The mitochondrial amidoxime reducing component (mARC) activates amidoxime prodrugs by reduction to the corresponding amidine drugs. This study analyzes relationships between the chemical structure of the prodrug and its metabolic activation and compares its enzyme-mediated vs. electrochemical reduction. The enzyme kinetic parameters K_M and V_max for the N-reduction of ten para-substituted derivatives of the model compound benzamidoxime were determined by incubation with recombinant proteins and subcellular fractions from pig liver followed by quantification of the metabolites by HPLC. A clear influence of the substituents at position 4 on the chemical properties of the amidoxime function was confirmed by correlation analyses of ¹H NMR chemical shifts and the redox potentials of the 4-substituted benzamidoximes with Hammett’s σ. However, no clear relationship between the kinetic parameters for the enzymatic reduction and Hammett’s σ or the lipophilicity could be found. It is thus concluded that these properties as well as the redox potential of the amidoxime can be largely ignored during the development of new amidoxime prodrugs, at least regarding prodrug activation.

Manganese oxides are considered to be very promising materials for water oxidation catalysis (WOC), but the structural parameters influencing their catalytic activity have so far not been clearly identified. For this study, a dozen manganese oxides (MnO_x) with various solid-state structures were synthesised and carefully characterised by various physical and chemical methods. WOC by the different MnO_x was then investigated with Ce⁴⁺ as chemical oxidant. Oxides with layered structures (birnessites) and those containing large tunnels (todorokites) clearly gave the best results with reaction rates exceeding 1250   h⁻¹ or about 50 μmol_O2 m⁻² h⁻¹. In comparison, catalytic rates per mole of Mn of oxides characterised by well-defined 3D networks were rather low (e.g., ca. 90   h⁻¹ for bixbyite, Mn₂O₃), but impressive if normalised per unit surface area (>100  m⁻² h⁻¹ for marokite, CaMn₂O₄). Thus, two groups of MnO_x emerge from this screening as hot candidates for manganese-based WOC materials: 1) amorphous oxides with tunnelled structures and the well-established layered oxides; 2) crystalline Mn^III oxides. However, synthetic methods to increase surface areas must be developed for the latter to obtain good catalysis rates per mole of Mn or per unit catalyst mass.

Chara corallina freshwater algae produce brown deposits of manganese oxides on their cell wall surfaces when growing in manganese-rich media. We report on the formation, topology, composition, atomic structure, and catalytic activities of these biogenic manganese oxides (BMOs). The deposits are volcano shaped and exhibit 3–5 μm craters in their centers. Microfocus X-ray irradiation and detection of characteristic X-ray fluorescence lines allowed elemental mapping at 5 μm spatial resolution and the identification of the volcano-shaped deposits as a Mn–Ca oxide. X-ray absorption spectroscopy (XAS) revealed a high-valent Mn^III/IV oxide. The structural analysis involved XAS spectra collected for a single volcano at room temperature and for single cells at 20 K. On the basis of the XAS data, the oxides were identified as members of the birnessite family of layered manganese oxides containing di-μ-oxido-bridged Mn^III/IVO₆ octahedra as central building units. The deposits share structural motifs with synthetic water-oxidizing Mn–Ca oxides and with the Mn₄Ca complex of photosystem II, the biological water-oxidation catalyst. Model reactions demonstrate low, but clearly detectable, activity of the manganese deposits for water-oxidation catalysis. The biogenic manganese oxides on the cell walls of Chara corallina thus represent an intriguing object to study how manganese-based catalysts for water oxidation are formed in a biological environment. The formation of BMOs in relation to cellular ion transport and the possibility of BMOs to fulfill a detoxification function in plants were also examined.

A mild screen-printing method was developed to coat conductive oxide surfaces (here: fluorine-doped tin oxide) with micrometer-thick layers of presynthesized calcium manganese oxide (Ca–birnessite) particles. After optimization steps concerning the printing process and layer thickness, electrodes were obtained that could be used as corrosion-stable water-oxidizing anodes at pH 7 to yield current densities of 1 mA cm⁻² at an overpotential of less than 500 mV. Analyses of the electrode coatings of optimal thickness (≈10 μm) indicated that composition, oxide phase, and morphology of the synthetic Ca–birnessite particles were hardly affected by the screen-printing procedure. However, a more detailed analysis by X-ray absorption spectroscopy revealed small modifications of both the Mn redox state and the structure at the atomic level, which could affect functional properties such as proton conductivity. Furthermore, the versatile new screen-printing method was used for a comparative study of various transition-metal oxides concerning electrochemical water oxidation under “artificial leaf conditions” (neutral pH, fairly low overpotential and current density), for which a general activity ranking of RuO₂>Co₃O₄≈(Ca)MnO_x≈NiO was observed. Within the group of screened manganese oxides, Ca–birnessite performed better than “Mn-only materials” such as Mn₂O₃ and MnO₂.