Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces

Подождите немного. Документ загружается.

600 Charged Particle and Photon Interactions with Matter

–

, H

–

HCOO

–

GSH

–

TyrO

(protein)

Tryp

dGMP

CCl

ONOO

–

dGMP–OH

–

2Br

–

dGMP

CCl

–

•

sCheme 22.2 Generation of ROS by radiolysis of aqueous solutions.

table 22.2

important

adiation

Chemical r

eactions

along with r

ate

Constants

for the g

eneration

of r

eactive

xygen

pecies

s.no. reaction

rate Constant

−1

)

2 2

O e OH OH N

+  → + +

− • −

9.1 × 10

• •

( )

−  →

( )

− +OH CH C OH CH C CH OH H O

2 3

6.0 × 10

• − •−

+  → +OH HCOO CO H O

2 2

3.2 × 10

• − •−

+  → +H HCOO CO H

2 2

2.1 × 10

CO O O CO

2 2 2 2

•− •−

+  → +

3.5 × 10

e O O

− •−

+  →

2 2

2 × 10

• •

( )

−  →

( )

− +

OH CH CH OH CH C OH H O

1.9 × 10

NO OH NO OH

2 2

− • −

+  → +

6.0 × 10

NO e NO

aq3 3

2− − −

+  →

9.2 × 10

e NO NO

− − −

+  →

2 2

4.1 × 10

• − • −

+  → +H NO NO OH

7.1 × 10

• − •

−

+  → +OH CO CO OH

3.0 × 10

• − •−

+  → +OH HCO CO H O

3 3 2

8.5 × 10

• − −

+  → +

OH N N OH

3 3

1.4 × 10

• − •− −

+  → +OH Br Br OH2

1.1 × 10

e Thym T T

− •−

+  →ine( )

1.7 × 10

• •

+ → −OH dGMP dGMP OH

6.8 × 10

• •

+ → +OH T RPH TRP H O

1.3 × 10

• •

+ → +OH GSH GS H O

2.3 × 10

Redox Reactions of Antioxidants 601

Recently, many laboratories involved in radiation chemistry programs have taken up antioxidant

research and published a number of papers in the literature. The rate constants for the scavenging reac-

tions of oxidizing and reducing free radicals by antioxidants were determined at near physiological

pH conditions. The antioxidant radicals have been characterized by transient spectra, decay kinetics,

prototropic equilibrium constants (pK

), conductivity changes, and reactions with other molecules like

oxygen. Such studies have been found to be useful to quantify reaction kinetics, estimate the reactiv-

ity of antioxidant substances, and also in identifying the site of free-radical attack on the antioxidant

molecule. One of the greatest contributions of pulse radiolysis is the estimation of one-electron reduc-

tion potentials of transient free radicals. Establishing reversible electron transfer between two couples

(Wardman, 1989), one-electron reduction potentials of several antioxidants have been reported.

In addition to the radicals given in Scheme 22.2 and Table 22.2, radiation chemists have also

designed methods to study reactions of secondary radicals from amino acids of proteins and DNA

radicals with antioxidants (Butler et al., 1984; Solar et al., 1984; O’Neill and Chapman, 1985;

Sonntag, 1987; Asmus and Bonifacic, 1999; Li et al., 2000; Santus et al., 2001; Filipe et al., 2002,

2004; Zhao et al., 2002, 2003). The most commonly employed aromatic amino acid radicals are

the indolyl radicals of tryptophan (TRP

•

) and the tyrosine phenoxyl radicals (TYRO

•

) and sulfur-

centered radicals from amino acids like methionine, glutathione, etc. Reactions of antioxidants with

these radicals have been used to evaluate their ability to protect proteins from oxidative damage,

especially

for hydrophobic antioxidants, which show preferential afnity toward proteins.

evaluate the ability of an antioxidant to protect DNA from oxidative damage, studies on the

repair and electron transfer reactions of several antioxidants have been carried out with secondary

DNA radicals (O’Neill and Chapman, 1985; Zhao et al., 2001, 2002, 2003). The following are impor-

tant DNA radicals that could be generated by pulse radiolysis:

•

OH radical adducts of deoxyguanosine

monophosphate (dGMP-OH

•

), deoxyadenosine monophosphate (dAMP-OH

•

), polyadenylic acid (poly

A-OH

•

), polyguanylic acid (poly G-OH

•

) and single- or double-stranded DNA (DNA-OH

•

), radical

anions of thymine (T

•−

) and thymidine monophosphate (TMP

•−

), and radical cations of dGMP and

dAMP (dGMP

•+

and dAMP

•+

). Another major contribution of the radiation chemists to antioxidant

research has been in studying

i−

radical reactions and development of SOD mimics.

In the present chapter, an attempt has been made to summarize the important aspects of the

research carried out in the last three decades on pulse radiolysis studies of antioxidants. Complete

coverage

of the reactions of antioxidants is beyond the scope of this chapter. However, studies with

the most promising antioxidant systems have been discussed in detail. It is expected that the infor-

mation provided in the chapter would be useful for a new radiation chemist to initiate work in this

multidisciplinary

research area.

22.6 redox studies oF antioxidants by pulse radiolysis

The redox reactions of antioxidants studied by employing pulse radiolysis belong to two broad and

general types: natural antioxidants and synthetic antioxidants. Of the two, natural products have

attracted the attention of many more scientists as they are present in several food products, and such

antioxidants can be developed as neutraceuticals because they are consumed through the diet (Shi

and Noguchi, 2000). Natural antioxidants can be further classied as phenolic and non-phenolic

compounds; the phenolic compounds outnumber the non-phenolic compounds and are therefore

discussed

separately in detail.

22.6.1 phenolic antioxidantS

Important antioxidants belonging to the class of phenolic compounds that have been studied by pulse

radiolysis are simple phenols, benzoic acid derivatives, cinnamic acid derivatives (Lin et al., 1998;

Bors et al., 2003), methoxy-phenols (Priyadarsini et al., 1998; Bors et al., 2002; Mercero et al., 2002),

602 Charged Particle and Photon Interactions with Matter

salicylic acid derivatives (Joshi et al., 2005b), gallic acid derivatives (Bors and Michel, 1999), tocoph-

erols (Packer et al., 1979), avonoids (Bors et al., 1994, 1995; Jovanovic et al., 1994, 1996; Li and Fang,

1998; Miao et al., 2001a,b), curcuminoids (Priyadarsini, 1997), resveratrol (Stojanovic and Brede,

2002), ellagic acid (Priyadarsini et al., 2002), sesamol (Joshi et al., 2005a), xanthones (Mishra et al.,

2006a), anthocyanins, and tannins (Bors et al., 2001a,b), folate (Joshi et al., 2001), and many other

plant-derived polyphenols (Bors et al., 2001a,b; Shi and Noguchi, 2000; Singh et al., 2009).

The antioxidant action of phenols proceeds by donating an electron or hydrogen atom to

•

radicals, specic one-electron oxidants, inorganic radicals, and chain propagating peroxyl radicals.

Most of these reactions produce a phenoxyl radical as the phenolic OH group in these compounds

has maximum electron density and the O–H bond is the weakest to break. Depending on the inter-

action of the unpaired electron on the phenoxyl radical with other substituents, the phenoxyl radical

gets resonance stabilized and the stability of this radical decides the efcacy of the phenol as an

antioxidant. In addition to this, a free radical may add to the aromatic ring of phenols producing

radical adducts, which may react with oxygen producing peroxyl radicals. For example,

•

OH radi-

cals add to the aromatic ring of simple phenol, where the majority of it goes to ortho, para positions,

while a minor fraction goes to meta and ipso positions (Mvula et al., 2001). These radical adducts

can participate in different reactions like acid- and base-catalyzed elimination of water to yield

phenoxyl radicals, absorbing at ∼400nm or reacting with oxygen to form peroxyl radicals (Alfassi

et al., 1994; Mvula et al., 2001). In general, the phenoxyl radicals do not react with oxygen and

therefore are not converted to peroxyl radicals. The formation of phenoxyl radicals accounts for

the antioxidant action, as this reaction converts the more reactive

•

OH radicals to the less reactive

phenoxyl radicals, but formation of peroxyl radicals may implicate a pro-oxidant effect. Therefore,

one has to look into all these aspects while exploring phenolic compounds as antioxidants. The reac-

tion

•

OH radical with phenol is summarized in Scheme 22.3.

There are many natural phenolic compounds, which show excellent antioxidant activity, and

their ability to neutralize free-radical oxidants is dependent on the substitutions. Compounds like

cinnamic acid, caffeic acid, and chlorogenic acid have two hydroxyl groups, and pulse radiolysis

studies have been employed to conrm their excellent free-radical-scavenging activity (Kono etal.,

1997; Lin et al., 1998; Lu and Liu, 2002; Bors et al., 2003). Several natural antioxidant com-

pounds have methoxy phenolic acids, compounds like ferulic acid, eugenol, isoeugenol, and vanil-

lin are ortho-methoxy phenolic acids found in food spices, exhibiting potent antioxidant activity.

The compounds react with most of the oxidizing free radicals including chain-breaking peroxyl

radicals (Priyadarsini et al., 1998; Guha and Priyadarsini, 2000; Bors, 2001; Mercero, 2002; Barik

etal., 2004). In most of these reactions, phenoxyl radicals are produced that get stabilized by reso-

nance through the aromatic ring and also by interacting with the lone pair on the methoxy oxygen.

H OH

48%

36%

max

= 400 nm

+ +

+ OH

–H

H OH

Peroxyl radical

Phenoxyl radical

•

sCheme 22.3 Hydroxyl radical reactions with phenol formation of phenoxyl and peroxyl radicals.

Redox Reactions of Antioxidants 603

Further,compounds like syringic acid and sinapic acid having two methoxy groups ortho to the

phenolic OH groups have been found to exhibit very good antioxidant activity. In such compounds,

substitution of additional methoxy groups not only helps in the stabilization of the phenoxyl radicals

but also increases their lipid solubility. Chemical structures of important phenolic antioxidants are

included

in Scheme 22.4.

Among

the natural vitamins, vitamin C (ascorbic acid) and vitamin E (α-tocopherol) are the

most promising antioxidants and are some of the rst compounds recognized. Pulse radiolysis con-

tributions

on these two compounds are discussed in detail.

22.6.1.1

vitamin

C and v

itamin

Vitamin C occurs as l-ascorbic acid and dehydroascorbic acid in fruits and vegetables. Ascorbic acid

is the most effective water-soluble antioxidant in the plasma and it is an excellent free-radical scav-

enger. The chemical structure of ascorbic acid is such that it can be considered as both a phenolic

and a non-phenolic antioxidant. In biological systems, oxidation of ascorbic acid to dehydroascorbic

acid proceeds through the formation of ascorbyl radicals. Radiation chemical experiments performed

several decades ago (Bielski et al., 1971, 1981; Schuler, 1977) using pulse radiolysis conrmed that

ascorbic acid radicals produced during the oxidation of ascorbic acid by

•

OH radicals and many other

oxidizing radicals showed two pK

values of 1.10 and 4.25 with the protonated form (

), neutral

radical (AH

•

), and ascorbate radical anions (A

•−

) in prototropic equilibrium (Scheme 22.5). At neutral

pH, A

•−

radicals absorb at 360nm with a molar extinction coefcient of 3300M

−1

. The decay of

ascorbate radicals is very well documented, and from the studies on effect of pH, ionic strength, and

buffers, it is proposed that A

•−

radical is in equilibrium with a dimer. The dimer reacts with other pro-

ton donors like water and buffers, and produces products like ascorbate ion and dehydroascorbic acid

by disproportionation. At pH 7, the reduction potentials of the couple A

•−

, H

/AH

−

is ∼0.32V vs. NHE

and that for dehydroascorbic acid/A

•−

is ∼−0.17V vs. NHE (Buettner, 1993). Due to this low redox

potential, ascorbic acid can act both as a reducing agent and an oxidizing agent.

The reactions of A

•−

with various biological molecules have also been investigated using pulse

radiolysis (Kobayashi et al., 1991). The A

•−

radical reacted with fully reduced and semiquinone

CO R'

Methoxyl phenols

R'= –CHO, o-Vanillin

R = –CH

–CH = CH

, Eugenol; R' = H

R = –CH

= CH–CH

, Isoeugenol; R' = H

COOH

R' R

CH=CH–COOH

R = –H; R' = –OCH

; Vanillic acid

R = –OCH

; R' = –OCH

; Syringic acid

R = –OH; R' = –OH; Gallic acid

R = –H; R' = –OCH

; Ferulic acid

R = –OCH

; R' = –OCH

; Sinapic acid

R = –OH; R' = –H; Caeic acid

Cinnamic acid derivativesBenzoic acid derivatives

sCheme 22.4 Some important dietary phenolic antioxidants.

– e

–

R = (CH

OH)

–

•

–

•

sCheme 22.5 Ascorbic radicals generated from the one-electron oxidation of ascorbic acid.

604 Charged Particle and Photon Interactions with Matter

forms of hepatic NADH-cytochrome b5 reductase with second-order rate constants of 4.3 × l0

and 3.7 × 10

−l

−1

, respectively. It however did not react with the ferrous form of cytochrome b5,

whereas it induced oxidation of cytochrome b5 in the presence of ascorbate oxidase, suggesting that

the rate constant of A

•−

with the ferrous cytochrome b5 must be several orders of magnitude smaller

than that of the disproportionation of A

•−

. On the other hand, A

•−

could reduce Fe

-EDTA with a

second-order

rate constant of 4.0 × 10

−1

−l

but did not reduce ferric hemoproteins.

Due to the low redox potential, the ascorbate ion acts as a source of

•

OH radicals and partici-

pates in Fenton reactions. This led to the speculation whether ascorbate is a pro-oxidant or an

antioxidant. It is now conrmed that ascorbate denitely acts as an antioxidant at low concentra-

tions but may become pro-oxidant at high concentrations especially in presence of free metal ions

(Yen et al., 2002).

Vitamin E, obtained from nuts, seeds, and cereals, is a collective term for eight compounds: α-, β-,

γ-, and δ-tocopherol and α-, β-, γ-, and δ-tocotrienols, but α-tocopherol accounts for 90% of vitaminE.

All the tocopherols contain a phenolic-OH group that enables them to donate hydrogen to a free radi-

cal. Vitamin E is readily incorporated into cell membranes and it is the rst known chain-breaking

antioxidant. Pulse radiolysis studies of vitamin E have been reported several years ago (Packer etal.,

1979; Jore et al., 1986; Bisby, 1990). Vitamin E reacts with almost all the oxidizing free radicals and

the phenoxyl radicals produced during oxidation reactions absorb at ∼425nm. Since vitamin E is

insoluble in water, most of the radiation chemical studies could be carried out either in alkaline water

or in aqueous–organic solutions or in micellar solutions. Vitamin E reacts with many different types

of peroxyl radicals and the rate constants range from 10

to 10

−1

, depending on the solvent and

hydrophobicity of the peroxyl radical. The phenoxyl radicals of vitamin E have long lifetimes and

decay by second-order reactions with 2k values of ∼10

−1

, and the lifetime varied signicantly

with the polarity of the medium. The one-electron reduction potential of vitamin E is ∼0.50V vs. NHE

(Buettner, 1993). The chemical structure of the vitamin E phenoxyl radical is given in Scheme 22.6.

The phenoxyl radicals of vitamin E, sometimes called as α-tocopheroxyl radicals, are highly

stabilized and several research groups have investigated quantitative structure–activity studies

(Buettner, 1993; VanAcker et al., 1993). Based on these studies, the structural features responsible

for the stability of the phenoxyl radical have been understood as follows: In the lipid bilayer, vitamin

E is orientated with the chroman head group toward the surface and the hydrophobic phytyl side

chain buried within the hydrocarbon region. The phenoxyl radical produced after donating hydro-

gen atom to the lipid peroxyl radical acquires stability by sharing its electron with nearby atoms.

The lone pair containing the p-orbital of the heterocyclic oxygen is almost perpendicular to the

aromatic plane. This lone pair overlaps with the singly occupied molecular orbital of the phenoxyl

radical and provides extra stability. The polarity of the vitamin E phenoxyl radical is such that it

moves

to the surface of the lipid, where it is regenerated by vitamin C.

The

regeneration reaction of vitamin E phenoxyl radicals back to vitamin E by the water-soluble

antioxidant vitamin C was rst reported by pulse radiolysis, in which direct decay of the phen-

oxyl radicals of vitamin E absorbing at 425nm was followed by simultaneous formation of ascor-

byl radicals absorbing at 360nm, and the rate constant for the regeneration reaction was found

to be 1.55× 10

−1

(Packer et al., 1979). The long lifetime of phenoxyl radicals allows the

–H

max

= 425 nm

(

)

–e

–

(

)

•

sCheme 22.6 Phenoxyl radical generated from the one-electron oxidation vitamin E.

Redox Reactions of Antioxidants 605

regeneration reaction to compete with other radical reactions. The resultant ascorbic acid radical

produced during the regeneration process is either recycled by NADH or converted to non-reactive

products (Kobayashi et al., 1991). The overall chain-breaking antioxidant mechanism involving

lipid-soluble vitamin E and water-soluble vitamin C in cells is represented by the sequential electron

transfer

process as represented in Scheme 22.7.

Based

on the kinetic and thermodynamic parameters for the reactions of polyunsaturated fatty

acid (PUFA-H) with its peroxyl radical, PUFA-OO

•

, and with vitamin E, it was possible to provide

a convincing explanation on how vitamin E when present in very small concentrations (even at a

ratio of PUFA and vitamin E as 1000:1) prevents the oxidation of bulk fatty acids and protects the

cell

membrane very effectively (Buettner, 1993).

22.6.1.2

water-soluble

nalogues

of v

itamin

Trolox C is a water-soluble analogue of vitamin E, which also exhibits similar scavenging reactions

with several types of peroxyl radicals and oxidizing free radicals (Davies et al., 1988). The stud-

ies employing pulse radiolysis showed that trolox C rapidly undergoes electron transfer to produce

phenoxyl radicals absorbing at 435 nm. The one-electron reduction potential for the conversion of

trolox C to its phenoxyl radicals is 0.48V vs. NHE. Trolox C phenoxyl radicals are readily repaired

by ascorbate with a rate constant of 8.3 × 10

−1

and also by some thiols with rate constants

of <10

−1

. Kinetic measurements also conrmed that trolox C repairs oxidized proteins. The

phenoxyl radicals of trolox C react with

i−

radicals (Cadenas et al., 1989) with a rate constant of

4.5 × 10

−1

, by electron transfer, converting back to trolox C. The chemical structure of trolox

and its phenoxyl radicals are given in Scheme 22.8.

Both

vitamin E and trolox C are used as standards for evaluating the antioxidants. A term called

trolox equivalent antioxidant capacity (TEAC) is used to compare the antioxidant ability of new

compounds (Kohen and Nyska, 2002;). Recently, a glycosylated derivative of α-tocopherol has been

synthesized and the pulse radiolysis studies have been reported in aqueous solutions. The stud-

ies conrmed that the free-radical chemistry of the glycosylated derivative is similar to that of

α-tocopherol

(Kapoor et al., 2002).

22.6.1.3

Flavonoids

Flavonoids are another large and diverse group of naturally occurring phenolic compounds ubiqui-

tous in plants and common in a great variety of fruits, vegetables, and beverages. The basic structure

of avonoids consists of two aromatic rings linked through a furan ring, denoted by the three rings

A, B, and C, and their chemical structures as given in Scheme 22.9 differ mainly in the presence

of double bond at 2,3-position and the 3-OH substitution. Examples of avonoids that have been

ROO

ROOH

Vitamin E

Vitamin C

NADH

Vitamin E

Vitamin C

NADH

•

sCheme 22.7 Redox recycling of antioxidants: Vitamin E and vitamin C.

COOH

–H

max

= 435 nm

–e

–

•

sCheme 22.8 Phenoxyl radicals generated from the one-electron oxidation of trolox C.

606 Charged Particle and Photon Interactions with Matter

extensively studied are green tea polyphenols, catechins, epicatechins, hesperidine, taxifolin, que-

recetin, rutin, silybins, etc. This list does not include other similar class of compounds like isoa-

vones,

neoavones, anthocyanins, xanthones, or biavonoids.

Flavonoids

participate in redox reactions with both reducing and oxidizing free radicals and also act

as metal chelators. With suitable hydroxyl substitutions, avonoids act as excellent hydrogen donors to

reactive oxygen free radicals (Bors and Saran, 1987; Bors et al., 1994, 2001a,b; Jovanovic et al., 1994,

1995, 1996; Li and Fang, 1998; Shi and Noguchi, 2000). Using the pulse radiolysis technique, reac-

tions of oxidizing radicals with several structurally related hydroxy-avonoids have been studied and

the absorption spectra of the avonoid phenoxyl radicals have been reported (Bors and Saran, 1987;

Bors et al., 1994, 2001a,b; Jovanovic et al., 1994, 1996. Mishra et al., 2003, 2006a; Zielonka et al.,

2003; Tamba and Torreggiani, 2004; Fu et al., 2008). Depending on the hydroxyl group substitution,

the absorption spectrum of the phenoxyl radical showed distinct features. Although the available liter-

ature on the absorption spectral details cannot be used with certainty as a characteristic tool to identify

different avonoid phenoxyl radicals, a generalized and empirical approach could be adopted to know

whether the phenoxyl radical is derived from the A ring or the B ring. For example, if the phenoxyl

radical is from the catechol of the B ring, it absorbs at 340–390nm and if the phenoxyl radical is

derived from the resorcinol moiety of the A ring, it absorbs at 420–480nm (Jovanovic et al., 1994,

1996; Cren-Olive et al., 2002). Depending on the structure, the phenoxyl radicals have a pK

between

∼4 and 6, and the absorption maximum of the anionic form of the radical is redshifted. The absorp-

tion spectrum in the 350nm region is quite sharp but that in the visible region is broad and extends up

to 600nm depending on the substitution. In avonoids like catechins, where the conjugation through

the C ring is not present, two distinct absorption bands due to two different phenoxyl radicals were

observed (Cren-Olive et al., 2002; Tamba and Torreggiani, 2004).

Although hydroxyl substitution on the three rings makes avonoids reactive toward peroxyl radi-

cals, the catechol hydroxyl group of the B-ring makes it more feasible to undergo oxidation. The one-

electron reduction potentials for the formation of phenoxyl radicals for a number of hydroxy avonoids

have been reported at neutral pH conditions and the values ranged between 0.3 and 0.7V vs. NHE

(Jovanovic et al., 1994, 1996; Bors et al., 1995). The reduction potential is signicantly reduced when

the 3-hydroxy group in the C ring is in conjugation with the catechol moiety. DFT calculations of dif-

ferent avonoids also supported the results from pulse radiolysis results (Zielonka et al., 2003).

Flavonoid phenoxyl radicals in aqueous solutions showed lifetimes of several hundred millisec-

onds. The long lifetime and the reduction potential values suggest that avonoids can be regenerated

back from their phenoxyl radicals by vitamin C and vitamin E. Indeed, both the reactions could be

observed by pulse radiolysis. Although, thermodynamically, reactions of avonoid phenoxyl radical

with vitamin E are feasible, such reactions would not be expected at the physiologically relevant

concentrations of vitamin E in the cell membrane. However, regeneration by ascorbate inside the

cells provides synergism like that observed with vitamin E. Because of this ascorbate-protective

role, avonoids are sometimes called as vitamin P (Bors et al., 1995). Although the long lifetime of

the avonoid phenoxyl radical favors the regeneration reaction, it is also proposed that this may also

allow some of the chain propagating reactions like lipid peroxidation, indicating its contribution to

the

pro-oxidant effects.

OOH

Flavonols

Flavans

Flavanones

Flavones

sCheme 22.9 Basic structure of different classes of avonoids.

Redox Reactions of Antioxidants 607

Based on all these pulse radiolysis studies and many relevant redox reactions, three structural

moieties have been identied to be important for antioxidant and radical-scavenging activity of

avonoids. These are: an o-hydroxy group in the B-ring, a 2,3-double bond combined with 4-oxo

group in the C-ring, and hydroxyl group at positions 3 and 5 of ring A (Bors et al., 2001a,b).

Accordingly, quercetin, with all these structural features showed promising antioxidant effects,

both in animals and humans. The pulse radiolysis studies on quercetin have been briey sum-

marized here.

Reactions of radicals like

•

OH,

, NO

i−

, peroxyl radicals, 1-hydroxylethyl radicals, etc.,

with quercetin have been studied and the transients characterized by absorption spectroscopy,

conductivity measurements, and Raman spectroscopy (Jovanovic et al., 1994, 1996; Miao et al.,

2001a,b; Marfak et al., 2004; Torreggiani et al., 2005). Two types of phenoxyl radicals were

observed (as shown in Scheme 22.10) during one-electron oxidation of quercetin and from pH-

dependent absorption changes, a pK

of 5, for the deprotonation of the catechol hydroxyl group

has been proposed. The reduction potential of quercetin at pH 7 is 0.33V vs. NHE. Therefore,

quercetin is not a good electron donor to ascorbate, but it can reduce vitamin E radicals thereby

helping in the regeneration of vitamin E. Quercetin could repair TRP

•

radicals produced during

the oxidation of low density lipoprotein (LDL) and human serum albumin (HSA) (Filipe et al.,

2002; Santus et al., 2001). HSA-bound quercetin could repair urate radicals by electron transfer,

both in presence and absence of copper (II) (Filipe et al., 2004). Quercetin could repair the

•−

radical and dGMP-OH radical adduct (Zhao et al., 2001, 2002, 2003), indicative of its ability

to repair damaged DNA.

While the antioxidant effects of avonoids are related to the phenolic moieties, their pro-oxidant

effects are related to the reactions leading to the production of semiquinone and quinonoid products,

i−

, and H

generation. Unlike the oxidation studies, not many reports are available on one-

electron reduction of avonoids, mainly because the anion radicals show weak absorption in the

UV–VIS region. The reaction of

−

and reducing radicals with a number of avonoids indicated

that the anion radicals of all the avonoids showed absorption at wavelength <400nm (Cai et al.,

1999; Fu et al., 2008). Flavonoids like naringin and quercetin containing C4 keto group showed

highest reactivity toward

−

radicals, while hydroxyl substitution in the B ring, and 2,3-double

bond, which are crucial for antioxidant activity, showed no inuence on the

−

scavenging activity.

–e

–

A C

~ 5.0

Quercetin

–H

max

= 350 nm λ

max

= 550 nm

A C

•

sCheme 22.10 Different types of quercetin phenoxyl radicals formed by one-electron oxidation.

608 Charged Particle and Photon Interactions with Matter

22.6.1.4 resveratrol

Trans-resveratrol (trans-3,5,4′-trihydroxystilbene), a non-avonoid polyphenol responsible for

the antioxidant activity of red wine, is found in grapes, mulberries, and other food products

(Stojanovic and Brede, 2002; Stojanovic et al., 2001; Mahal and Mukherjee, 2006). In addition to

antioxidant activity, resveratrol could inhibit platelet aggregation and showed anticancer activity.

The phenoxyl radicals of resveratrol produced during oxidation by

•

OH radicals, one-electron

oxidants, and peroxyl radicals showed absorption maximum at 410 nm. Comparing the spectral

and kinetic properties of the transients derived from trans-resveratrol and its analogues, it has

been concluded that in the neutral and acidic solution, the para hydroxy group of trans-resveratrol

is more reactive than the meta-hydroxy groups (Scheme 22.11). Quantum chemical studies on

resveratrol derivatives conrmed that the 4′-hydroxyl group of resveratrol is more reactive than

the ones at the 3- and 5- positions because of the resonance effects (Cao et al., 2003). Resveratrol

could repair the nucleic acid radicals and amino acid radicals with rate constants ranging from

to 10

−1

(Mahal and Mukherjee, 2006). Some of these reports also indicate that trans-

resveratrol is a better radical scavenger than vitamins E and C and is as efcient as some of the

avonoids.

22.6.1.5 Curcumin

Curcumin is a major phenolic pigment derived from turmeric, which is commonly used as a

spice and as a household medicine in India. Recent scientic research has shown that curcumin

is a ten times more effective antioxidant than vitamin E. It is also a potent antitumor agent and at

present several phase I and phase II clinical trials are being carried out on curcumin for the treat-

ment of different types of cancers. The greatest advantage of curcumin is the pharmacological

safety to humans even at a dose of 8 g day

−1

(Shishodia et al., 2005). Pulse radiolysis studies on

reactions of

i−

radicals, CCl

•

radicals, lipidperoxyl, methyl, and methyl peroxyl radicals,

glutathione radicals, tryptophan radicals, etc., with curcumin have been reported (Gorman et al.,

1994; Priyadarsini, 1997; Khopde et al., 1999; Jovanovic et al., 2001; Kapoor and Priyadarsini,

2001; Priyadarsini et al., 2003). The rate constants for the reactions of these radicals and several

other oxidants have been found to be in the range of 10

to 10

−1

in aqueous/aqueous–

organic solutions. In all these reactions, a transient showing a strong absorption band with a

maximum at 500 nm and another weak band in the UV region was observed. The 500 nm absorb-

ing transient was characterized as the phenoxyl radical, which acquires resonance stabilization

through the α,β-unsaturated β-diketone structure. The lifetime of the phenoxyl radical is a few

hundred milliseconds in membrane models (Priyadarsini, 1997). The phenoxyl radicals of cur-

cumin could be converted back to the parent curcumin by ascorbic acid and the rate constant is

comparable to that of vitamin E. Curcumin has two possible sites for free-radical attack, these

are: the central methylenic (CH

) group and the phenolic OH group (Scheme 22.12). Coupling

pulse radiolysis studies on free-radical reactions, with in vitro and in vivo antioxidant activities

and quantum chemical calculations with several curcumin derivatives, it has been conrmed that

the phenolic-OH is mainly involved in the free-radical-scavenging activity and the antioxidant

activity of curcumin.

–e

–

–H

max

= 410 nm

•

sCheme 22.11 Phenoxyl radicals, generated from the one-electron oxidation of resveratrol.

Redox Reactions of Antioxidants 609

22.6.1.6 Folic acid

Folic acid, also known as vitamin B9, is essential for normal body functions and is involved in many

biochemical processes including nucleotide synthesis. Folate, the mono anion of folic acid is present

in leafy vegetables and fruits. Although folic acid was recognized as an essential vitamin, its role

as an antioxidant was recognized much later. Folic acid exists in keto–enol tautomerism in solution.

Using the pulse radiolysis technique, the reactions of oxidizing free radicals

CCl O

3 2

i−

•

OH, and O

•−

with folic acid were studied at different pH conditions (Joshi et al., 2001; Patro

et al., 2005). All these radicals react with folic acid to produce a phenoxyl radical having absorp-

tion maximum around 430nm, which undergoes cleavage to form smaller products. Additionally

it repairs thiyl radicals at physiological pH, a reaction important in contributing to the antioxidant

mechanism of folic acid. The one-electron reduction of folic acid by hydrated electron and isopro-

ylketyl radicals was studied (Moorthy and Hayon, 1976), in the pH range 0–14, and reported forma-

tion of transient exhibiting characteristic spectra in the UV–Vis region with prominent maximum

∼465nm. The one-electron reduced radicals have four pK

values at 1, 6.6, 8.0, and 10.3, and the

radicals act as powerful reducing agents. The oxidation reactions of folic acid at pH 7 are summa-

rized

in Scheme 22.13.

22.6.2 non-phenolic antioxidantS

Unlike phenolic antioxidants, non-phenolic natural products with antioxidant capacity are much less

in number. Compounds like melatonin, carotenoids, retinal, and thiols, and synthetic compounds like

cyclic nitroxides, thiols, and selenium compounds are some of the well-studied systems reported in

literature. The radiation chemical contributions to some of these systems are summarized below.

22.6.2.1 melatonin

Melatonin (N-acetyl-5-methoxytryptamine) is an endogenous compound whose antioxidant poten-

tial has been recognized recently. It is a hormone secreted by the pineal gland of vertebrates. It

plays an important role in regulating circadian rhythms and sleep. Melatonin exhibits immuno-

modulatory properties and is a potent antioxidant and protects organisms from free-radical damage

OCH

–e

–

–H

Curcumin

Phenoxyl radical

Methylene radical

max

= 500 nm

max

~ 300 nm

–H

•

sCheme 22.12 Possible free radical induced oxidation reaction pathways on curcumin.

Glu-PABA

–e

–

–H

max

= 430 nm

•

sCheme 22.13 Phenoxyl radicals generated from the one-electron oxidation of folic acid at neutral pH.