## 4 Natural Antioxidants and Their Role in Cosmetics
### Vitamins  
#### 4.1.1 Vitamin C (Ascorbic Acid)  
Vitamin C (Figure 2), also known as ascorbic acid, is a highly versatile and biologically active vitamin with a wide range of effects on human physiology. It exhibits potent reducing properties, allowing its conversion to L-dehydroascorbic acid, the oxidized form of ascorbic acid, which retains equivalent vitamin activity. This compound plays a critical role in numerous biochemical transformations and metabolic processes, facilitating essential cellular functions [23]. Ascorbic acid acts as an excellent electron and hydrogen donor due to its structural composition, which includes adjacent hydroxyl and carbonyl groups. It can donate two electrons, making it a crucial cofactor in various enzymatic reactions. Upon oxidation, ascorbic acid is converted into the ascorbic anion and, through electron donation, forms the ascorb  
Figure 2: Chemical structures of antioxidant vitamins: **(1)** vitamin C, **(2)** vitamin E, **(3)** vitamin A, and **(4)** vitamin B5. Notably, within the intracellular environment, vitamin C serves as the principal antioxidant in the cytosolic fraction. It can cross the blood-brain barrier in its oxidized form via glucose transporters, maintaining its antioxidant capacity within the central nervous system [25]. Vitamin C is particularly concentrated in the brain, where it primarily reduces reactive oxygen species and oxidized tocopherols. Dehydroascorbate is released by neurons, captured by astrocytes, and then transformed back into its reduced form before being supplied to neurons. Oxidized ascorbate is regenerated by glutathione, NADPH, GSH-dependent dehydrogenase, and omega-class GST [26]. Furthermore, ascorbic acid is essential for maintaining the reduced form of metal ions that are part of some enzymes, though this action can have harmful effects. During the two-step oxidation of ascorbate in the presence of Cu\({}^{2+}\) and Fe\({}^{3+}\), Cu\({}^{1+}\) and Fe\({}^{2+}\) are formed. These species react with hydrogen peroxide in the Fenton reaction, producing hydroxyl radicals. In addition to its antioxidant functions, vitamin C is involved in the biosynthesis of collagen, serotonin, steroid hormones, dopamine, and noradrenaline [27; 28]. In cosmetics, vitamin C serves multiple essential roles, acting as an antioxidant, pH regulator, preservative, skin protectant, and ultraviolet (UV) filter with potential photoprotective effects. When applied in its free ascorbic acid form at low pH, it exhibits keratolytic effects. Additionally, vitamin C derivatives are widely used to prevent skin hyperpigmentation and stimulate collagen synthesis. It is a key ingredient in dermocosmetic products for sensitive skin and skin prone to capillary fragility and rosacea. Ascorbic acid also exhibits anti-inflammatory properties, enhancing overall skin health [29]. When combined with vitamin E, vitamin C functions as a synergistic antioxidant, amplifying protection against oxidative stress [30; 31]. As a potent antioxidant, it effectively neutralizes reactive oxidants generated by environmental pollutants and UV radiation. This effect is particularly significant in the epidermis, where vitamin C concentrations are higher. Repeated UV exposure depletes ascorbic acid (AA) levels in the skin. Although AA does not absorb UVA (320-400 nm) or UVB (290-320 nm) radiation, its topical application provides photoprotective benefits due to its strong antioxidant and anti-inflammatory properties [29]. A deficiency in vitamin C can lead to various dermatological symptoms, including skin fragility, corkscrew-shaped hairs, and impaired wound healing, indicative of early-stage scurvy. While vitamin C has been shown to mitigate radiation-induced skin damage from acute UV exposure, its potential for reversing photoaging caused by chronic sun damage remains an area of ongoing research [30].