Reversible Interconversion of CO₂ and Formate by a Molybdenum-Containing Formate Dehydrogenase

CO₂ and formate are rapidly, selectively, and efficiently interconverted by tungsten-containing formate dehydrogenases that surpass current synthetic catalysts. However, their mechanism of catalysis is unknown, and no tractable system is available for study. Here, we describe the catalytic properties of the molybdenum-containing formate dehydrogenase H from the model organism Escherichia coli (<i>Ec</i>FDH-H). We use protein film voltammetry to demonstrate that <i>Ec</i>FDH-H is a highly active, reversible electrocatalyst. In each voltammogram a single point of zero net current denotes the CO₂ reduction potential that varies with pH according to the Nernst equation. By quantifying formate production we show that electrocatalytic CO₂ reduction is specific. Our results reveal the capabilities of a Mo-containing catalyst for reversible CO₂ reduction and establish <i>Ec</i>FDH-H as an attractive model system for mechanistic investigations and a template for the development of synthetic catalysts

Single-Molecule Charge Transport and Electrochemical Gating in Redox-Active Perylene Diimide Junctions

A series of redox-active perylene tetracarboxylic diimide (PTCDI) derivatives have been synthesized and studied by electrochemical cyclic voltammetry and electrochemical scanning tunnelling microscopy break junction techniques. These PTCDI molecules feature the substitution of pyrrolidine at the bay (1,7-) position of perylene and are named pyrrolidine-PTCDIs. These moieties exhibit a small bandgap (2.1 eV) compared with the “normal” (unsubstituted) PTCDI molecule (2.5 eV). Pyrrolidine-PTCDIs were functionalized with different anchoring groups (thiol, amine, pyridine) for building metal–molecule–metal (m–M–m) junctions. The single-molecule conductance values of pyrrolidine-PTCDIs have been determined by analyzing a large number of molecular (m–M–m) junctions created between an STM tip and substrate using a statistical method. Furthermore, we studied the gate dependence of the single-molecule conductance by trapping a molecule between the two electrodes and recording the current as a function of electrochemical gate potential. The experimentally determined conductance values for these bay-substituted pyrrolidine-PTCDI molecules are about twice as much as the unsubstituted PTCDI molecules. The present work shows that single-molecule conductance can be tuned by the bandgap of a molecular system without significantly altering the conductance pathway

Ambipolar Transport in an Electrochemically Gated Single-Molecule Field-Effect Transistor

Charge transport is studied in single-molecule junctions formed with a 1,7-pyrrolidine-substituted 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) molecular block using an electrochemical gate. Compared to an unsubstituted-PTCDI block, spectroscopic and electrochemical measurements indicate a reduction in the highest occupied (HOMO)–lowest unoccupied (LUMO) molecular orbital energy gap associated with the electron donor character of the substituents. The small HOMO–LUMO energy gap allows for switching between electron- and hole-dominated charge transports as a function of gate voltage, thus demonstrating a single-molecule ambipolar field-effect transistor. Both the unsubstituted and substituted molecules display similar n-type behaviors, indicating that they share the same n-type conduction mechanism. However, the substituted-PTCDI block shows a peak in the source–drain current <i>vs</i> gate voltage characteristics for the p-type transport, which is attributed to a two-step incoherent transport <i>via</i> the HOMO of the molecule

Survival.Raw Data.Zinc.xlsx

This dataset is the raw honey bee survivorshipship data resulting from varying levels of zinc exposure. These data are associated with a manuscript titled:    Zinc and laboratory emerge date impact honey bee gut microbiota and survival. Abstract of manuscript:  Honey bees may be exposed to a wide variety of chemicals in the environment, including pesticides, antibiotics, and metals. Zinc, for example, is commonly included in fertilizers, pesticides, and feed additives, and is found in agricultural runoff. Honey bees can be exposed to zinc directly or indirectly by consuming zinc-contaminated nectar and pollen. However, there is a paucity of studies addressing the putative effects of zinc on honeybee´s health. In this study, we tested the effects of zinc on honey bee survivorship and gut microbiota. To evaluate survivorship, we exposed bees to six concentrations of zinc (0, 50, 100, 250, 500, or 1000 mg/L) and assessed survival daily for 10 days. To evaluate effects of zinc on gut microbiota, we exposed bees to 5 or 100 mg/L zinc. Bees were sampled before (day 0) and after zinc exposure (days 3, 6, and 9). Abdominal contents underwent DNA extraction and 16S rRNA sequencing (V3-V4) on an Illumina MiSeq. Sequences were filtered and processed through QIIME2 and DADA2. Zinc treatment had minimal effects on bacterial DNA concentrations and absolute cell counts while emerge date (the date a bee emerged from the brood comb) had a significant effect with decreased bacterial concentrations and cell counts observed at later emerge dates. Survival was only minimally impacted (>89% survival) at zinc concentrations up to 100 mg/L. Zinc had limited effects on overall gut microbial composition, diversity, and taxonomic abundances, with the greatest differences noted in the bee group exposed to the higher concentration of zinc (100 mg/L). In this group, several beneficial taxa (Lactobacillus, Rhizobiaceae, Gilliamella) were found at reduced abundances, while Paenibacillus, a potentially pathogenic taxa, was found at increased abundances. This suggests that zinc exposure, even at relatively low levels, may negatively impact honey bee health, even if survivorship is not dramatically impacted. Notably, emerge date effects were also observed in microbial composition. These results demonstrate the need to include assessments of honeybee gut microbiota in addition to other metrics of honey bee health and survivorship when evaluating the potential effects of agrochemicals on honey bees</p

Catalytic Turnover of [FeFe]-Hydrogenase Based on Single-Molecule Imaging

Hydrogenases catalyze the interconversion of protons and hydrogen according to the reversible reaction: 2H⁺ + 2e^– ⇆ H₂ while using only the earth-abundant metals nickel and/or iron for catalysis. Due to their high activity for proton reduction and the technological significance of the H⁺/H₂ half reaction, it is important to characterize the catalytic activity of [FeFe]-hydrogenases using both biochemical and electrochemical techniques. Following a detailed electrochemical and photoelectrochemical study of an [FeFe]-hydrogenase from Clostridium acetobutylicum (<i>Ca</i>HydA), we now report electrochemical and single-molecule imaging studies carried out on a catalytically active hydrogenase preparation. The enzyme <i>Ca</i>HydA, a homologue (70% identity) of the [FeFe]-hydrogenase from Clostridium pasteurianum, CpI, was adsorbed to a negatively charged, self-assembled monolayer (SAM) for investigation by electrochemical scanning tunneling microscopy (EC-STM) techniques and macroscopic electrochemical measurements. The EC-STM imaging revealed uniform surface coverage with sufficient stability to undergo repeated scanning with a STM tip as well as other electrochemical investigations. Cyclic voltammetry yielded a characteristic cathodic hydrogen production signal when the potential was scanned sufficiently negative. The direct observation of the single enzyme distribution on the Au-SAM surface coupled with macroscopic electrochemical measurements obtained from the same electrode allowed the evaluation of a turnover frequency (TOF) as a function of potential for single [FeFe]-hydrogenase molecules