Temperature dependent electrochemical investigations of molybdenum and tungsten oxobisdithiolene complexes

Abstract

To achieve a better understanding why thermophilic and hyperthermophilic organisms use tungsten instead of molybdenum within the active sites of their molybdopterin dependent oxidases, electrochemical investigations of model complexes for the active sites of enzymes belonging to the DMSO reductase (molybdenum) and the aldehyde oxidoreductase (tungsten) family have been undertaken. Cyclic voltammetry and differential pulse voltammetry of four pairs of molybdenum and tungsten oxobisdithiolene compounds show huge differences in the response of their redox potentials to rising or decreasing temperatures, depending on the substituents at the dithiolene group. The mnt²⁻ compounds (1a, 1b) respond with decreasing redox potentials E_1/2 to rising temperatures whereas all other compounds show positive gradients δE/δT. In every case the values for the gradients for the tungsten compounds are greater than those for the molybdenum compounds. Six of the investigated compounds are known in the literature and two compounds were newly synthesized. These two new compounds include the pyrane subunit of the native molybdopterin ligand and should therefore be even better models for the active site of the molybdopterin containing enzymes. The molybdenum/tungsten pair with these new ligands shows a remarkably small difference for the redox potentials of the transition M^IV↔ M^V of only 30 mV at 25 °C and the reversion of the usual order with higher potentials for the molybdenum than the tungsten compound at a temperature of 70 °C; a temperature that is in the range where usually tungsten containing enzymes instead of molybdenum containing ones are found.