**2. Antioxidants: compounds with antioxidant activity**

#### **2.1 Antioxidants: vitamins, provitamins with antioxidant activity**

*Vitamin A* - Vitamin A is a group of unsaturated nutritional organic compounds that includes retinol, retinal, and several provitamins A carotenoids (most notable beta-carotene) [20–22] (**Figure 1**).

Generally, the three major antioxidant vitamins are beta-carotene (precursor of vitamin A), vitamin C, and vitamin E. We will find them in colourful fruits and vegetables, especially those with purple, blue, red, orange, and yellow hues [23].

The active form (retinol) comes from animal sources such as milk, eggs, meat, and fatty fish, all of which may be high in fat and cholesterol. But it also comes from plants, in the form of beta-carotene and other carotenoids, which are converted into vitamin A in the body [24].

Depending on the environment, vitamin A can be converted to an ester (a) or oxidized to aldehyde (b). The chain can continue to oxidize the aldehyde of vitamin A to the specific acid [25] (**Figure 2** and **Table 2**).

The main carotenes are showed in **Figure 3** (right side) (**Figure 4**).

The group of *xanthophylls* includes (among many other compounds) lutein, zeaxanthin, neoxanthin, violaxanthin, flavaxanthin, and α- and β-cryptoxanthin (**Figures 5** and **6**).

*Vitamin E* is a fat-soluble vitamins group with 4 tocopherols and 4 tocotrienols. The tocopherol content of animal and vegetable fats (oils) is strictly influenced by animal feed (**Figure 7**).

For alpha(α)-tocopherol each of the three "R" sites has a methyl group (CH3) attached. For beta(β)-tocopherol: R1 = methyl group, R2 = H, R3 = methyl group. For gamma(γ)-tocopherol: R1 = H, R2 = methyl group, R3 = methyl group. For delta (δ)-tocopherol: R1 = H, R2 = H, R3 = methyl group. The same configurations exist for the tocotrienols, except that the hydrophobic side chain has three carbon– carbon double bonds whereas the tocopherols have a saturated side chain [43]. For alpha(α)-tocotrienol each of the three "R" sites has a methyl group (CH3) attached.

**Figure 1.** *Structure of vitamin A (Retinol).*

**Figure 2.** *Changes of vitamin A (to ester (a) or to aldehyde (b)).*


*Natural Compounds with Antioxidant Activity-Used in the Design of Functional Foods DOI: http://dx.doi.org/10.5772/intechopen.97364*

#### **Table 2.**

*The Vitamins A, provitamins with antioxidant activity.*

R1 = methyl group, R2 = H, R3 = methyl group in beta(β)-tocotrienol. R1 = H, R2 = methyl group, R3 = methyl group – in gamma(γ)-tocotrienol. R1 = H, R2 = H, R3 = methyl group, in delta(δ)-tocotrienol. Palm oil is a good source of alpha and gamma tocotrienols (**Figures 8** and **9** and **Table 3**) [58].

**Figure 3.** *The main carotenes.*

**Figure 4.** *Lycopene is a key intermediate in the biosynthesis of many carotenoids.*

**Figure 5.** *The Chemical Structure of Lutein and Zeaxanthin.*

*Natural Compounds with Antioxidant Activity-Used in the Design of Functional Foods DOI: http://dx.doi.org/10.5772/intechopen.97364*

**Figure 6.** *The action of carotenoids on free radicals.*

**Figure 7.** *General chemical structure of tocopherols (a) and tocotrienols (b).*

**Figure 8.** *Conversion of α-tocopherol to hydroxy alkyl quinone [44].*

#### **Figure 9.**

*Tocopherols function by donating H atoms to radicals (X) [45].*

Oxidation of L-ascorbic acid to dehydroascorbic acid (**Figures 10** and **11**) depends on several parameters: oxygen partial pressure, pH, temperature and the presence of metal ions (**Figure 12**). Traces of metal ions - especially Cu2+ and Fe3+ result from losses or transfers of substances (insufficiently controlled reaction media, insufficiently protected packaging).

#### *2.1.1 Vitamins K*

Actively participates in cellular oxidations, by reversing the transition from oxidized to reduced form, ensuring the transport of hydrogen non-enzymatically (**Figure 13**).


## **Table 3.**

*The Vitamins E and C activity.*

A number of other vitamins (B vitamins, vitamin K) together with their precursors are active in redox processes in the body and can be very good antioxidants, especially in reduced forms (hydrogenated forms). These bio-compounds are the first to oxidize, protecting the cellular environment from free radical attack.
