**4.1 Free radical damage to the deoxyribose moiety of DNA**

The highly reactive hydroxyl radical (**·**OH) reacts with the sugar moiety of DNA causing structural modification and strand breaks by a variety of mechanisms [21]. The OH radical reacts with the 2′-deoxyribose sugar residue in DNA by abstracting H• from all its carbon atoms forming five carbon-centered radicals. The H4′ and H5′ atoms are more accessible to H• abstraction by the OH radical than the H1′, H2′, and H3′. The C4′ C-centered radical appears to be the major radical generated by H• abstraction from 2′-deoxyribose in DNA [22]. These radicals undergo further reactions, producing a variety of 2′-deoxyribose oxidative adducts. While some products detach from DNA, others remain tethered as end groups of fragmented DNA strands [22]. In the absence of oxygen and as depicted in **Figure 2**, one of the byproducts formed from C4′-radical of 2′-deoxyribose as an end group of a severed DNA strand is 2,5-dideoxypentose-4-ulose. The product is formed by heterolytic cleavage of the phosphate group at C5′ to give a C4′/C5′-radical cation which in turn undergoes hydration and subsequent one-electron reduction and base elimination (**Figure 2**). Other products formed from the C4′ radical include 2-deoxypentose-4-ulose and 2,3-dideoxypentose-4-ulose. However, in the presence of oxygen, rapid addition of O2 to the C4′-radical forms a peroxyl radical which undergoes a series of fragmentation reactions yielding 3′-phosphoglycolate as an end group [23].

Oxidation of the C1′, C2′, and C5′ radicals yields products such as 2-deoxypentonic acid lactone, erythrose, 2-deoxytetradialdose, and 5′ aldehyde [22].

**Figure 2.**

*Mechanism of product formation from reactions of the C4*′*-radical of 2*′*-deoxyribose, leading to 2,5-dideoxypentose-4-ulose as an end group of a broken DNA strand.*

#### **4.2 Free radical damage to DNA bases**

Besides reacting with the sugar moiety of DNA, the highly reactive hydroxyl radical (**·**OH) reacts with the heterocyclic bases guanine (**Figure 3**), thymine, and cytosine, causing free radical-induced DNA damage by several different pathways. Guanine, however, possesses the lowest reduction potential (1.29 V) among the four DNA bases, rendering the motif the best electron donor and prone to preferential oxidization [24]. The hydroxyl radical reacts with the C4-, C5-, and C8-positions of guanine and to a lesser extent with the C2-position, generating a plethora of products. Interestingly, the HO-adduct radicals generated from the addition reactions of HO. may exhibit reducing or oxidizing properties (redox ambivalence), yielding the relevant products accordingly. Hence, while the C5-OH– and the C8-OH–adduct radicals are reducing, the C4-OH–adduct radical is predominantly oxidizing. The last two adduct radicals form in yields of 17% and 65–70%, respectively, whereas the yield of the C5-OH– adduct radical is lower than 10% [25]. Although formed in relatively lower yields, the C8-OH–adduct radical produces the major byproducts of guanine reactions (**Figure 3**). Thus, as shown in **Figure 3** and following reaction of the hydroxyl radical with the C-8 position of guanine, one-electron oxidation of the resulting C8-OH–adduct radical yields the enol form of 8-hydroxyguanine which undergoes tautomerization to generate the predominant keto form [21]. The latter may also form via a pathway involving 1,2-hydride-shift and subsequent oxidation of the C8-OH–adduct radical. The 1,2-hydride-shift radical product may also undergo single electron reduction, followed by ring opening reaction to form 2,6-diamino-4-hydroxy-5- formamidopyrimidine. The preceding radical damage to DNA has been directly correlated to several disease states such as genetic mutation, atherosclerosis, Alzheimer's disease, and the aging process [26, 27].

**99**

*Nonenzymatic Exogenous and Endogenous Antioxidants DOI: http://dx.doi.org/10.5772/intechopen.87778*

of hydrogen from a lipid molecule (LH) by an initiator (R**.**

of physiological conditions and tissue injuries [28].

E functions by reducing lipid peroxyl radicals (LOO**.**

based free radical (L**.**

peroxyl radical (LOO**.**

**exogenous antioxidants**

*General process of lipid peroxidation.*

**5.1 Vitamins**

**Figure 4.**

*5.1.1 Vitamin E*

**4.3 Free radical damage to polyunsaturated fatty acid groups of cell membranes**

While free radicals react with all major classes of biomolecules, peroxidation of the polyunsaturated fatty acid groups (PUFA) of cell membranes comprises the main target of oxidative damage, resulting in a destructive self-propagating chain reaction. The general mechanism of PUFA peroxidation involves abstraction

the peroxyl radical reacts with PUFA moieties, producing lipid hydroperoxides (LOOH) and perpetuating the chain reaction. The hydroperoxides can further dissociate to dangerous radical species like bioactive aldehydes which inflict damage on other cellular components. Lipid hydroperoxidation has been linked to a number

**5. Regulation of free radicals with nonenzymatic small natural** 

Vitamin E is a collection of optically active methylated phenolic compounds comprising four tocopherols and four tocotrienols [29] where α-tocopherol is the most common and biologically active species (**Figure 5**) [30]. The structures feature two primary parts: a densely substituted polar chromanol aromatic ring and a lipophilic long polyprenyl side chain. The main chemical structural difference between different forms of Vitamin E is that tocotrienols feature unsaturated isoprenoid hydrocarbon side chains with three carbon-carbon double bonds versus saturated isoprenoid side chains for tocopherols. Within each group, the vitamers are differentiated by the number and positions of the methyls in the chromate ring. The polyprenyl precursor for the biosynthesis of tocopherols and tocotrienols is phytyl pyrophosphate (PPP) and geranylgeranyl pyrophosphate (GGPP), respectively [31]. Vitamin E is biosynthesized though the shikimate pathway, and while α-tocopherol and α-tocotrienol are considered structurally unique, the remaining compounds in each class are constitutional isomers. The presence of three stereogenic centers (position C2 of the chromate ring, position C4 and C8 of the phytyl side chain) produces 8 different stereoisomers (four pairs of enantiomers) depending on the position and orientation of the groups in each of the chiral centers. Since the discovery of vitamin E in 1920, it has been shown to be the most powerful membrane-bound antioxidant utilized by cells to scavenge reactive nitrogen and oxygen species with consequent disruption of oxidative damage to cell membrane phospholipids during cellular lipid peroxidation of the polyunsaturated fatty acids (PFA) and low-density lipoprotein (LDL) [32]. The antioxidant is liposoluble and localized to cell membranes. Vitamin

hydrogen atom of the chroman ring (**Figure 5**), resulting in a relatively stable and unreactive resonance-stabilized tocopheroxyl radical which is unable to trigger

) which reacts rapidly with molecular oxygen to form the

) known to propagate the chain reaction (**Figure 4**). As such,

) to generate a carbon-

) by transferring the phenolic

**Figure 3.** *Oxidative and reductive product formation from reactions of the C8-OH–adduct radical of guanine.*

### **4.3 Free radical damage to polyunsaturated fatty acid groups of cell membranes**

While free radicals react with all major classes of biomolecules, peroxidation of the polyunsaturated fatty acid groups (PUFA) of cell membranes comprises the main target of oxidative damage, resulting in a destructive self-propagating chain reaction. The general mechanism of PUFA peroxidation involves abstraction of hydrogen from a lipid molecule (LH) by an initiator (R**.** ) to generate a carbonbased free radical (L**.** ) which reacts rapidly with molecular oxygen to form the peroxyl radical (LOO**.** ) known to propagate the chain reaction (**Figure 4**). As such, the peroxyl radical reacts with PUFA moieties, producing lipid hydroperoxides (LOOH) and perpetuating the chain reaction. The hydroperoxides can further dissociate to dangerous radical species like bioactive aldehydes which inflict damage on other cellular components. Lipid hydroperoxidation has been linked to a number of physiological conditions and tissue injuries [28].

**Figure 4.** *General process of lipid peroxidation.*
