is found that the heterogeneous electron transfer rate (k0) of the s-tetrazine molecules and Li-ion diffusivity (DLi) in the s-tetrazine electrodes are much faster than conventional electrode active materials. [ABSTRACT FROM AUTHOR]

In recent years, defective TiO2-based composite nanomaterials have received much attention in the field of photocatalysis. In this work, TiB2 was used as a precursor to successfully prepare Ti3+ defective TiO2 (TiO2-B) with a truncated bipyramidal structure by a one-step method. Then, the SnS2 nanosheets were assembled onto the as-prepared TiO2-B through simple hydrothermal reaction. TiO2-B exhibits strong visible light absorption properties, but the recombination rate of the photo-generated electron-hole pair was high and does not exhibit ideal photocatalytic performance. Upon introducing SnS2, the heterojunction catalyst SnS2-Ti3+ defective TiO2 (SnS2/TiO2-B) not only possesses the strong light absorption from UV to visible light region, the lowest photo-generated charge recombination rate but also achieves a more negative conduction band potential than the reduction potential of CO2 to CO, and thereby, exhibits the significantly enhanced selectivity and yield of CO in photocatalytic CO2 reduction. Notably, SnS2/TiO2-B produces CO at a rate of 58 µmol·h−1·g−1 with CO selectivity of 96.3% under visible light irradiation, which is 2 and 19 times greater than those of alone TiO2-B and SnS2, respectively. Finally, a plausible photocatalytic mechanism on SnS2/TiO2-B was proposed that the electron transfer between TiO2 and SnS2 follows the Z-scheme mode. Our results present an effective way to gain highly efficient TiO2 based photocatalysts for CO2 reduction by combining different modification methods of TiO2 and make full use of the synergistic effects. [ABSTRACT FROM AUTHOR]

Quasi-linear anionic 3d-metal(I) silylamides are a new and promising class of molecules. Due to their highly negative reduction potential we wanted to test their capability to reduce substrates under coordination of their monoanionic radicaloid form. In a proof of principle study, we present the results of the reaction of metal(I) silylamides of chromium to cobalt with 2,2′-bipyridine (bipy), the redox non-innocence and reducibility of which was already established. In the course of these studies complexes of the type K{18-crown-6}[M(hmds)2(bipy)] (hmds = –N(SiMe3)2) were obtained. These compounds were isolated and thoroughly characterized to confirm the electron transfer onto the bipyridine ligand, which now acts as a radical monoanion. For comparison of the structural changes of the bipyridine ligand, the analogous zinc complexes were also synthesized. Overall our results indicate that anionic metal(I) silylamides are capable of reducing and ligate substrates, even when the electrochemical reduction potential of the latter is by up to 1 V higher. [ABSTRACT FROM AUTHOR]