*2.2.1 Phenols and Polyphenols with antioxidant activity*

Flavonoids (or bioflavonoids; from the Latin word flavus, meaning yellow, their colour in nature) are a class of polyphenolic secondary metabolites found in plants, and thus commonly consumed in diets. Flavonoids are a well-known family of plant polyphenolic compounds. Flavonoids are represented by 6 major subclasses, present in the basic diet in humans: anthocyanidins, flavan-3-ol, flavonols, flavanones, flavones and isoflavones, flavonols.

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

#### **Figure 10.**

*Forms of ascorbic acid: I-L-Ascorbic acid, II-Dehydroascorbic Acid, III-2,3-Dicetogulonic acid; Hydrated IVhemiacetal.*

#### **Figure 11.**

*L-ascorbic acid is a powerful reducing agent.*

**Figure 12.** *The electron transfer during metal catalysis.*

#### **Figure 13.** *The redox system of vitamin K.*

Anthocyanidins are vegetable pigments, similar to anthocyanins but lacking in the carbohydrate side (**Table 4**). Their activity is based on that of the flavylium cation and the oxonium ion, which have various replacement groups of hydrogen atoms. Depending on the pH, these pigments can have various colours: red, purple, blue, and bluish green [59] (**Figures 14** and **15**).

*Flavan-3-ols* (sometimes referred to as flavanols) are derivatives of flavans that possess a 2-phenyl-3,4-dihydro-2H-chromen-3-ol skeleton.

These compounds include catechin, epicatechin-gallate, epigallocatechin, epigallocatechin gallate, pro-anthocyanidins, theaflavins, thearubigins (**Figure 16**).

Until 2013, both the Food and Drug Administration and the European Food Safety Authority did not issue restrictions on the use of catechins, nor did they approve any catechin-based medicines. [60] (**Figure 17**).

*Flavonols* are a class of flavonoids that have the 3-hydroxyflavone backbone (IUPAC name: 3-hydroxy-2-phenylchromen-4-one). Their diversity stems from the different positions of the phenolic -OH groups. Flavonols are present in a wide variety of fruits and vegetables. In Western populations, estimated daily intake is in the range of 20–50 mg per day for flavonols. Individual intake varies depending on the type of diet consumed [61]. The most used flavonols: Isorhamnetin, Kaempferol, Myricetin, Quercetin (**Figure 18**).


#### **Table 4.**

*The main anthocyanidins and their substitution radicals.*

**Figure 14.** *Flavylium Cation (a) and general pyramidal oxonium ion (b).*

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

**Figure 15.** *Basic Structure of Anthocyanidins (R3' and R4' = -OH).*

**Figure 16.** *Chemical Structure of Flavan-3-ols.*

**Figure 17.** *Chemical Structure of Catechins.*

Flavones (derived by the Latin flavus "yellow") are a class of flavonoids based on the nucleus of 2-phenylchromen-4-one (2-phenyl-1-benzopyran-4-one). Apigenin (4 ', 5,7-trihydroxyflavone), luteolin (3', 4 ', 5,7-tetrahydroxyflavone), tangeritine (4', 5,6,7,8-pentamethoxyflavone), chrysin (5,7-dihydroxyphlavone)) and 6-hydroxyflavone are compounds that belong to the class of flavones [62].


#### **Figure 18.**

*Chemical Structure of the main Flavonols and their substitution radicals.*

In plants, a number of flavonoid glycosides often appear, which are in fact colourless aromatic ketones, derived from flavone (flavanone). [63].

Isoflavones are substituted derivatives of isoflavone, a type of naturally occurring isoflavonoids [64] many of which act as phytoestrogens in mammals [65]. Isoflavones are produced almost exclusively by the members of the bean family, Fabaceae (*Leguminosae*) (**Figures 19** and **20**).

**Figure 20.** *Chemical Structure of the main isoflavones.*

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

The consumption of isoflavones-rich food or dietary supplements is under preliminary research for its potential association with lower rates of postmenopausal cancer [66, 67] and osteoporosis in women [68]. Use of soy isoflavone dietary supplements may be associated with reduction of hot flashes in postmenopausal women [67, 68] (**Figure 21**).

#### **2.3 Antioxidants: acids, amino acids and other compounds with antioxidant activity**

#### *2.3.1 Lipoic acid*

Lipoic acid (LA) is an organo-sulfurized compound of caprylic acid (octanoic acid). It is also known - in the technical literature and as α-lipoic acid (ALA) and thioctic acid [69] (**Figures 22** and **23**).

In cells, α-LA can be reduced to dihydrolipoic acid, the more bioactive form of LA, involved in antioxidant processes that lead to decreased redox activities of iron and copper ions in solutions. [70]. Recent research has shown that the anti-aging and cellular disease prevention effects are mainly due to genetic mechanisms that improve the antioxidant state of the cell. However, this likely occurs via prooxidant mechanisms, not by radical scavenging or reducing effects [71–73]. α-Lipoic acid is an antioxidant that acts in both forms (both oxidized and reduced) on tissues and lipo- and water-soluble substances. It can be easily reduced by breaking the disulfide bridge with the formation of sulfhydryl groups. The di-hydrolipoic form of α-lipoic acid is regenerated by the redox mechanisms of vitamins C and E.

**Figure 21.** *The main structures of flavonoids.*

*Functional Foods - Phytochemicals and Health Promoting Potential*

**Figure 22.** *Structure of α-Lipoic Acid.*

**Figure 23.** *α-lipoic acid (α-LA) and the two forms in which it is found (oxidized and reduced).*

#### *2.3.2 Folic acid*

The tetrahydrofolate (II) derivative of folic acid (I) is the enzymatic cofactor that can transfer a carbon unit in various oxidation states (such as in formyl or hydroxymethyl residues) (**Figure 24**).

Folate contributes major to spermatogenesis. In women, folate is important for oocyte quality and maturation, implantation, placentation, fetal growth and organ development [74].

#### *2.3.3 Cysteine*

Cysteine (symbol Cys) [75] is a semi essential [76] proteinogenic amino acid with the formula HOOC-CH-(NH2)-CH2-SH. The thiol side chain in cysteine often participates in enzymatic reactions, as a nucleophile. Due to the ability of thiols to undergo redox reactions, cysteine has antioxidant properties. Its antioxidant properties are typically expressed in the tripeptide glutathione, which occurs in humans and other organisms. The systemic availability of oral glutathione (GSH) is negligible; so, it must be biosynthesized from its constituent amino acids, cysteine, glycine, and glutamic acid [77]. While glutamic acid is usually sufficient because amino acid nitrogen is recycled

**Figure 24.** *Folic acid (I) and tetrahydrofolate derivative (II).*

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

through glutamate as an intermediary, dietary cysteine and glycine supplementation can improve synthesis of glutathione [78]. Cysteine and cystine - form an important redox system, whose steady-state depends on oxidation conditions (**Figure 25**).

#### *2.3.4 Glutathione*

Glutathione (γ-L-glutamyl-L-cysteinyl-glycine) is found in both animals, plants, and microorganisms (**Figure 26**).

The active group of glutathione is -SH, through which glutathione can participate in redox reactions, having a reduced form marked with G-SH and an oxidized one (with disulfide bridge, G-S-S-G, according to the Eq. (1)):

$$\begin{array}{ll}\text{2GSH} \xleftarrow{\longrightarrow} \text{G-S-S-G} + 2\text{H} \\ \text{Reduced form} \end{array} \tag{1}$$

GSH protects cells by neutralizing single reactive oxygen species [79–81]. This transformation is found in the reduction of peroxides:

**Figure 25.** *The redox mechanism Cysteine-Cystine.*

**Figure 26.** *Structure of Glutathione.*

$$2\text{ GSH} + \text{R}\_2\text{O}\_2 \rightarrow \text{GSSG} + 2\text{ ROH} \left(\text{R} = \text{H}, \text{alkyl}\right) \tag{2}$$

and with free radicals:

$$\text{GSH} + \text{R.} \rightarrow \text{0.5 GSSG} + \text{RH} \tag{3}$$

It maintains exogenous antioxidants such as vitamins C and E in their reduced (active) states [81].

#### **2.4 The main oxidoreductases with antioxidant activity**

*a. FAD-dependent oxidoreductases* are enzymes of a heteroproteinic nature from the group of aerobic dehydrogenases having as active groups derivatives of vitamin B2 (riboflavin or 7,8-dimethyl-10-ribithyl-isoaloxazine), namely: flavin adenine mononucleotide (FMN) and flavin dinucleotide (FAD). Flavin enzymes (FMN, FAD) are involved in electron and proton transfer reactions mediated by the isoalloxazine nucleus. They accept either an electron or a pair of electrons (unlike NAD and NADP which only accept electron pairs) (**Figure 27**).

$$\text{FAD} \left( \text{FMN} \right) + 2 \text{ H} + + 2 \text{e} - \Leftrightarrow \text{FADH}\_2 \left( \text{FMNH}\_2 \right) \tag{4}$$

Flavin-enzymes have the standard redox potential Eo between +0.19 V (oxidants stronger than NAD +) and � 0.49 V (reducing agent stronger than NADH), which shows a wide range of variation of redox properties depending on environmental conditions and the nature of the substrate (**Figure 28**). For some flavin -enzymes that also contain a metal (molybdenum or iron) in their molecule, it can stabilize the semi-quinone form by pairing the electron alone with unpaired electrons existing in metal ions; the metal can transport electrons to the respective flavin enzymes.

*b. NAD-dependent oxidoreductases* are enzymes from the class of anaerobic dehydrogenases and have as coenzymes, Nicotinamide Adenine Dinucleotide (NAD+) or reduced (NADH + H+) and Nicotinamide Adenine Dinucleotide Phosphate Oxidate (NADP+) or reduced (NADPH). These coenzymes consist of a derivative of vitamin PP, nicotinamide and an adenine-derived nucleus (**Figure 29**).

**Figure 27.** *Structure of Flavin Adenine Dinucleotide (FAD).*

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

**Figure 28.** *Mechanisms of FAD (a-left) and (b-down).*

#### **Figure 29.** *Structure of NAD and NADP.*

NAD<sup>+</sup> and NADP<sup>+</sup> are anaerobic, because the transferred hydrogen acceptor is not oxygen, but another element. They catalyze redox reactions by the generally reversible transfer of protons. The transfer of hydrogen in the redox reactions catalyzed by NAD<sup>+</sup> and NADP<sup>+</sup> is carried out at the level of the nicotinamide component in the structure of these coenzymes (**Figure 30**).

Preservation of antioxidant characteristics can be achieved by using special techniques: Mild Food Processing, Supercritical Fluid Extraction (SFE), separation in active plasma field, separation in magnetic and gravitational field.

**Figure 30.** *NAD (P) – redox mechanism.*

#### **Figure 31.**

*Sepparation a synthetic food preservative from a liquid food using, nano-plasma field, SFE and antioxidant agent (©).*

Using the properties of compounds with antioxidant activity (from certain redox systems in food), an improved SFE process at the nanomolecular level - with the help of a nano-plasma field, Professor Savescu Petre succeeded in separating (in the form of crystals) a synthetic food preservative from a liquid food (**Figure 31**). The advanced separation was performed by a personal technique (under innovative patent by PhD. Habil. Professor Petre Săvescu), within the INCESA Research Hub of the University of Craiova, Romania.

#### **3. Conclusions**

Antioxidants are valuable bio compounds that can increase both the nutritional value of the functional food and the therapeutic value of this important product.

For dietary supplements and functional foods, it is important to use only natural antioxidants. Synthetic antioxidants can cause a number of consumer health problems. In the design and construction of a functional food it is important to use only inoculated and even organic raw materials. All used raw materials, food additives, and technological adjuvants must be analysed before processing the food supplement - to avoid unwanted reactions and the appearance of compounds with a potential risk to the health of the consumer.

It is forbidden to use raw materials, food additives, technological auxiliaries which can contain traces of antibiotics, plant or animal hormones, pesticides, heavy *Natural Compounds with Antioxidant Activity-Used in the Design of Functional Foods DOI: http://dx.doi.org/10.5772/intechopen.97364*

metals. For their analysis will be used complex chromatography techniques (GC, LC), advanced separation techniques (using supercritical fluids and plasma fields), optical methods of analysis (UV-Viz, NIR, FT-IR) with Certified Reference Materials and Pure Analysis Substances and modern standardized methods of electrochemistry. Antioxidants can have the functions of immune-modulatory compounds, food preservatives, and food colouring, sequestering/chelating agents for heavy metals.
