HypA and HypB are maturation proteins required for incorporation of nickel into the hydrogenase large subunit. To examine the functions of these proteins in nickel insertion, the hybF gene, which is a homolog of hypA essential for maturation of hydrogenases 1 and 2 from Escherichia coli, was overexpressed, and the product was purified. This protein behaves like a monomer in gel filtration and contains stoichiometric amounts of zinc but insignificant or undetectable amounts of nickel and iron. In filter binding assays radioactively labeled nickel binds to HybF with a K D of 1.87 M and in a stoichiometric ratio. To identify amino acid residues of HybF involved in nickel and/or zinc binding, variants in which conserved residues were replaced were studied. An H2Q replacement eliminated both in vivo activity and in vitro binding of nickel. The purified protein, however, contained zinc at the level characteristic of the wild-type protein. When E3 was replaced by Q, activity was retained, but an E3L exchange was detrimental. Replacement of each of the four conserved cysteine residues of a zinc finger motif reduced the cellular amount of HybF protein without a loss of in vivo activity, indicating that these residues play a purely structural role. A triple mutant deficient in the synthesis or activity of HypA, HybF, and HypB was constructed, and it exhibited the same responsiveness for phenotypic complementation by high nickel as mutants with a single lesion in one of the genes exhibited. The results are interpreted in terms of a concerted action of HypB and HybF in nickel insertion in which HybF (as well as its homolog, HypA) functions as a metallochaperone and HypB functions as a regulator that controls the interaction of HybF with the target protein.

An experimental and theoretical study of the influence of solution chemistry on the electrodeposition of copper from complexing citrate baths is proposed and discussed. The behavior of the system is described in terms of the relative distribution of various copper-citrate complexes, combined with a model mechanism for electrodeposition kinetics involving an adsorbed blocking intermediate. Studies of partial-current polarization curves for copper deposition over a wide range of solution pH and free citrate concentration substantiate the mechanism and offer convincing evidence for the significant role of solution chemistry in the electroreduction process. In addition to the copper system, the mechanism proposed offers a framework that may be useful for the study of other metals and alloys electrodeposited from complexing baths containing citrate or citrate-like molecules.

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The use of micro structured reactors is an accepted technology in fundamental chemical research as well as in industrial applications. The application of electrochemical microreactors (ECMR) has not attracted as much attention as continuously performed reactions in a confined space. Nevertheless ECMRs are in use to perform electro-organic reactions. In this review, different aspects of ECMRs with structured electrodes and interelectrode distances of mainly B100 lm are investigated and discussed, together with various manufacturing techniques and prototypes described therein. Based on representative examples described in various publications for electrolysis (for direct and indirect electrolysis) advantages and disadvantages of electrochemical microreactors are presented and compared with those of conventional electrochemical reactors. List of symbolsA e Electrode surface A e = Lw (m 2 ) a e Volume specific electrode area (m -1 ) a inv Specific investment costs (€/m 2 h) A, B, C, D Reagent or product of the electrochemical reaction b A Kinetic constant of the reaction involving reagent A (V -1 ) b inv Current costs (€/kW h) C A Concentration of reagent A (mol m -3 or mol L -1 ) D Inter-electrode gap (m) d e Electrode thickness (m) d eq Equivalent or hydraulic diameter D Diffusion coefficient (m 2 s -1 ) E c , E a Cathodic, anodic potential (V) E c e ; E a e Cathodic, anodic equilibrium potential (V) E cell Cell voltage (V) E tn Thermoneutral cell voltage F Faraday constant (96,486 A s mol -1 ) i Current density (A m -2 )

An electrochemical microreactor for organic electrosynthesis has been investigated for the anodic synthesis of 4-methylanisole to 4-methoxy-benzaldehydedimethylacetal in methanol solution. Selectivity and conversion in the single-pass thin-gap flow reactor were examined as a function of the composition of the electrolyte solution, the flow rate and the applied current. The experimental results indicate that potassium fluoride currently used for industrial synthesis and providing higher yields than sodium perchlorate, exerts an influence on the reaction mechanism: high KF concentrations facilitate the undesired oxidation of the diacetal. Nevertheless, a feed solution containing 0.1 M anisole in 0.01 M KF can be converted at 90% in the 100 lm thin-gap cell with acceptable voltages and a measured selectivity of nearly 87%. The selectivity observed substantially higher than that typically observed in conventional electrochemical cells.

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