Coenzyme B12 model compounds: A quantitative electrochemical comparison

Spin-chemical effects on intramolecular photoinduced charge transfer reactions in bisphenanthroline copper(i)-viologen dyad assemblies

Two covalently linked donor-acceptor copper phenanthroline complexes (C-A dyads) of interest for solar energy conversion/storage schemes, [Cu(i)(Rphen(OMV)2 4+)2]9+ = RC+A4 8+ with RC+ = [Cu(i)Rphen2]+ involving 2,9-methyl (R = Me) or 2,9-phenyl (R = Ph)-phenanthroline ligands that are 5,6-disubstituted by 4-(n-butoxy) linked methylviologen electron acceptor groups (A2+ = OMV2+), have been synthesized and investigated via quantum chemical calculations and nanosecond laser flash spectroscopy in 1,2-difluorobenzene/methanol (dfb/MeOH) mixtures. Upon photoexcitation, charge transfer (CT) states RC2+A+A3 6+ are formed in less than one ns and decay by charge recombination on a time scale of 6-45 ns. The CT lifetime of RC2+A+A3 6+ has a strong dependence on MeOH solvent fraction when R = Me, but is unaffected if R = Ph. This solvent effect is due to coordination of MeOH solvent in MeC+A4 8+ (i.e. exciplex formation) allowed by conformational flattening of the ligand sphere, which cannot occur in PhC+A4 8+ having bulkier Phphen ligand framework. Interestingly, the decay time of the CT state increases for both species at low magnetic fields with a maximum increase of ca. 30% at ca. 150 mT, then decreases as the field is increased up to 1500 mT, the highest field investigated. This magnetic field effect (MFE) is due to magnetic modulation of the spin dynamics interconverting 3CT and 1CT states. A quantitative modeling according to the radical pair mechanism involving ab initio multireference calculations of the complexes revealed that the spin process is dominated by the effect of Cu hyperfine coupling. The external magnetic field suppresses the hyperfine coupling induced spin state mixing thereby lengthening the CT decay time. This effect is counteracted by the field dependent processes of T0-S mixing through the Δg-mechanism and by a local mode spin-orbit mechanism. Further, the maximum MFE is limited by a finite rate of direct recombination of 3CT states and the spin-rotational mechanism of spin relaxation. This study provides a first comprehensive characterization of Cu(ii)-complex spin chemistry and highlights how spin chemistry can be used to manipulate solar energy harvesting and storage materials.

The inhibition of corrinoid-catalyzed oxidation of mercaptoethanol by methyl iodide: mechanistic implications

The cobalamin coenzymes (5'-deoxyadenosyl- and methylcobalamin) and their cobinamide counterparts (5'-deoxyadenosyl- and methylcobinamide) catalyze the oxidation of 2-mercaptoethanol to its disulfide with hydrogen peroxide formation under aerobic conditions. The reactions are blocked by methyl iodide. Inhibition by methyl iodide is apparently due to the formation of the trans dialkyl corrinoids: methyl(adenosyl)cobalamin, dimethylcobalamin, methyl(adenosyl)cobinamide, and dimethylcobinamide, respectively. When the reaction system is illuminated with visible light, inhibition is released and a dramatic enhancement in the rate of oxygen consumption occurs. For reactions catalyzed by adenosyl- and methylcobalamin and then inhibited by methyl iodide, the rates observed during photolysis approach those obtained with aquacobalamin. For reactions catalyzed by adenosyl- and methylcobinamide and then inhibited by methyl iodide, the rates observed during photlysis approach those obtained with diaquacobinamide. Thus, both trans axial carbon-cobalt bonds in the putative dialkyl corrinoid are homolyzed during photolysis. In contrast to these results, the catalysis of the aerobic oxidation of 2-mercaptoethanol by aquacobalamin is only weakly inhibited by methyl iodide. This observation suggests that aquacob(II)alamin is produced during the catalysis of this reaction. Superoxide, the anticipated product of the reaction between aquacob(II)alamin and dioxygen, is formed during aquacobalamin-catalyzed 2-mercaptoethanol oxidation since superoxide dismutase decreases the rate of oxygen consumption by 50%. However, the enzyme has no effect on oxygen uptake during reactions catalyzed by cobalamin coenzymes and their cobinamide counterparts. These corrinoid catalysts apparently transfer two electrons to dioxygen from cobalt(I) intermediates formed during the reactions. Nitrogenous bases inhibit corrinoid-catalyzed thiol oxidation by competing with 2-mercaptoethanol for axial-ligand coordination sites on the catalyst. In contrast to the inhibition observed with methyl iodide, visible light has no effect on the inhibition obtained with nitrogenous bases.