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13Research Results in Pharmacology
Research Results in Pharmacology 8(3): 41-48 UDC: 615.015:547.785.5 DOI 10.3897/rrpharmacology. 8.85498
Search for compounds with antioxidant and antiradical activity among N9-substituted 2-(biphenyl-4-yl)imidazo[1,2-^]benzimidazoles
Alexander A. Spasov1, Anastasia A. Brigadirova1, Olga N. Zhukovskaya2, Anatoly S. Morkovnik2, Yuliya V. Lifanova1
1 Volgograd State Medical University, 1 Pavshikh Bortsov Sq., Volgograd 400131, Russia
2 Institute of Physical and Organic Chemistry of the Southern Federal University 194/2 Stachki Ave., Rostov-on-Don 344090, Russia Corresponding author: Yuliya V. Lifanova (j_semenova_pharm@mail.ru)
Academic editor: Mikhail Korokin ♦ Received 18 April 2022 ♦ Accepted 2 August 2022 ♦ Published 18 August 2022
Citation: Spasov AA, Brigadirova AA, Zhukovskaya ON, Morkovnik AS, Lifanova YuV (2022) Search for compounds with antioxidant and antiradical activity among N9-substituted 2-(biphenyl-4-yl)imidazo[1,2-a]benzimidazoles. Research Results in Pharmacology 8(3): 41-48. https://doi.org/10.3897/rrpharmacology.8.85498
Introduction: Biphenyl and imidazobenzimidazole derivatives attract ongoing attention as a combination of these two privileged substructures with promising pharmacological activities. The aim of this study was to synthesize and investigate in vitro antioxidant activity of promising novel compounds: 2-(biphenyl-4-yl)imidazo[1,2-a]benzimidazoles.
Materials and methods: The newly synthesized compounds were characterized by IR, 1H NMR and CHBr(Cl)NO analyses. All newly synthesized compounds were screened for their in vitro antioxidant activity: inhibition of lipid peroxidation (LPO), 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS^+) radical cation decolorization and inhibition of hemoglobin (Hb)-H2O2-induced luminol chemiluminescence.
Results and discussion: 2-Amino-3-[(2-biphenyl-4-yl)-2-oxo-ethyl)]-1-R-1H-benzimidazolium bromides were synthesized, and their cyclization into functionalized imidazo[1,2-a]benzimidazole derivatives was studied. The resulting compounds showed LPO inhibitory activity comparable to that of dibunol. Compounds 1a and 1d (see graphical abstract), containing a methyl or dimethylaminoethyl substituent in the N9 position also proved to be equally highly active in the Hb-H2O2-induced luminol chemiluminescence model, while compound 1a was somewhat more active than 1d in the ABTS^ radical scavenging assay.
Conclusion: The study showed that compounds 1a and 1d have the highest antioxidant activity. Thus, this new class of 2-(biphenyl-4-yl)imidazo[1,2-a]benzimidazole derivatives represents a valuable leading series with great potential for use as antioxidants and as promising candidates for further efficacy evaluation.
Graphical abstract:
Abstract
In
Copyright Spasov AA et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Keywords
antioxidant activity, cyclization, imidazo[1,2-a]benzimidazoles, quaternary benzimidazolium salts.
Introduction
One of the main mechanisms of the normal development of the body is to maintain a balance between the processes of free radical and peroxidation of various substrates and the state of antioxidant protection. Free radical oxidation is a necessary process for natural physiological reactions to occur in body cells, but it is also one of the universal mechanisms of their damage (Lankin et al. 2001; Jones 2008). Intensive formation of free radicals with insufficient activity of the endogenous antioxidant compensating system of the body leads to the occurrence of oxidative stress, which is involved in the development of numerous pathologies, for example, tumors (Kinnula and Crapo 2004; Valko et al. 2006), atherosclerosis (Förstermann et al. 2017; Kattoor et al. 2017; Marchio et al. 2019), cardiovascular diseases (Golikov et al. 2003; Petrie et al. 2018; Zhao et al. 2021), neurodegenerative diseases (Guidi et al. 2006; Chen and Zhong 2014; Tönnies and Trushina 2017), diabetes mellitus (Petrie et al. 2018; Luc et al. 2019; Zhang et al. 2020), non-alcoholic fatty liver disease (Cichoz-Lach and Michalak 2014; Masarone et al. 2018; Chen et al. 2020), etc. In cases where the mechanism that prevents and eliminates the consequences of damage caused by free radical oxidation, namely the endogenous antioxidant system, including antioxidants present in the cell in low concentrations, cannot cope with the pathological process, protection against the action of free radical oxidation can be enhanced by the intake of antioxidants.
By chemical nature, antioxidants represent a wide class of compounds: phenols and polyphenols (tocopherols, eugenol, pyrocatechol, gallic acid derivatives), flavonoids (rutin, quercetin), steroid hormones (lecithin, cephalin) and many other compounds (Belviranli and Okudan 2015; Neha et al. 2019). In addition, the N9-substituted imidazo[1,2-a]benzimidazole derivatives, which we are actively studying, also demonstrate antioxidant properties, which allows us to consider this group of compounds as promising for further modification and development of new antioxidants (Anisimova et al. 2007, 2016; Kosolapov et al. 2013; Spasov et al. 2017).
When developing new pharmacologically active compounds, considerable attention is paid to the so-called "privileged" substructures (DeSimone et al. 2004). In continuation of research on the search for new pharmacologically active compounds, the present work describes the synthesis of previously unknown imidazo[1,2-a]benzim-idazoles containing a biphenyl group in position 2, as well as various substituents at the nitrogen atom N9. Biphenyls are of interest as pharmacologically important substructures whose derivatives are characterized by a number of
pharmacological effects, including antioxidant ones (Sev-erinsen et al. 2008; Jain et al. 2017). Their antioxidant potential lies in the ability to serve as scavengers of reactive oxygen species and inhibit lipid peroxidation (LPO) (Maddila et al. 2012; Shashikumar et al. 2014; Rikhi et al. 2015). In this regard, it is promising to study the activity of a combination of these two privileged substructures (Kim et al. 2014; Schneider and Schneider 2017), biphenyl and imidazobenzimidazole derivatives, for which high antioxidant activity should also be expected.
Materials and methods
Synthesis
IR spectra (n/cm-1) of compounds obtained were recorded on a Varian Excalibur 3100 FT-IR spectrophotometer (Varian, USA), using the method of attenuated total reflection in powder; 'H NMR spectra were recorded on Varian Unity-300 (Varian, USA) and Bruker Avance 600 N (Bruker, USA) spectrometers. Chemical shifts for 1H are given relative to the signals of residual protons of a deuterated solvent (DMSO-d6 and CDCl3, 5 2.49 and 7.24, respectively). Melting points were measured on a Fisher-Johns Melting Point Apparatus (Fisher Scientific, USA). Elemental analysis was carried out using a classical method (Gel man et al. 1987). Reaction progress and purity of synthesized compounds were monitored by TLC (plates with Al2O3 III degree of activity, eluent CHCl3, visualization with iodine vapors in a moist chamber).
Generalprocedure for synthesizing 2-amino-3-[(2-biphenyl-4-yl)-2-oxoethyl]-1-R-1H-benzimidazolium bromides 2
To a hot solution of 3 mmol of the corresponding amine 1 in acetone or acetonitrile at room temperature, 3 mmol of 4-(bromoacetyl)biphenyl was added. The reaction mixture was kept for 6-8 h at 25 °C. The hydrobromide precipitate was filtered off and washed thoroughly with acetone. The resulting chromatographically pure salts were dried in air and used in the next step without further purification. The structure of salts 2c-f was confirmed by their transformation into imidazo[1,2-a]benzimidazoles, as well as by spectroscopic data.
2-Amino-3-[2-(biphenyl-4-yl)-2-oxoethyl]-1-butyl-1H-benzimidazolium bromide (2c)
Yield 97%, mp 246-248 °C. Found (%): C 64.48; H 5.45; Br 17.03; N 8.93. C25H26BrN3O. Calculated (%):
25 26 3 v '
C 64.66; H 5.64; Br 17.21; N 9.05. IR spectrum, n/cm-1:
3207, 3240 (NH2), 1687 (C=O). 1H NMR spectrum (300 MHz, DMSO-d6), 5, ppm, J (Hz): 0.90-0.95 (t, 3H, CH-2CH2CH2CH3, J=7.5); 1.34-1.41 (c, 2H, CH2CH2CH2CH3, J=6.9); 1.69-1.74 (t, 2H, CH2CH2CH2CH3, J=7.5); 4.224.26 (t, 2H, NBzm-CH2, J=6.0); 6.01 (s, 2H, CH2CO); 7.277.39 (m, 2H, HJ; 7.46-7.57 (m, 3H, HJ; 7.63-7.68 (t, 2H, HAr, J=7.5); 7.80-7.82 (d, 2H, HAi, J=6.0); 7.95-7.98 (d, 2H, HAr, J=9.0); 8.2 (s, 2H, HJ 8.88 (br. s, 2H, N+H2).
2-Amino-3-[(2-biphenyl-4-yl)-2-oxoethyl)]-1-[2-(dimethylamino)ethyl-1H-benzimidazolium bromide (2d)
Yield 85%, mp 190-193 °C. IR spectrum, n/cm-1: 3208, 3245 (NH2), 1687 (C=O). Found (%): C 62.45; H 5.50; Br 16.48; N 11.50. C25H27BrN4O. Calculated (%): C 62.63; H 5.68; Br 16.67; N 11.69. 'H NMR spectrum (300 MHz, DMSO-d6), 5, ppm, J (Hz): 2.26 (s, 6H, N(CH3)2), 2.67 (s, 2H, N-CH2 exocycle); 4.33-4.37 (t, 2H, NHet-CH2, J=5.9); 6.01 (s, 2H, CH2CO); 7.27-7.38 (m, 2H, HJ; 7.46-7.65 (m, 5H, HAr); 7.80-7.82 (d, 2H, HAr, J=7.2); 7.95-7.98 (d, 2H, HAr, J=8.4); 8.16-8.19 (d, 2H, HAr, J=8.4); 9.0 (s, 2H,
n+h2). r
2-Amino-3-[(2-biphenyl-4-yl)-2-oxoethyl)]-1-[2-(diethylamino)ethyl-1H-benzimidazolium bromide (2e)
Yield 98%, mp 208-210 °C. IR spectrum, n/cm-1: 3208, 3245 (NH2), 1687 (C=O). Found (%): C 63.80; H 6.23; Br 15.63; N 10.95 C27H31BrN4O. Calculated (%): 63.92; H 6.15; Br 15.74; N 11.04. >H NMR spectrum (300 MHz, DMSO-d6), 5, ppm, J (Hz): 0.81-0.85 (t, 6H, N(CH2CH3)2, J=7.05), 2.54-2.50 (t, 4H, N(CH2CH3)2, J=6.45), 2.742.77 (t, 2H, N-CH2 exocycle, J=5.25); 4.33-4.30 (t, 2H, NHet-CH2, J=5.25); 6.01 (s, 2H, CH2CO); 7.26-7.38 (m, 2He, HAr); 7.46-7.65 (m, 5H, HJ; 7.80-7.82 (d, 2H, HAr J=7.2); 7.95-7.98 (d, 2H, HAr, J=8.4); 8.16-8.19 (d, 2H, HAi, J=8.4), 9.0 (s, 2H, N+H2).
2-Amino-3-[(2-biphenyl-4-yl)-2-oxoethyl)]-1-[2-(morpholino)ethyl-1H-benzimidazolium bromide (2f)
Yield 98.2%, mp 219-221 °C. IR spectrum, n/cm-1: 3208, 3245 (NH2), 1688 (C=O). Found (%): C 62.09; H 5.65; Br 15.26; N 10.65. Calculated (%): C 62.19; H 5.57; Br 15.35; N 10.75. >H NMR spectrum (300 MHz, DM-SO-d6), 5, ppm, J (Hz): 2.4 (br. s, 4H, CH2NCH2), 2.7 (s, 2H, CH2), 3.3 (br. s, 4H, CH2OCH2), 4.3(5 (s, 2H, CH2), 6.01 (s, 2H, CH2CO), 7.27-7.39 (m, 2H, HAr), 7.47-7.57 (m, 3H, HJ, 7.62-7.65 (m, 2H, HAr), 7.8-7.83 (t, 2H, HAi, J=7.2), 7.96-7.98 (d, 2H, HAr, J=8.4), 8.16-8.19 (d, 2H, HAi, J=8.1), 8.97 (s, 2H, N+H2).
Synthesis of 2-(biphenyl-4-yl)-9-[2-(dimethylamino)ethyl]-9H-imidazo[1,2-a]benzimidazole hydrochloride (1d)
A mixture of 1 mmol of bromide 2d and 2 mmol of fused sodium acetate was refluxed in 7 mL of glacial acetic acid until the reaction was completed (3-4 h). The precipitate formed during cooling was filtered off, washed with water, and dried in air. The resulting base was purified by recrystallization from DMF. It was then converted to the hydrochloride by the action of concentrated HCl. Yield
80%, mp 225-227 °C. Found (%): C 72.12; H 6.15; Cl 8.36; N 13.54. C25H24N4 HCl. Calculated (%): From 72.02; H 6.04; Cl 8.50; N 13.44. 'H NMR spectrum (300 MHz, DMSO-d6), 5, ppm, J (Hz): 2.95 (s, 6H, 2CH3), 3.69-3.73 (t, 2H, CH2N(CH3)2, J=6.2) , 4.98 (s, NHet-CH2), 7.34-7.55 (m, 5H, HJ; 7.74-7.76 (d, 2H, HAr, J=7.2); "7.81-7.84 (d, 2H, HAr, J=8.4) 7.93-8.09 (m, 4H, HAr); 8.64 (s, 1H, HAr), 10.68 (br. s, 1H, N+H).
Pharmacological activity Inhibition of LPO
Antioxidant activity in vitro was studied in the ascor-bate-induced LPO model (Lankin et al. 1975). A 4% rat liver homogenate was used as a substrate. The LPO reaction was induced by adding 50 mM of ascorbic acid (Chemapol, Czech Republic). The rate of oxidation was judged by the accumulation of products that give a positive reaction with 2-thiobarbituric acid (Fluka, Switzerland) (TBA-positive products). The optical density of the colored sample was measured at a wavelength of 532 nm on a spectrophotometer PD-303UV (APEL, Japan) in a cuvette with an optical path length of 10 mm. The activity of the studied compounds was expressed as a percentage relative to the control sample (without adding compounds). Bu-tylated hydroxytoluene (dibunol) (Merck, Germany) and trolox (Sigma, USA) were used as reference substances. All compounds were tested in the concentration range from 0.1 to 10 ^M to evaluate the concentration-effect relationship and calculate the median inhibitory concentration (IC50).
ABTS^+ radical cation decolorization
Antiradical activity in vitro was studied on the model of the oxidation reaction of 2,2'-azino-bis-(3-ethylbenzothi-azoline-6-sulfonic acid) (ABTSO (Rice-Evans and Miller 1994). The reaction medium with a total volume of 3 mL contained 0.1 mg of hemoglobin (hemoglobin, Hb) and 0.1 mg of ABTS^ (Sigma, USA) in phosphate-buffered saline (pH 6.8). The oxidation of ABTS^ was induced by adding a solution of H2O2 (0.612 mM) in phosphate-buffered saline. The optical density of the sample was measured at a wavelength of 734 nm for 30 min with an interval of 5 min on a spectrophotometer PD-303UV (APEL, Japan) in a cuvette with an optical path length of 10 mm. The activity of the studied compounds was expressed as a percentage relative to the control sample (without adding compounds) at the tenth minute of the reaction. Trolox (Sigma, USA) was used as a reference substance. All compounds were tested in the concentration range from 10 to 100 ^M to evaluate the concentration-effect relationship and calculate the IC50.
Inhibition of Hb-H2O2-induced luminol chemiluminescence
In addition, antiradical activity in vitro was studied in the model of free radical formation in the Hb-H O -luminol
system by measuring the chemiluminescence kinetics (Te-selkin et al. 1997), which was recorded at 37 °C for 10 min on a Lum-100 chemiluminometer (OOO DISoft, Russia). The reaction medium with a total volume of 1 mL contained 0.01 mg Hb and 1 ^M luminol (Serva, Germany) in phosphate-buffered saline (50 mM KH2PO4, 100 ^M EDTA, pH 7.4). Free-radical oxidation of luminol was induced by adding 0.025% H2O2 solution in phosphate-buffered saline. EDTA was added to the buffer to prevent the decomposition of H2O2 by heavy metals present in trace amounts in water and chemical reagents. For all obtained chemiluminescence kinetic curves, the integral under the kinetic curve was calculated for a time equal to 10 min. The activity of the studied compounds was expressed as a percentage relative to the "control kinetics" of chemilu-minescence of the model system without the addition of compounds. Trolox (Sigma, USA) was used as a reference substance. All compounds were tested in the concentration range from 0.1 to 10 ^M to evaluate the concentration-effect relationship and calculate the IC50.
Statistical data processing
Statistical processing of the results was carried out using the non-parametric Kruskal-Wallis test with Dunns multiple comparisons post-test and the regression analysis method for analyzing the concentration-effect relationship and calculating IC50 in the GraphPad Prism 6.0 (GraphPad Software Inc., San Diego, CA, USA).
Results and discussion
The synthesis of 9^-imidazo[1,2-a]benzimidazoles 1a-f containing a biphenyl group directly linked to the benzimidazole tricycle is shown in Fig. 1.
The synthesis of biphenyl derivatives 1a-f was carried out in two stages by quaternization of 1-R-2-aminoben-imidazoles 3a-f with 4-(bromoacetyl)biphenyl, followed by acid-catalyzed cyclization of the resulting 1-R-(4-bi-phenoyl)-methyl-2-iminobenzimidazole hydrobromides 2a-f. Quaternary salts 2a-f are formed in almost quantitative yield (92-95%) and can be used in the next step without further purification. The cyclization of bromides 2a-f
was carried out by boiling in acetic acid in the presence of fused sodium acetate for 4 h. The tricycles 1a-f that precipitated from the reaction mass on cooling were washed with water, dried, and purified by crystallization. Mixing tests of these compounds did not show depression with the compounds previously prepared by basic catalyzed cyclization. Physicochemical characteristics of compounds 1a-c,e,f and 2a,b were published in (Spasov et al. 2017).
The structure of the obtained biphenyl derivatives 1d, 2a-f was confirmed by IR and *H NMR spectroscopy and elemental analysis. IR spectra of quaternary benzim-idazolium salts 2a-f are characterized by the presence of absorption bands of the immonium group >N+=C (16871688 cm-1) and stretching vibrations of the primary amino group (two bands in the region of 3150-3240 cm-1). In the *H NMR spectra of bromides 2a-f, in addition to other signals, there are two-proton singlets of the protons of the methylene groups of the biphenoylmethyl fragments (5 5.9-6.0 ppm) and the protonated imino group (5 8.83-8.88 ppm). In the spectra of cyclization products, imidazobenzimidazole bromohydrates 1a-f, such signals are absent, but downfield signals of the N+H fragment and the H(3) proton of the imidazole ring formed during the reaction are observed.
First of all, the antioxidant activity of the newly synthesized 2-(biphenyl-4-yl)imidazo[1,2-a]benzimidazoles 1a-1f was studied in vitro using an ascorbate-induced LPO model. This model is a widely used method for primary testing chemical compounds for the presence of antioxidant activity and belongs to the so-called enzyme-independent methods (Alam et al. 2013; Romulo 2020). According to the results of the experiment, it was found that all the studied compounds 1a-1f at a maximum concentration of 10 ^M significantly suppressed the process of ascorbate-induced LPO (Table 1). Their activity was comparable to the activity of the reference substance dibunol and statistically significantly (¿><0.05) exceeded the activity of the other reference substance trolox by almost 2 times. At a lower concentration of 1 ^M, only two compounds, 1a and 1d, containing a methyl or dimethylaminoethyl substituent at the N9 position of imidazo[1,2-a]benzimidazole, respectively, retained
Figure 1. Scheme of 9H-imidazo[1,2-a]benzimidazole derivatives synthesis. 1-3: R=H (a); CH3 (b); (CH2)2CH3 (c); CH2N(CH3)2 (d); CH2N(C2H5)2 (e); CH2N(CH2CH2)2O (f). Reagents and conditions: i. 4-(bromoacetyl)biphenyl, acetone; ii. CH3COONa, glacial acetic acid CH3COOH, boiling; iii. HCl.
high antioxidant activity. At the same time, the IC50 of compounds 1a and 1d turned out to be similar to that for dibunol and was lower than the IC50 of trolox by 95 and 50 times, respectively.
Next, the antiradical properties of compounds 1a and 1d with the highest antioxidant activity according to the results of the first experiment were studied in vitro using the ABTS^ oxidation reaction model (Rice-Evans and Miller 1994). When ABTS^ is incubated in the presence of Hb and H2O2, a relatively stable ABTS^+ radical cation is formed, and compounds with antiradical properties reduce ABTS^+radical cation to ABTS^ and decolorize its solution (Alam et al. 2013; Romulo 2020). According to the experimental results, both the reference substance trolox and compounds 1a and 1d at high concentrations of 50 and 100 ^M significantly suppressed the ABTS^ oxidation reaction. At a lower concentration of 10 ^M, compounds 1a and 1d, but not trolox, also showed little antiradical activity (Table 2). At the same time, the IC50 of compound 1a was slightly more than 2 times lower than the IC50 of trolox, while the IC50 of compound 1d was similar to that of trolox.
In addition, the antiradical properties of compounds 1a and 1d were studied in vitro in a free radical formation model in the Hb-H2O2-luminol system by measuring the chemiluminescence kinetics (Teselkin et al. 1997). When
interacting with some reactive molecules (free radicals, reactive oxygen species), luminol undergoes oxidation, during which chemiluminescence quanta are emitted. In this regard, luminol is used as a luminescent probe for reactive oxygen species. The introduction of inhibitors of free radical oxidation into the model chemiluminescence system leads to a change in the parameters of the chemiluminescence kinetics of luminol. This change is manifested in an increase in the latent period, a decrease in the light sum and the intensity of the glow. The nature of the change in these parameters depends on the mechanism of action of the test compound (Kobayashi et al. 2001). Compounds 1a and 1d and reference substance trolox were able to scavenge reactive oxygen species and luminol radicals formed in the reaction system in the model of Hb-H2O2-induced luminol chemiluminescence. At the same time, in compounds 1a and 1d, the antiradical properties turned out to be statistically significantly (¿><0.05) more pronounced than in trolox, which is also confirmed by their IC50 values, which were approximately 5.5-5.9 times lower than that of trolox (Table 3).
The high antioxidant properties of 9H-imidazo[1,2-a] benzimidazoles are explained by the fact that fused benzimidazole derivatives are polynuclear aromatic compounds (Pozharskiy 1985) with a complex
Table 1. Antioxidant activity of N9-substituted-2-biphenylimidazo[1,2-a]benzimidazoles and reference substances in the model of ascorbate-induced LPO in vitro
Compound R Inhibition of ascorbate-induced LPO, mean±SE, n=6 (%) 10 ^M 1 ^M IC50 (^M)
1a CH3 94.27±2.70* 73.06±0.26 0.19
1b C2H5 91.75±1.06* 25.06±1.20# 2.37
1c C4H9 91.50±1.31* 46.12±1.29 1.22
1d CH2CH2N(CH3)2 92.17±1.13* 67.99±3.47 0.36
1e CH2CH2N(C2HA 93.50±6.17* 33.64±1.39# 1.88
1f CH2CH2N(CH2CH2)2O 92.66±0.83* 49.93±0.60 1.01
Dibunol - 92.95±0.78 77.58±2.49 0.27
Trolox - 48.22±0.19 18.1
Note: Statistical significance: *p<0.05 vs. Trolox, *p<0.05 vs. Dibunol (Kruskal-Wallis test with Dunn's multiple comparisons post-test); ■ - not tested.
Table 2. Effect of N9-substituted-2-biphenylimidazo[1,2-a]benzimidazoles and reference substance on ABTS^ oxidation reaction in vitro
Compound R Inhibition of the ABTS^ oxidation reaction, mean±SE, n=6 (%) IC50 (^M)
50 ^M 10 ^M
1a CH3 90.87±1.19* 31.48±1.24* 22.4
1d CH2CH2N(CH3)2 67.17±1.41 12.72±1.63 50.1
Trolox - 67.04±2.56 0.77±0.32 49.5
Note: Statistical significance: *p<0.05 vs. Trolox (Kruskal-Wallis test with Dunn's multiple comparisons post-test).
Table 3. Antiradical activity of N9-substituted-2-biphenylimidazo[1,2-a]benzimidazoles and reference substances in the model of
Hb-H2O2-induced luminol chemiluminescence in vitro
Compound R Chemiluminescence inhibition, m±SE, n=6 (%) 1 ^M IC5„ (^M)
1a CH3 97.37±0.31* 0.27
1d CH2CH2N(CH3)2 97.23±0.46* 0.29
Trolox - 40.04±5.11 1.6
Note: Statistical significance: *p<0.05 vs. Trolox (Kruskal-Wallis test with Dunn's multiple comparisons post-test).
n-electron system with unpaired electrons, which gives this condensed system the properties of "electron redundancy" and makes it vulnerable to attack by electrophilic particles (Grandberg and Nam 2016). The structure of imidazo[1,2-a]benzimidazole contains a 14n-electron system and two pairs of unpaired electrons in orbitals perpendicular to the n-system. Thus, imidazobenzimidazole derivatives have a high n-redundancy and can be donors of electron pairs that are not part of the aromatic n-system (Avdyunina 1979), and, therefore, are characterized by high reactivity and the ability to inhibit free-radical oxidation processes.
Thus, 2-(biphenyl-4-yl)imidazo[1,2-a]benzimidazoles showed pronounced antioxidant properties in the model of ascorbate-induced LPO, comparable with the activity of the reference substance dibunol. When evaluating the antiradical properties of the two most active compounds in the model ofascorbate-induced LPO, containing a methyl or dimethylaminoethyl substituent - 1a and 1d, respectively, in the N9 position of imidazo[1,2-a]benzimidazole, they also turned out to be equally highly active in the model of Hb-H2O2-induced luminol chemiluminescence, whereas in the ABTS^ oxidation reaction model, compound 1a was slightly more active than 1d.
imidazo[1,2-a]benzimidazole derivatives (1a-1f) in good yields. All synthesized compounds were tested for their antioxidant activity. The study showed that compounds 1a and 1d have the highest antioxidant activity. Thus, this new class of 2-(biphenyl-4-yl) imidazo[1,2-a]benzimidazole derivatives represents a valuable leading series with great potential for use as antioxidants and as promising candidates for further efficacy evaluation.
Funding
The chemical section was financially supported by the Ministry of Science and Higher Education of the Russian Federation (state task in the field of scientific activity, Southern Federal University, 2020, project FENW-2020-0031 [0852-2020-0031]).
Conflict of interests
The authors have declared that no competing interests exist.
Conclusion
In conclusion, we have described a simple and efficient protocol for the synthesis of novel 2-(biphenyl-4-yl)
Acknowledgments
This work was carried out using the equipment of the Common Use Center of the Southern Federal University.
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Author Contribution
■ Alexander A. Spasov, Doctor Habil. of Medical Sciences, Professor, Member of the Russian Academy of Sciences, Head of the Department of Pharmacology and Bioinformatics; e-mail: aspasov@mail.ru, ORCID ID http://orcid. org/0000-0002-7185-4826. The author suggested the concept and design of the manuscript and approved of the final version of the manuscript.
■ Anasyasia A. Brigadirova, PhD in Medical Sciences, Associate Professor of the Department of Pharmacology and Bioinformatics; e-mail: a.brigadirova@gmail.com, ORCID ID http://orcid.org/0000-0003-0957-7087. The author conducted the biological experiments, analyzed their results, and wrote the pharmacological part of the manuscript.
■ Olga N. Zhukovskaya, PhD in Chemical Sciences, Senior Scientist at the Laboratory of Organic Synthesis, Institute of Physical and Organic Chemistry; e-mail: zhukowskaia.ol@yandex.ru, ORCID ID http://orcid.org/0000-0003-2485-2139. The author synthesized the compounds, studied their structure, and wrote the main manuscript text.
■ Anatoly S. Morkovnik, Doctor Habil. of Chemical Sciences, Head of the Laboratory of Organic Synthesis; e-mail: asmork@mail.ru, ORCID ID http://orcid.org/0000-0002-9182-6101. The author was engaged in editing the chemical part of the manuscript.
■ Yuliya V. Lifanova, PhD student in Medical Sciences, Assistant Professor of the Department of Pharmacology and Bioinformatics; e-mail: j_semenova_pharm@mail.ru, ORCID ID https://orcid.org/0000-0001-9663-5067. The author obtained the data and revised the manuscript.
