Prediction of methyl viologen redox states for biological applications

• MV can have both positive and adverse effects on biological processes.
• A thermodynamic model was effectively used to predict MV redox states.
• An electrode system can be used to control the MV redox states.
• Slight changes in potential can greatly affect the forms of MV.
• MV can exist as MV2+, MV+, or MV0, MVCl2, MV22+, MVH2+, and as crystalline films.

Methyl viologen (MV) is an electron mediator that has been shown to be beneficial for enhancing product formation in biofuel processes. For example, increased yields have been observed for ethanol and butanol production. MV has also been used in bioremediation processes such as removal of pollutants from groundwater. However, MV has also been shown to have detrimental effects including inhibition of cell growth. Unfortunately, studies have shown little information regarding the MV redox state. A thermodynamic model is presented and validated in order to predict the most likely redox state of MV depending on the redox potential, pH, and degree of MV dimerization. Model results showed that there are narrow potential ranges in which MV can change from one redox state to another. During fermentation processes in which the potential is in these narrow ranges, slight perturbations in the redox potentials can drastically change the redox state of MV. This may explain the observed variations in the positive and negative effects of MV. Thus, caution should be used when applying MV to any biological system. If needed, an electrode system may prove useful for controlling the redox potential and the associated MV redox state.