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lron("0") porphyrins catalyze the electrochemical reduction of C02. The main reduction product is CO. In DMF, with tetraalkylammonium salts as supporting electrolyte, the porphyrin is however destroyed by carboxylation and/or hydrogenation of the ring after a few catalytic cycles. The presence of a hard electrophile such as Mg2+ ion dramatically improves the rate of the reaction, the production of CO, and, most importantly, the stability of the catalyst. The reaction mechanism involves the introduction of one molecule of C02 into the iron coordination sphere. The addition of a second molecule of C02 acts as a Lewis acid and then allows the breaking of one C-0 bond of the first C02 molecule thus leading to CO. This process is accelerated by Mg2+ ions in a way that depends upon the temperature. At low temperatures (-40 °C), the Mg2+ ions facilitate the decomposition of the complex containing two molecules of C02, whereas, at room temperature, Mg2+ ions triggers the breaking of the bond at the level of the complex containing a single molecule of C02 in its coordination sphere. The combined action of iron("0") porphyrins and of Mg2+ ions offers a remarkable example of a bimetallic catalysis where an electron-rich center starts the reduction process and an electron-deficient center assists the transformation of the bond system. catalytic reactions and on metal surfaces.

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Clostridium cellulolyticum produces cellulolytic complexes (cellulosomes) made of 10-13 cell wall degrading enzymes tightly bound to a scaffolding protein (CipC) by means of their dockerin domain. It has previously been shown that the receptor domains in CipC are the cohesin domains and that the cohesin/dockerin interaction is calcium-dependent. In the present study, surface plasmon resonance was used to demonstrate that the free cohesin1 from CipC and dockerin from CelA have the same K(D) (2.5 x 10(-)(10) M) as that of the entire CelA and a larger fragment of CipC, the latter of which contains, in addition to cohesin1, a cellulose binding domain and a hydrophilic domain of unknown function. This demonstrates that neither the catalytic domain of CelA nor the noncohesin domains of CipC have any influence on the interaction. Dockerin domains are composed of two conserved segments of 22 residues: removal of the second segment abolishes the affinity for cohesin1, whereas modified dockerins having twice the first segment, twice the second, or both segments but in a reverse order have K(D) values for cohesin1 in the same range as that observed for wild-type dockerin. These data indicate that if two segments are required for the complexation with the cohesin, segments 1 and 2 are similar enough to replace each other. Calcium overlay experiments revealed that the dockerin domain has one calcium binding site per conserved segment. Circular dichroism performed on wild-type and mutant dockerins indicates that this domain is well structured and that removal of calcium only weakly affects the secondary structure, which remains 40-45% helical.

The mechanism of the electrochemical reduction of vitamin Bi2r into Bi2S is analyzed using mainly cyclic voltammetry. Adsorption of the neutral Bi2r forms is minimized by addition of a solubilizing salt which allows a systematic investigation as a function of pH and sweep rate. The reduction mechanism involves, according to pH, the intermediacy of the protonated and neutral base-off Bi2r, II-OH+ and II-O, the base-on Bj2r, II-C, the protonated and neutral base-off Bi2S, I-OH and I-O-, the base-on Bi2S, I-C-, and the cobalt hydride, I-OH2-. Below pH 2.9 the reduction is fast, with II-OH+ giving rise to a mixture of I-OH and I-OH 2+, the latter being formed predominantly below pH 1. Kinetic control by the base-on/ base-off reaction appears above pH 2.9 and increases up to pH 4.7, reaching then a steady magnitude. The first reduction path then involves II-O continuously regenerated from II-C by the rate-determining coordination of the nucleotide side chain. Direct rate-determining electron transfer to II-C is observed at more negative potentials leading to I-Cwhich is immediately converted into I-O-. Thermodynamic and kinetic data featuring quantitatively the oxidation-reduction process are given.