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Graphic abstract Iron is a fundamental element for virtually all forms of life. Despite its abundance, its bioavailability is limited, and thus, microbes developed siderophores, small molecules, which are synthesized inside the cell and then released outside for iron scavenging. Once inside the cell, iron removal does not occur spontaneously, instead this process is mediated by siderophore-interacting proteins (SIP) and/or by ferric-siderophore reductases (FSR). In the past two decades, representatives of the SIP subfamily have been structurally and biochemically characterized; however, the same was not achieved for the FSR subfamily. Here, we initiate the structural and functional characterization of FhuF, the first and only FSR ever isolated. FhuF is a globular monomeric protein mainly composed by α-helices sheltering internal cavities in a fold resembling the “palm” domain found in siderophore biosynthetic enzymes. Paramagnetic NMR spectroscopy revealed that the core of the cluster has electronic properties in line with those of previously characterized 2Fe–2S ferredoxins and differences appear to be confined to the coordination of Fe(III) in the reduced protein. In particular, the two cysteines coordinating this iron appear to have substantially different bond strengths. In similarity with the proteins from the SIP subfamily, FhuF binds both the iron-loaded and the apo forms of ferrichrome in the micromolar range and cyclic voltammetry reveals the presence of redox-Bohr effect, which broadens the range of ferric-siderophore substrates that can be thermodynamically accessible for reduction. This study suggests that despite the structural differences between FSR and SIP proteins, mechanistic similarities exist between the two classes of proteins. Supplementary Information The online version contains supplementary material available at 10.1007/s00775-021-01854-y.

International audience; *corresponding authors: frederic.barriere@univ-rennes1.fr and thomas.flinois@univ-rennes1.fr Abstract: 5-aminosalicylic acid and salicylic acid have been used to form redox active films onto glassy carbon electrodes through recurrent cyclic voltammetry. The variation of the formal potential of the film obtained from 5-aminosalicylic acid as a function of pH is linear over the entire pH range studied (pH 2 to 10) with a slope of-80 mV per pH unit. Salicylic acid-based redox active films permit the detection of sodium and potassium ions (with a slope of-10 mV per 1 mM of cation) and chloride (with a slope of +11 mV per 1 mM of chloride). A lipid deposit of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) onto these modified electrodes allowed the integration of ionophores (valinomycin and nigericin) and the monitoring of the pH and potassium ion concentration variation at the modified electrode / lipid deposit interface.

International audience; Electrografted anthraquinone on graphite was used as a probe to monitor the pH change at the biofilm-electrode interface at the anode of a microbial fuel cell inoculated with wastewater. The grafting procedure was optimized so that the pH-dependent electrochemical response of the grafted quinone did not overlay with that of the electroactive biofilm. The variation of the formal potential of the grafted quinone as a function of pH was linear over the pH range 1-10 with a slope of - 64 mV. This allowed to monitor the interfacial pH change over three weeks of biofilm colonization of the electrode. During that time the interfacial pH decreased from neutrality to 5.3 while the anolyte only acidified down to pH 6.2. This finding is relevant as local pH change usually leads to alterations of the bioenergetics process of microbial communities and hence on the performance of bioelectrochemical devices.

The electroactivity of adsorbed flavocytochrome c3, a tetraheme FAD-containing flavoenzyme isolated from the bacterium Shewanella putrefaciens, is investigated by cyclic voltammetry at an edge plane pyrolytic graphite electrode before and after modification with grafted catechol serving as an efficient pH sensor based on a redox readout. Flavocytochrome c3 adsorption onto the unmodified or modified electrode surface is successfully achieved by cyclic voltammetry (100 consecutive cycles) in a flavocytochrome c3 solution containing polymyxin as co-adsorbate. The immobilized flavocytochrome c3 retains its electrochemical activity and its catalytic fumarate reductase activity. The redox activity of the protein arises from its FAD and four hemes cofactors. The experiments evidence that the hemes' redox potential of flavocytochrome c3 from Shewanella putrefaciens, for which no crystal structure is yet available, depend on pH which is at variance with data from the other strains Shewanella frigidimarina or Shewanella oneidensis. authorsversion published

The electrochemical reduction of aqueous pyridinium and N-methyl pyridinium ions is investigated in the absence and presence of CO2 and electrolysis reaction products on glassy carbon, Au, and Pt electrodes are studied. Unlike pyridinium, N-methyl pyridinium is not electroactive at the Pt electrode. The electrochemical reduction of the two pyridine derivatives was found to be irreversible on glassy carbon. These results confirmed the essential role of the N-H bond of the pyridinium cation. In contrast, the electrochemical response of N-methyl pyridinium ion at the glassy carbon electrode suggests that a specific interaction occurs between the glassy carbon surface and the aromatic ring of the pyridinium derivative. For all electrodes, an enhancement of current was observed in the presence of CO2 . However, NMR spectroscopy of the solutions following electrolysis showed no formation of methanol or other possible byproducts of the reduction of CO2 in the presence of either pyridinium derivative ion.

We show that discrete collisions of zeptoliter emulsion droplets on an ultramicroelectrode (UME) can be used as individual controlled potential coulometry experiments, lasting between 100 and 500 milliseconds. By loading a highly hydrophobic toluene droplet with a hydrophobic analyte of interest and knowing the volume of the droplet to a high degree of precision, Faraday’s Law can be employed to calculate the number of electrons passed during the electrolysis. Monodisperse (±15 % of the average size) emulsion systems were created by ultrasonication followed by filtering through a 200 nm porous filter. Discrete droplet collision events were observed in the amperometric i-t curve. Each of these collisions are interpreted as individual coulometry experiments, implying that several bulk electrolyses can be carried out over the course of one collision experiment. Herein, we show calculations of the electron stoichiometry for the ferrocene oxidation reaction, which agrees well with the expected value of 1 electron. We further extend the methodology to more complicated systems, such as the oxidation of tetrathiafulvalene (TTF), tertiary aliphatic amines, such as tripropylamine (TPrA), and a 1,2-diphenylhydrazine (DPH) molecule. This electroanalytical methodology allows for fast, nanoscale electrolysis in low dielectric media.

International audience

Redox properties have been imparted to activated carbon with a high degree of functionalization by chemical grafting of 2-amino-4,5-dimethoxybenzoic acid in situ diazotized. The diazotization reaction was accomplished in the presence or in the absence of HCl for estimating the positive or negative effect of the carboxylic acid substituent on the grafting yield. Thermal gravimetric analysis, X-ray photoelectron spectroscopy and cyclic voltammetry experiments show that when the carboxylic acid group participates to the diazotization reaction, the grafting yield is improved and becomes even better than when the carboxylic group is not present, increasing the capacitance of pristine carbon electrode from 120 to 200 F/g. Keywords: Activated carbon, Diazonium salt, Catechol, Chemical grafting, Supercapacitor

International audience; A polyol process assisted by pulse microwave activation was used to prepare efficient Pt/C electrocatalysts for PEMFC applications with reducing cost. Catalysts from pulsed microwave method were compared with a catalyst issued from a classical method, in terms of active surface area, platinum loading and activity towards the oxygen reduction reaction. A design of experiments (DOE derived from the Taguchi method) has been implemented to optimize experimental parameters only related to pulse microwave activation, the intrinsic synthesis parameters (concentration of platinum salt, platinum/carbon weight ratio and pH) being kept constant. Controlled parameters were duration of microwave pulse, maximum temperature and total duration of the synthesis. Considered responses were catalyst active surface area and the Pt/C loading. An optimized configuration of synthesis parameter was proposed. The confirmation experiment revealed a trend in agreement with that expected. Three catalysts (two from pulsed microwave synthesis method and one prepared by the classical method) were characterized by transmission electron microscopy, cyclic voltammetry and CO stripping. Catalysts from pulsed microwave method display higher characteristics than the one prepared by the classical method. The Pt/C catalyst from the confirmation experiment displays the highest catalytic activity toward oxygen reduction reaction.