الفهرس 
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Abstract
The present study was concerned mainly with the antioxidant activity of some phenolic compounds, aniline compounds and heterocyclic compounds by two different methods. The relationship between the chemical structure and the antioxidant activity was studied. Finally, studying the protective role of the most active tested groups using linoleic acid model was investigated. The obtained results are classified into two parts.
The obtained results from part one could be summarized in the following:
1- The investigated phenolic compounds included 36 compounds, which the chemical structures are varied. Aniline compounds group included 17 different compounds. The structural differences among aniline compounds involved type of substituent on the aromatic ring which is varied in the number and the position on the ring. Heterocyclic compounds group included 11 different compounds. This group included vitamin C (L-ascorbic acid) and vitamin E (α-tocopherol). The structural differences among heterocyclic compounds included type of heterocyclic ring, beside number and position of the substituents on the ring.
2- The antioxidant activity of all investigated compounds was estimated by determination of their scavenging activity towards H2O2 and DPPH.
3- The H2O2 scavenging activity of all tested groups was determined at different concentrations. There are wide ranges of activity
Eslam, S., Bendary, Ph.D., 2014
among groups and inside each group. The obtained results were confirmed by the EC50 “efficient concentration” value.
4- The percent of H2O2 scavenging activity of phenolic compounds was determined at concentrations ranged from 0.04167 to 4 mM. 2-Naphthol was the most active phenolic compounds while phenol was the lowest one.
5- The percent of H2O2 scavenging activity of aniline compounds was determined at concentrations ranged from 0.0833 to 0.8333mM. 2,3-Diaminophthaline was the most active aniline compounds while 4-aminobenzoic acid was the lowest compound. On the other hand, 3-nitroaniline has no H2O2 scavenging activity.
6- The percent of H2O2 scavenging activity of 11 heterocyclic compounds was determined at concentrations ranged from 0.0425 to 3.3333 mM. L-Ascorbic acid was the most active heterocyclic compound while histidine was the lowest one. While, proline, hydroxyproline, imidazol, 4-hydroxycoumarin and pyridine have no H2O2 scavenging activity.
7- The more favorable mechanism for scavenging H2O2 is singlet electron transfer (SET). Therefore, the compound that is easier to lose electron, which the more active it is. In general, anilines were more active than phenols due to their lower IP value. Different substituents on phenolic compounds increased H2O2 scavenging activity due to the decrease of IP of phenolic OH. In contrast, most of substituents on aniline compounds decreased activity due to the increase of IP of NH2 group. Therefore, aniline analog structure is more active than phenolic due to its aniline structure.
Eslam, S., Bendary, Ph.D., 2014
8- The presence of electron withdrawing group at ortho or para–position decreased IP of phenolic OH or NH2 group, which had enhanced the activity. But at the same time, it had decreased the stabilization of the formed radical. Some electron withdrawing group at ortho-position was capable of forming iHB. Strong electron withdrawing group at meta-position decreased their negative effect on the H2O2-scavenging activity of phenolic compounds. Strong electron withdrawing group at meta-position decreases the H2O2 scavenging activity of aniline compounds. Substitution with aromatic ring decreases IP of phenolic OH and NH2 group. At the same time, the stabilization of the formed radical was increased due to resonance. Therefore, the H2O2 scavenging activity of phenolic and aniline compounds will be increased dramatically.
9- Electron repelling group at ortho or para-position increases the phenoxy radical stabilization. On the contrary, it decreases IP of NH2 group. Electron repelling group at ortho-position has more negative effect on IP of phenolic OH than at para-position. ortho-Methoxy group increases activity due to its ability to form iHB. On the contrary, para-methoxy group on anilines has more negative effect on IP of NH2 which translated to decrease of the activity.
10- The H2O2 scavenging activity increases in the presence of two unlike active groups than in the presence of two like active groups. Second like active group at ortho-position increases the activity due to the formation of iHB. In contrast, second like active group
Eslam, S., Bendary, Ph.D., 2014
at meta-position decreases activity, while second unlike active group at para-position increases its activity.
11- The comparison between anilines, phenolics compounds and heterocyclic compounds is difficult due to the structural heterogeneity of the tested heterocyclic compounds.
12- DPPH scavenging activity of all tested groups was determined at different concentrations. There are wide ranges of activity among groups and inside each group. The obtained results were confirmed by the EC50 “efficient concentration” value, antiradical efficiency (AE) and the antioxidant activity index (AAI).
13- The percent of DPPH scavenging activity of phenolic compounds was determined at concentrations ranged from 0.00258 to 96.03 mM. Caffeic acid was the most active phenolic compounds, while 4-nitrophenol was the lowest active one. Phenol-4-sulfonic acid, 2-hydroxy-acetophenone, p-methoxybenzoic acid and cinnamic acid had no AE. The phenolic compounds could be classified into two groups according to AAI results; very strong antioxidants group and poor antioxidants group.
14- The percent of DPPH scavenging activity of aniline compounds was determined at concentrations ranged from 0.011 to 33.003 mM. 2, 3-Diaminophethaline was the most active aniline compound while p-anisidine was the lowest active one. While, N,N-dimethyl-4-nitroso-aniline, 2-nitroaniline, 3-nitroaniline, 4-nitroaniline, 3-bromoaniline and 4-chloroaniline had no AE. The aniline compounds could be classified into two groups according to AAI results; very strong antioxidants group and poor antioxidants group.
Eslam, S., Bendary, Ph.D., 2014
15- The percent of DPPH scavenging activity of heterocyclic compounds was determined at concentrations ranged from 0.0055 to 82.51. α-Tocopherol was the most active compound, while hydroxyproline was the lowest one. On the other hand, imidazole, histidine and pyridine had no AE. The heterocyclic compounds could be classified into three groups according to AAI: very strong antioxidants group, strong antioxidants group and poor antioxidants group.
16- The DPPH assay was believed to involve hydrogen atom transfer reaction. So, the more favorable mechanism for DPPH scavenging is hydrogen atom transfer (HAT). In this mechanism, the bond dissociation energy (BDE) of the OH bonds is an important parameter in evaluating the antioxidant activity, because the weaker the OH bond the easier will be the reaction of free radical inactivation.
17- The most active phenolic compound as antioxidant is o-dihydroxy configuration. The antioxidant activity of o-dihydroxy configuration was affected with the substitution at para position. In addition, type of substituents had affected the activity.
18- Phenolic compounds exhibited higher activity as antioxidant than aniline compounds in DPPH-scavenging assay. The higher antioxidant activity of phenolics is due to the lower bond dissociation energies (BDE) of OH than that of NH2. Therefore, phenols analog structure is more active than anilines due to its phenolic structure. Different substituents on phenolic and aniline compounds had affected DPPH scavenging activity positively or
Eslam, S., Bendary, Ph.D., 2014
negatively due to its effect on BDE of phenolic OH group or NH2 group.
19- The presence of withdrawing group at ortho or para-position decreased activity of phenolic and aniline compounds sharply. In anilines, some withdrawing groups at ortho-position are capable of forming iHB, so DPPH scavenging activity will be increased. The negative effect of withdrawing group on the phenols activity had decreased when it is at meta-position then para-position followed by ortho-position. Substitution with aromatic ring decreased BDE of phenolic OH and NH2 group and the stabilization of the formed radical increased due to resonance. So, the activity of phenolic and aniline compounds will be increased.
20- Electron donor group at ortho or para-position increased the stabilization of the formed radical. In addition, it decreased BDE of OH and NH2 group which both positively reflected on the activity. Electron donor group at ortho has more positive effect on BDE of phenolic OH than para-position. The presence of methoxy group at ortho-position is an exception because it has two opposite effects. Ortho-methoxy group decreased the activity due to its ability to form iHB that restrike the H atom transferee. On the other hand, it decreases the BDE of OH that translated into more activity. Therefore, the net of the two opposite effects will be reflected on DPPH scavenging activity of the compounds.
21- The DPPH scavenging activity increased in the presence of two unlike active groups. Active groups at ortho-position increased activity due to formation of iHB followed by at para-position. NH2 group at ortho-position increased its activity.
Eslam, S., Bendary, Ph.D., 2014
22- Aminophenol has a different response according to the reaction mechanism. Whereas, the H2O2-scavenging activity depends on the second mechanism and the compound able to lose electron (ET), so the amino group in this assay is the responsible group for aminophenol antioxidant activity. In contrast, the DPPH-scavenging activity depends on the first mechanism and the compound is able to hydrogen atom transfer (HAT), so the hydroxyl group in this assay is the responsible group for aminophenol antioxidant activity. Therefore, aminophenol is able to form two different radicals depending on the suitable mechanism.
The obtained results from part two could be summarized in the following:
1- The protective role of the most active phenolic and aniline compounds in linoleic acid model was also examined at different concentrations. There are wide ranges of activity between groups and inside each group. The obtained results were confirmed by the EC50 “efficient concentration” value.
2- The inhibition percentage of linoleic acid peroxidation (inhibition of MDA formation) of phenolic compounds was determined at concentrations ranged from 0.02 to 24 mM. 1-Naphthol and 2-aminophenol were the most active phenolic compounds while 3-nitrophenol was the lowest active one. While, 4-nitrophenol, 3-hydroxybenzoic acid, 4-hydroxybenzoic acid and cinnamic acid showed no antioxidant activity. In general, investigated phenolic compounds were less active than α-tocopherol.
Eslam, S., Bendary, Ph.D., 2014
3- The inhibition percentage of linoleic acid peroxidation of aniline compound was determined at concentrations ranged from 0.1 to 8 mM. 2,3-Diaminophthaline was the most active aniline compounds while 2-nitroaniline was the lowest compound.
4- Different substituents on phenolic and aniline compounds affect the protective role due to their effect on the solubility properties firstly and the reaction mechanism secondarily. HAT mechanism is more suitable to scaveng RO● and ROO● under linoleic acid conditions.
5- Phenolic and aniline compounds that can react with only SET mechanism have very week protective role. In addition, phenolic and aniline compounds that prefer SET mechanism more than HAT mechanism have lower protective role than that prefer HAT mechanism.
6- In general, anilines are more active in linoleic acid model protective activity than phenols due to their hydrophobic properties. However, when anilines and phenols have a closer hydrophobicity, the effect of reaction mechanism appeared strongly to determine the protective role. Where, phenols are more active than anilines due to their lower BDE of OH so, HAT mechanism will take place, which is obvious, when the phenolic compounds and their analog aniline compounds are compared.