A theoretical study of ascorbic acid oxidation and HOO˙/O₂˙^- radical scavenging

Area of Science:

• Biochemistry
• Chemical Physics

Background:

• Ascorbic acid (vitamin C) is a potent antioxidant and radical scavenger.
• Its oxidation involves sequential electron and proton loss, forming a reactive ascorbate radical.
• The ascorbate radical exhibits unusual reactivity, primarily disproportionation or reaction with other radicals.

Purpose of the Study:

• To elucidate the oxidation mechanism of ascorbic acid.
• To calculate key physicochemical properties like pK_a and reduction potentials.
• To understand the pH-dependent reactivity of ascorbic acid and its radical.

Main Methods:

• Utilized B3LYP/6-31+G(d,p) and CBS-QB3 computational methods.
• Employed the SMD implicit solvent model with explicit water molecules.
• Constructed a potential-pH (Pourbaix) diagram using calculated data.

Main Results:

• The most stable form of dehydroascorbic acid in water is the bicyclic hydrated structure.
• Thermodynamically favored disproportionation of the ascorbate radical at physiological pH.
• Proposed a mechanism involving dimerization, internal electron transfer, cyclization, and hydrolysis.

Conclusions:

• The calculated mechanism explains ascorbic acid radical reactions, including with superoxide.
• Disproportionation is a key pathway for ascorbate radical deactivation.
• Computational chemistry provides insights into antioxidant redox mechanisms.