Open access peer-reviewed chapter

EPR Analysis of Antioxidant Compounds

By Betül Çalişkan and Ali Cengiz Çalişkan

Submitted: November 24th 2017Reviewed: January 23rd 2018Published: August 1st 2018

DOI: 10.5772/intechopen.74294

Downloaded: 96

Abstract

A free radical is a molecular species having an unpaired electron and it is a highly reactive entity and unstable. A free radical is a molecule with one or more unpaired electrons in its outer shell. Free radicals can be formed by chemical bond breakage from molecules or by redox reactions. When cells use oxygen, the oxidative stress occurs. The oxidative stress causes free radical formation. Free radicals can also be generated from ionizing radiations, ozone, heavy metal poisoning, cigarette smoking, and chronic alcohol intake. These free radicals are highly reactive and oxidize biomolecules leading to tissue injury and cell death. They also cause toxic effects and diseases. Antioxidants neutralize free radicals resulting from oxidative stress. Antioxidants play an important role in the treatment of diseases. The most suitable method for the analysis of free radicals is the electron paramagnetic resonance (EPR) spectroscopy method. The EPR method detects a paramagnetic center with a single electron. It gives information about the interactions with other nuclei around one electron. It provides information on the structure and environment of radicals.

Keywords

  • free radical
  • radiation damage center
  • antioxidant
  • electron paramagnetic resonance

1. Introduction

Free radicals are an atom or a molecule that bears an unpaired electron and is extremely reactive, capable of engaging in a rapid change reaction that destabilizes other molecules and generates many more free radicals. In plants and animals, these free radicals are deactivated by antioxidants. These antioxidants act as an inhibitor of the process of oxidation, even at relatively small concentrations, and thus have diverse physiological roles in the body. The body is constantly exposed to the negative and sometimes lethal effects of oxidants during normal physiological processes. The harmful free radicals such as hydroxyl, peroxyl and the superoxide anion are constantly being produced as a result of metabolic reactions in living systems. On a daily basis, up to 5% of inhaled oxygen may be converted to reactive oxygen species (ROS). These ROS have the ability to bind to cellular structures and have been implicated in a number of pathological processes such as aging, inflammation, reoxygenation of ischemic tissues, atherosclerosis, cancer and even Parkinson’s disease in men [1]. Two processes, which produce free radicals in vivo, have been identified and named the Fenton reaction and the Haber-Weiss reaction [2]. Antioxidants play an important role in animal health. Conventional antioxidants have been shown to improve animal performance during conditions characterized by increased tissue oxidant levels such as stress, injury and infections [3].

Free radicals can be classified as reactive oxygen species (ROS), reactive nitrogen species (RNS) and reactive sulfur species (RSS). Even though free radicals are mainly produced through regular metabolic routes, there are also some external factors that promote their production, including smoking, environmental pollutants, radiation and drugs, among others [4]. In healthy organisms, there is a delicate balance between the production and the removal of free radicals, which guarantees that they remain in adequate concentrations. However, when this balance is broken, these reactive species start producing chemical damages to proteins, lipids, DNA, RNA and sugars generating the so-called oxidative stress of body cells [4]. These processes have been associated with several diseases including cardiovascular, liver, neurological and renal disorders, as well as cancer, auto-immune deficiency and degenerative disorders associated with aging, diabetes, obesity, autism, Alzheimer’s, Parkinson’s and Huntington’s diseases [4]. Although humans have developed different endogenous defense mechanisms to protect cells from the excess of free radicals and to avoid the oxidative stress, often these mechanisms are not enough. Therefore, to increase protection from oxidative damage, dietary supplements with antioxidants are recommended as a way to maintain the concentration of free radicals as low as possible [5].

Antioxidants destroy free radicals. Antioxidants neutralize the radical, and thus the radical-antioxidant association maintains itself in a stationary state. For all these situations, antioxidants must be in a structure that will quench the radical. Therefore, unpaired electrons in the structure of antioxidants must be either radical ions or free radicals. So, they must form a paramagnetic center. Paramagnetic structures are analyzed by electron paramagnetic resonance (EPR) spectroscopy. Thus, the EPR analysis of antioxidants is as valuable and important as the investigation of the paramagnetic centers of radicals. There are many studies done for this purpose. Among these studies, especially the works we have done recently are noteworthy.

Potassium hydroquinone monosulfonate (PHM), succinic anhydride and 3-nitroacetophenone compounds are antioxidants. Paramagnetic centers formed by gamma irradiation effect on single crystals of these materials were analyzed by the EPR method [6, 7, 8].

2. EPR studies of antioxidant compounds

2.1. Gamma-irradiated potassium hydroquinone monosulfonate single crystal

Free radicals or other odd electrons have often been postulated as intermediates in the decomposition of organic compounds by ionizing radiation. It is still important for formulating mechanisms in radiation chemistry to identify the radicals that are formed. If several organic compounds are irradiated and resulting radicals are identified, one may hope that generalizations can be drawn concerning modes of bond rupture.

The quinone structure is widespread in nature: for example, quinones play an integral role in many biological electron-transfer processes, particularly respiration and photosynthesis [9, 10]. Quinones have long been considered for their fungicide, antibacterial and anticancer properties [11]. The quinone derivatives are especially interesting because they have significant properties due to the ready reversibility of the quinone-semiquinone redox system [12].

The single crystal of PHM was irradiated with gamma rays at room temperature and studied by the EPR technique at 125 K. The spectra were dependent on the orientation of H in the planes, which are perpendicular to each other. The paramagnetic centers formed from PHM are shown in Figure 1 [6].

Figure 1.

Structure of two semiquinone anion radicals observed in PHM single crystal.

The experimental spectrum in Figure 2 has 1:1:1:2:1 intensity ratios and exhibits the superimposition due to the two PHM anion radicals. Because of the radical A, the spectra exhibit 2-lines with intensity ratios 1:1. Owing to the C6 β-proton, the spectra exhibit a doublet (1:1). Because of the radical B, the spectra exhibit 3-lines with intensity ratios 1:2:1. Owing to the C3 and C4 β-protons, the spectra exhibit a triplet (1:2:1). The spectrum in Figure 3 has 1:2:2:1 intensity ratios and exhibits the superimposition due to the two PHM anion radicals.

Figure 2.

EPR spectrum of gamma-irradiated PHM single crystal at 125 K when the magnetic field is in the ab plane at an angle 0° toward the axis.

Figure 3.

EPR spectrum of gamma-irradiated PHM single crystal at 125 K when the magnetic field is in the ac plane at an angle 70° toward the axis.

The simulation values of the hyperfine coupling constants of the spectra in Figures 2 and 3 are given in Table 1. The EPR parameters belonging to two semiquinone anion radicals observed in PHM are included in Tables 2 and 3.

Radical ARadical B
Figure 2aCHβA= 0.328 mTaCH2βB= aHβB= 0.572 mT
Center field = 335.5 mTCenter field = 335.5 mT
ν = 9.38 GHzν = 9.408 GHz
Line width = 0.2 mTLine width = 0.215 mT
Figure 3aCHβA= 0.327 mTaCH2βB= aHβB= 0.55 mT
Center field = 335.5 mTCenter field = 335.5 mT
ν = 9.38 GHzν = 9.401 GHz
Line width = 0.22 mTLine width = 0.22 mT

Table 1.

EPR parameters of simulated spectra.

Principal valuesDirection cosines
ACHβA(mT)Axx = 0.3470.807090
−0.198271
0.556143
0.452748
0.812426
−0.367402
−0.378980
0.548318
0.745467
Ayy = 0.324
Azz = 0.307
aav = 0.326
gAgxx = 2.016900.745218
0.353981
0.565109
0.366680
0.490298
−0.790667
−0.556952
0.796643
0.235582
gyy = 2.01479
gzz = 2.01263
gav = 2.01477

Table 2.

The EPR parameters of radical A observed in PHM at 125 K.

Note: The errors are estimated to be ±0.00005 and ±0.005 mT for all the calculated g- and A-values, respectively.

Principal valuesDirection cosines
AC2H2βBAxx = 0.6620.618669
−0.736026
0.274800
0.457813
0.053485
−0.887439
0.638480
0.674838
0.370051
Ayy = 0.56
Azz = 0.483
aav = 0.568
gBgxx = 2.011190.506864
0.787949
−0.349608
−0.262535
0.527402
0.808037
0.821075
−0.317780
0.474185
gyy = 2.01056
gzz = 2.00986
gav = 2.01054

Table 3.

The EPR parameters of radical B observed in PHM at 125 K.

Note: The errors are estimated to be ±0.00005 and ±0.005 mT for all the calculated g- and A-values, respectively.

The EPR measurements have shown the existence of two semiquinone anion radicals. The semiquinone anion radicals were stable. The observed semiquinone anion radicals were obtained from the oxidation of PHM single crystal.

The angular dependences of EPR spectra were obtained for different orientations of the static magnetic field with respect to the crystalline axes. For the radical A and radical B, the spectroscopic splitting factor and the hyperfine coupling constants are anisotropic. For the radical A, the average values of the g-factor and the hyperfine coupling constant were obtained as gA= 2.01477 and aCHβA= 0.326 mT, respectively. For the radical B, the average values of the g-factor and the hyperfine coupling constant were obtained as gB= 2.01054 and aC2H2βB= 0.568 mT, respectively.

2.2. Gamma-irradiated succinic anhydride single crystal

Succinate:quinone reductase (SQR) of complex II occupies a unique central point in the mitochondrial respiratory system as a major source of electrons driving reactive oxygen species (ROS) production. It is an ideal pharmaceutical target for modulating ROS levels in normal cells to prevent oxidative stress-induced damage or alternatively, increase ROS in cancer cells, inducing cell death [13].

The single crystals of succinic anhydride were irradiated gamma-rays at room temperature and studied by the EPR technique at 125 K [7]. The spectra were dependent on the orientation of H in the planes which are perpendicular to each other The paramagnetic centers formed from succinic anhydride are shown in Figure 4.

Figure 4.

Structure of two succinic anhydride anion radicals observed in succinic anhydride single crystal.

For only a few angles, the spectra have 1:2:1:1:2:1:2:4:2:2:4:2:1:2:1:1:2:1 intensity ratios. For many angles, the spectra have 1:1:1:1:1:1:1:1:2:2:2:2:2:2:2:2:1:1:1:1:1:1:1:1 intensity ratios. The experimental spectrum in Figure 5 has 1:2:1:1:2:1:2:4:2:2:4:2:1:2:1:1:2:1 intensity ratios and exhibits the superimposition due to the two CH2CH2 atom groups. Because of the radical A, the spectra exhibit 9-lines with intensity ratios 1:2:1:2:4:2:1:2:1. Owing to the β-protons, the spectra exhibit a triplet (1:2:1). Each of the lines (1:2:1) splits into triplets (1:2:1) due to the two equivalent γ-protons. Because of the radical B, the spectra exhibit 9-lines with intensity ratios 1:2:1:2:4:2:1:2:1. Owing to the β-protons, the spectra exhibit a triplet (1:2:1). Each of the lines (1:2:1) splits into triplets (1:2:1) due to the two equivalent γ-protons. Total 18-lines are observed due to the two anion radicals. The spectrum in Figure 6 has 1:1:1:1:1:1:1:1:2:2:2:2:2:2:2:2:1:1:1:1:1:1:1:1 intensity ratios and exhibits the superimposition due to the two (CH2)βCHγ1Hγ2 atom groups. Because of the radical A, the spectra exhibit 12-lines with intensity ratios 1:1:1:1:2:2:2:2:1:1:1:1. Owing to the β-protons, the spectra exhibit a triplet (1:2:1). Each of the lines (1:2:1) splits into doublets (1:1) due to the γ1-proton. Each of the lines (1:1:2:2:1:1) splits into doublets (1:1) due to the γ2-proton. The hyperfine structure splittings in the radical A are the same as that of the radical B. The total 24-lines are observed due to the two anion radicals.

Figure 5.

Experimental and simulated EPR spectra of gamma-irradiated succinic anhydride single crystal at 125 K when the magnetic field is in the bc plane at an angle 90° toward the axis.

Figure 6.

Experimental and simulated EPR spectra of gamma-irradiated succinic anhydride single crystal at 125 K when the magnetic field is in the ac plane at an angle 110° toward the axis.

We have obtained the computer simulations of the spectra that give the best agreement with experimental values. The simulations of the EPR spectra have been carried out using the Bruker’s WINEPR software.

The simulation values of the hyperfine coupling constants of the spectra in Figures 5 and 6 are given in Table 4. The EPR parameters belonging to two succinic anhydride anion radicals observed in succinic anhydride are included in Tables 5 and 6.

Radical ARadical B
Figure 5aCH2βA= aHβA= 3.26 mTaCH2βB= aHβB= 3.281 mT
aCHγ1Hγ2Hγ1A= aHγ1A= 0.47 mTaCHγ1Hγ2Hγ1B= aHγ1B= 0.505 mT
aCHγ1Hγ2Hγ2A= aHγ2A= 0.43 mTaCHγ1Hγ2Hγ2B= aHγ2B= 0.47 mT
Center field = 335.576 mTCenter field = 335.576 mT
ν = 9.368 GHzν = 9.423 GHz
Line width = 0.4 mTLine width = 0.4 mT
Figure 6aCH2βA= aHβA= 3.245 mTaCH2βB= aHβB= 3.253 mT
aCHγ1Hγ2Hγ1A= aHγ1A= 0.39 mTaCHγ1Hγ2Hγ1B= aHγ1B= 0.44 mT
aCHγ1Hγ2Hγ2A= aHγ2A= 0.52 mTaCHγ1Hγ2Hγ2B= aHγ2B= 0.5 mT
Center field = 335.576 mTCenter field = 335.576 mT
ν = 9.368 GHzν = 9.423 GHz
Line width = 0.3 mTLine width = 0.42 mT

Table 4.

EPR parameters of simulated spectra.

EPR parametersPrincipal valuesDirection cosines
ACH2βA= AHβA(mT)Axx = 3.3810.194135
−0.723568
0.662390
0.945231
−0.042637
−0.323606
0.262393
0.688935
0.675662
Ayy = 3.285
Azz = 3.227
aiso = 3.298
ACHγ1Hγ2Hγ1A= AHγ1A(mT)Axx = 0.4920.818223
0.016324
0.574669
0.479980
0.530794
−0.698482
−0.316433
0.847344
0.426473
Ayy = 0.44
Azz = 0. 37
aiso = 0.434
ACHγ1Hγ2Hγ2A= AHγ2A(mT)Axx = 0.620.325533
0.775337
−0.541185
−0.489384
0.627894
0.605188
0.809031
0.067839
0.583838
Ayy = 0.494
Azz = 0.414
aiso = 0.509
gAgxx = 2.017530.998258
−0.054529
−0.022518
0.021593
0.692912
−0.720698
0.054902
0.718957
0.692883
gyy = 2.01513
gzz = 2.01428
giso = 2.01565

Table 5.

The EPR parameters of radical A observed in succinic anhydride at 125 K.

EPR parametersPrincipal valuesDirection cosines
ACH2βB= AHβB(mT)Axx = 3.360.021180
0.829840
−0.557599
−0.990260
−0.059355
−0.125949
−0.137614
0.554835
0.820500
Ayy = 3.283
Azz = 3.251
aiso = 3.298
ACHγ1Hγ2Hγ1B= AHγ1B(mT)Axx = 0.5860.066077
−0.896800
0.437475
0.942855
−0.087383
−0.321541
0.326586
0.433722
0.839778
Ayy = 0.485
Azz = 0.379
aiso = 0.483
ACHγ1Hγ2Hγ2B= AHγ2B(mT)Axx = 0.6120.759274
−0.632874
−0.151571
−0.102410
0.113810
−0.988210
0.642663
0.765844
0.021600
Ayy = 0.464
Azz = 0.288
aiso = 0.455
gBgxx = 2.006600.977193
−0.096452
0.189184
0.210889
0.545205
−0.811343
−0.024888
0.832736
0.553111
gyy = 2.00410
gzz = 2.00300
giso = 2.00457

Table 6.

The EPR parameters of the radical B observed in succinic anhydride at 125 K.

The radical formation mechanism in the present work is the same as that of potassium hydroquinone monosulfonate [6]. The angular dependences of EPR spectra were obtained for different orientations of the static magnetic field with respect to the crystalline axes. For the radical A and radical B, the spectroscopic splitting factor and the hyperfine coupling constants of the Hβ, Hγ1 and Hγ2 protons are anisotropic. For the radical A, the average values of the g-factor and the hyperfine coupling constants were obtained as gA= 2.01565, aHβA= 3.298 mT, aHγ1A= 0.434 mT, aHγ2A= 0.509 mT, respectively. For the radical B, the average values of the g-factor and the hyperfine coupling constants were obtained as gB= 2.00457 and aHβB= 3.298 mT, aHγ1B= 0.483 mT, aHγ2B= 0.455 mT, respectively.

2.3. Gamma-irradiated 3-nitroacetophenone single crystal

Nitroaromatic compounds have widespread actual or potential use in medicine and cancer therapy [14]. Nitroaromatic compounds have been extensively studied and reviewed mainly because of their interesting biological activities as well as their extended use in the chemical industry [15].

The radical in 3NAP identified as 3-nitroacetophenone anion radical is shown in Figure 7. The spectra were found to be temperature dependent. When we examined the EPR spectra at a temperature range of 120–360 K, we decided that the radiation damage center in the sample was due to the break of the pi bond in the carbon-oxygen double bond. It was seen that the unpaired electron interacted with all the protons in the molecular structure [8].

Figure 7.

Structure of the 3NAP anion radical observed in 3NAP.

In the EPR spectra, especially the impact of the methyl protons was felt. The hyperfine coupling constants of the methyl protons were measured at 300 K. The angular variations of EPR parameters of the 3NAP only at 300 K could be investigated. The spectra were clear only in a few angles, at 120 K. Therefore, only the simulations of these spectra were made, at 120 K. In addition, the simulation was also made for a single angle, at 300 K.

The EPR spectra of irradiated 3-nitroacetophenone single crystals consist of four hyperfine structure lines with a 1:3:3:1 intensity ratio, which is caused by the interaction of the unpaired electron with the three equivalent protons of the methyl group, at most orientations of the crystal in the magnetic field, at 300 K. The H(2) and the H(6) protons are also considered approximately equivalent.

In Figures 811, the experimental spectra of the gamma-irradiated 3NAP single crystals and their simulation spectra were compared.

Figure 8.

EPR spectra of 60Co-γ irradiated 3NAP single crystal at 300 K when (a) the magnetic field is in the a*b-plane and is away 20° from the a*-axis and (b) simulation of the spectrum; line width is 0.34 mT.

Figure 9.

EPR spectra of 60Co-γ irradiated 3NAP single crystal at 120 K when (a) the magnetic field is in the a*c-plane and is away 140° from the a*-axis and (b) simulation of the spectrum; line width is 0.38 mT.

Figure 10.

EPR spectra of 60Co-γ irradiated 3NAP single crystal at 120 K when (a) the magnetic field is in the a*c-plane and is away 150° from the a*-axis and (b) simulation of the spectrum; line width is 0.32 mT.

Figure 11.

EPR spectra of 60Co-γ irradiated 3NAP single crystal at 120 K when (a) the magnetic field is in the a*c-plane and is away 165° from the a*-axis and (b) simulation of the spectrum; line width is 0.34 mT.

The simulation values of the hyperfine coupling constants of the spectra in Figures 811 are given in Table 7. The EPR parameters of the 3-nitroacetophenone anion radical are given in Table 8.

Figure 8Figure 9Figure 10Figure 11
aCH3βmT1.551.361.681.585
aH2aH6=aC2H2γmT0.380.430.450.46
aH3=aHδmT0.350.680.710.68
aH4=aHσmT0.250.390.2650.29

Table 7.

Coupling constants values of simulated spectra.

Principal valuesDirection cosines
aCH3β(mT)Axx = 1.7430.673208
0.572490
0.468023
−0.717129
0.659816
0.224430
−0.180325
−0.486721
0.854743
Ayy = 1.447
Azz = 1.015
aiso = 1.402
ρ = 0.52
ggxx = 2.0064430.732137
0.299886
0.611591
0.415100
0.515457
−0.749664
−0.540063
0.802728
0.252902
gyy = 2.006110
g zz = 2.004067
giso = 2.005540

Table 8.

The EPR parameters of the 3-nitroacetophenone anion radical observed in 3-nitroacetophenone at 300 K.

In the EPR spectra, especially the impact of the methyl protons was felt. Only the hyperfine coupling constants of the methyl protons could be experimentally measured at 300 K. The EPR spectra of irradiated 3-nitroacetophenone single crystals consist of four hyperfine structure lines with a 1:3:3:1 intensity ratio, which is caused by interaction of the unpaired electron with the three equivalent protons of the methyl group, at most orientations of the crystal in the magnetic field, at 300 K. The H(2) and the H(6) protons are also considered approximately equivalent. The hyperfine coupling for the CH3 group is anisotropic and their average values being aCH3= 1.402 mT.

The simulations of the EPR spectra were carried out using the Win-EPR software.

3. Conclusion

There are many factors that trigger the formation of free radicals. One of these is the radiation effect. The various compounds that undergo radiation effects have the radiation damage centers as a result of breaking bonds in their structures. The radiation damage center is a paramagnetic center, and the detection and motion of these centers are investigated in detail by EPR spectroscopy.

Antioxidants also show paramagnetic structure under radiation effect. EPR analysis of antioxidants is crucial to remove the effect of free radicals that cause many diseases. Potassium hydroquinone monosulfonate (PHM), succinic anhydride and 3-nitroacetophenone compounds show antioxidant properties. When EPR analyses of these materials were made, it was seen that their radical mechanisms were similar to each other. The anion radicals were observed in all three compounds. When the resonance structure and the formation mechanisms are carefully examined, it is seen that the oxygen atom takes an electron and forms a stable anion structure.

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Betül Çalişkan and Ali Cengiz Çalişkan (August 1st 2018). EPR Analysis of Antioxidant Compounds, Free Radicals, Antioxidants and Diseases, Rizvan Ahmad, IntechOpen, DOI: 10.5772/intechopen.74294. Available from:

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