**3. Ascorbic acid as an electron donor**

Ascorbic acid is a reducing agent and thus an electron donor. The electron donation ability of ascorbic acid is responsible for its biochemical and physiological roles [14]. The double-bond carbon between the second and third of the 6-carbon molecules of ascorbic acid give two electrons. Ascorbic acid is often referred to as an antioxidant owing to the fact that it prevents other molecules from being consumed by donating electrons [15]. Ascorbic acid as an antioxidant and its interaction with reactive nitrogen species and singlet oxygen has been extensively documented [16]. Ascorbic acid has been utilized as an antioxidant for the stabilization of oxidationprone medications. These medications include vitamin A, morphine, cholecalciferol, rifampin, promethazine, and sulphacetamide [17].

Electrons from ascorbic acid can react with metals such as copper and iron, leading to the generation of superoxide and hydrogen peroxide, which may later result in the formation of ROS. Consequently, in certain situations, ascorbic acid will give rise to an oxidizing agent by way of its operation as a reducing agent. This reaction occurs in the biological system when different pharmacological doses of ascorbic acid are absorbed in the plasma and extracellular fluid compartment as well as low doses of ascorbic acid in cell culture media containing metals [18].

When vitamin C donates electrons, the electrons are lost sequentially. The product formed after the loss of one electron is a free radical, semidehydroascorbic acid, or ascorbyl radical. When compared with other free radicals, the ascorbyl radical is different because its half-life can be measured in many seconds or even minutes depending on the absence or the presence of oxygen or electron acceptors, especially iron [19]. Ascorbyl radical is relatively stable and fairly reactive. This attribute explains why ascorbic acid may be a preferred antioxidant. In other words, a reactive and a harmful free radical can react with ascorbic acid [20]. The reduction of a reactive free radical with the generation of a less-reactive product is sometimes called free radical scavenging or quenching. Therefore, ascorbic acid is a good free radical scavenger due to its chemical composition [21]. For instance, under some conditions, ascorbyl radical could be estimated in blood and extracellular fluid samples [22].

Upon the loss of second electron, a more stable species, dehydroascorbic acid is formed when compared with ascorbyl free radical. The stability of dehydroascorbic acid depends on temperature and pH of the medium, but is often only in minutes. Dehydroascorbic acid has affinity for facilitated glucose transporters and is transported by a number of them [23]. Variety of oxidants in biological system mediate the formation of both ascorbyl radical and dehydroascorbic acid which includes molecular oxygen, superoxide, hydroxyl radical, hypochlorous acid, reactive nitrogen species, and trace metals like iron and copper. Both dehydroascorbic acid and ascorbyl radical could be reversibly broken down to ascorbic acid *via* different metabolic pathways and also through oxidizing compounds inside the body system including glutathione [24]. All the ascorbic acids consumed by human beings cannot be regained because there exists just fragmental depletion back to ascorbic acid.

**Figure 2.** *Redox chemistry of vitamin C.*

In case of nonreduction of dehydroascorbic acid to ascorbic acid, it will be broken down to 2, 3-diketogulonic acid irreversibly due to the rupture of the lactone ring structure that is a component of ascorbic acid, ascorbyl radical, and dehydroascorbic acid. The 2, 3-diketogulonic acid is metabolized further into other compounds such as xylose, xylonite, lyxonate, and oxalate [25]. The development of oxalate is clinically significant because some persons with hyperoxaluria (excessive oxalate excretion) develop oxalate kidney stones (**Figure 2**).
