Dimethylsulfide (DMS) dehydrogenase is a complex heterotrimeric enzyme that catalyzes the oxidation of DMS to DMSO and allows Rhodovulum sulfidophilum to grow under photolithotrophic conditions with DMS as the electron donor. The enzyme is a 164 kDa heterotrimer composed of an alpha-subunit that binds a bis(molybdopterin guanine dinucleotide)Mo cofactor, a polyferredoxin beta-subunit, and a gamma-subunit that contains a b-type heme. In this study, we describe the thermodynamic characterization of the redox centers within DMS dehydrogenase using EPR- and UV-visible-monitored potentiometry. Our results are compared with those of other bacterial Mo enzymes such as NarGHI nitrate reductase, selenate reductase, and ethylbenzene dehydrogenase. A remarkable similarity in the redox potentials of all Fe-S clusters is apparent.

The syntheses and characterization of two new redox active cyclam ligands ferrocenylmethyl-(6-methyl-1,4,8,11-tetraazacyclotetradec-6-yl)-amine (L 3 ) and 1,1Ј-ferrocenylmethyl-bis(6-methyl-1,4,8,11-tetraazacyclotetradec-6-yl)-amine (L 4 ) are reported. The compounds each possess a ferrocenyl group bearing one (L 3 ) or two (L 4 ) appended macrocycles linked by their exocyclic amino groups and the crystal structures of both compounds have been determined. Anion binding of L 3 and L 4 was investigated by electrochemical titrations where H-bonding to each macrocycle causing a shift in the Fc ϩ/0 redox potential was used as a reporter of guest binding. The Zn II complex of L 3 has also been isolated and characterized structurally. These compounds were analysed for their capacity to electrochemically recognize anions in both aqueous and non-aqueous solution. We have found that L 3 , L 4 and [ZnL 3 ] 2ϩ sense Cl Ϫ and AcO Ϫ anions in MeCN-CH 2 Cl 2 , a function that is lost in aqueous solution.

Dimethyl sulfide dehydrogenase isolated from the photosynthetic bacterium Rhodovulum sulfidophilum is a heterotrimeric enzyme containing a molybdenum cofactor at its catalytic site, as well as five iron-sulfur clusters and a heme b cofactor. It oxidizes dimethyl sulfide (DMS) to dimethyl sulfoxide in its native role and transfers electrons to the photochemical reaction center. There is genetic evidence that cytochrome c2 mediates this process, and the steady state kinetics experiments reported here demonstrated that cytochrome c2 accepts electrons from DMS dehydrogenase. At saturating concentrations of both substrate (DMS) and cosubstrate (cytochrome c2), Michaelis constants, KM,DMS and KM,cyt of 53 and 21 microM, respectively, were determined at pH 8. Further kinetic analysis revealed a "ping-pong" enzyme reaction mechanism for DMS dehydrogenase with its two reactants. Direct cyclic voltammetry of cytochrome c2 immobilized within a polymer film cast on a glassy carbon electrode revealed a reversible FeIII/II couple at +328 mV versus the normal hydrogen electrode at pH 8. The FeIII/II redox potential exhibited only minor pH dependence. In the presence of DMS dehydrogenase and DMS, the peak-shaped voltammogram of cytochrome c2 is transformed into a sigmoidal curve consistent with a steady-state (catalytic) reaction. The cytochrome c2 effectively mediates electron transfer between the electrode and DMS dehydrogenase during turnover and a significantly lower apparent electrochemical Michaelis constant K'M,DMS of 13(+/-1) microM was obtained. The pH optimum for catalytic DMS oxidation by DMS dehydrogenase with cytochrome c2 as the electron acceptor was found to be approximately 8.3.