Redox cycling of quinones reduced by ascorbic acid

Area of Science:

• Biochemistry
• Chemical Kinetics

Background:

• Quinone redox cycling in aqueous solutions with ascorbic acid and oxygen generates reactive oxygen species (ROS).
• This process is paradoxical due to unfavorable reduction potentials, requiring low product concentrations (monodehydroascorbate and superoxide).
• Existing disproportionation pathways are insufficient to manage these product levels.

Purpose of the Study:

• To elucidate the mechanism of ascorbate-driven quinone redox cycling.
• To understand how reactive oxygen species are generated in this system.
• To predict hydrogen peroxide generation rates under various conditions.

Main Methods:

• Parallel monitoring of quinone redox status and redox cycling rates.
• Investigating the influence of quinone and ascorbate concentrations.
• Analyzing the effect of quinone reduction potential on cycling rates.

Main Results:

• Evidence supports a mechanism where monodehydroascorbate is oxidized by the semiquinone.
• Quinone cycling (semiquinone/hydroquinone) facilitates rapid monodehydroascorbate disproportionation.
• The cycling rate depends on quinone, ascorbate concentrations, and quinone reduction potential.

Conclusions:

• A novel mechanism explains the control of ROS generation during ascorbate-driven redox cycling.
• This mechanism accounts for observed dependencies on reactant concentrations and redox potentials.
• The findings enable prediction of hydrogen peroxide formation rates with different quinones and conditions.