research article

The Antioxidant Activity of Dihydropyridine Derivatives

M. Akram Khan*, Vinay B Kola, Bodrul Noor, Jean Acco

Bimolecular Research Centre, Sheffield Hallam University, Howard Street, Sheffield S11WB, UK

*Corresponding author: M. Akram Khan, Bimolecular Research Centre, Sheffield Hallam University, Howard Street, Sheffield S11WB, UK

Received Date: 07 December, 2020; Accepted Date: 11 December, 2020; Published Date: 18 December, 2020

Citation: Khan MA, Kola VB, Noor B, Acco J (2020) The Antioxidant Activity of Dihydropyridine Derivatives. Curr Res Bioorg Org Chem 03: 124. DOI: 10.29011/2639-4685.100024

Abstract

1,4-Dihydropyridines 6a-i were synthesised in the laboratory by the Hantzsch reaction and purified by flash column chromatography. These molecules bear a close resemblance to the biological reducing agent Nicotinamide Adenine Dinucleotide (NAD) and were evaluated for their antioxidant activities by two complementary assays, DPPH and β-carotene/ linoleic acid. The relative antioxidant activity (RAA) results obtained by β-carotene/linoleic acid were more reliable and showed compounds with electron donating groups on the aromatic rings gave higher RAA values compared with L-ascorbic acid (AA). Compounds 6a, 6c, 6d and 6g possessed the most potent antioxidant activity of 71%, 80%, 78% and 45% respectively compared with AA with RAA 49%. It was found that the free radical scavenging ability of compounds 6b and 6f showed a small concentration dependence profile in DPPH assay but both had low antioxidant activity in both DPPH and b-carotene/linoleic assays.

Graphical Abstract

The antioxidant activities of 1,4-dihydropyridines synthesised by the Hantzsch reaction.


Keywords: Antioxidant activity; 1,4-Dihydropyridines; Hantzsch synthesis; NADH

Introduction

Aromatic heterocyclic molecules comprise the largest class of therapeutic compounds in clinical use and continue to feature as promising candidates in future drug discovery programmes. In this context, 1,4-dihydropyridines (1,4-DHP) constitute a large group of structurally diverse group of drugs that possess a wide variety of biological activities such as calcium-channel modulating, antihypertensive, antioxidant, antimicrobial, vasodilator, bronchodilator, anti-atherosclerotic, anti-aggregation, anti-ischemic, anti-diabetic, and antitumor agents [1,2] agents. For example, nifedipine 1, nitrendipine 2 and amlodipine 3 (Figure 1) are 1,4-DHP clinically important drugs used in pharmacology for the treatment of cardiovascular diseases, including hypertension [3].

As a result of the widely recognised well established pharmacological activities of 1,4-DHPs research interest in this area has continued to grow and generate new molecules. Calcium antagonist DHPs have been evaluated for their antioxidant activity via a competitive kinetic procedure. It was found that calcium antagonist DHPs such as nifedipine 1 and nitrendipine 2, possessed antioxidant activity related to their electron density on the DPH ring. The process of anti-oxidation took place via a primary one-electron accompanied by a fast proton release, resulting in the creation of a neutral radical undergoing an easier one-electron step. This results in the generation of a final product being a protonated form of the parent pyridine. Therefore, as the radical is more prone to be oxidised than reduced, this prevents the propagation of the oxidative chain reaction. In another study the synthesis of a new group of 1,4-DHPs has been reported for their antioxidant properties [2,4,5]. One interesting feature of 1,4-DHPs is their close resemblance to the biological reducing agent Nicotinamide Adenine Dinucleotide (NADH) and their interaction with cellular enzymes. Based on this, two new synthetic series of 1,4-DHP derivatives containing substituted pyrazole moiety have been reported for antioxidant activity. These compounds were found to possess both antioxidant and antimicrobial properties [4,6] and displayed good safety profiles when administered in in vivo experiments in large doses (2000mg).

Reactive oxygen species (ROS) are formed during normal cell aerobic respiration [7] and are the main cause of cell damage involved in chronic diseases like diabetes cancer, cardiovascular and others [8]. Antioxidants play an important role in neutralising (ROS) and protecting the cells from oxidative damage. In an on-going work on heterocyclic compounds, in our laboratories, we have synthesised some 1,4-DHPs and have developed assays to assess their antioxidant activities. In this endeavour, here, we report the antioxidant activities of nine 1,4-DHPs (6a-i) with considerable structural diversity (Figure 2).

Experimental section

Chemistry: materials and method

Melting points were recorded on Stuart SMP3 digital apparatus ; IR spectra were recorded on Perkin-Elmer Spectrum 100 FTIR spectrophotometer with a universal ATR sampling accessory; 1H NMR spectra were recorded on a Bruker AC 250 MHz and 13C NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometers. Mass spectra (MS) were obtained on VG 770E spectrometer operated in EI mode at 70 eV. TLC analyses were done using Merck aluminium coated silica gel sheets and flash chromatography was performed using BDH flash silica gel and the eluents are indicated in parenthesis for each compound.

General method for synthesis of dihydropyridine (6a-i) by the Hantzsch reaction.

A typical procedure is illustrated by the synthesis of diethyl 2,6-dimethyl-4-(thiophen-3-yl)-1,4-dihydropyridine-3,5-dicarboxalate (6f). To a round bottom flask equipped with a magnetic stirrer thiophene-3-carboxaldehyde (2.24g,0.02moles) was added ethyl acetoacetate (5.20g, 0.04moles), isopropanol (3ml) and conc. ammonia (0.5ml). The mixture was refluxed in an oil bath at 120°C for 3h and then rotary evaporated at 60°C to remove the solvent to give the crude product as a solid (4.70g) which was purified by silica gel flash chromatography [1:2, ethyl acetate: heptane] to give pure product (6f) (4.10g, 61.2%) as a brown coloured solid; Rf = 0.34 (1:2, ethyl acetate: heptane); m.p.168.8-169.5°C; FTIR (solid) n (cm-1) 3342.80 (NH), 1694.64 (ester >C=O), 1647.84 (alkene, C=C). 1H NMR: (d, ppm, CDCl3) 1.20 (6H, triplet, J=7.12 Hz, 2 x -OCH2-CH3), 2.30 (6H, s, 2 x -CH3), 4.12 (4H, q, J= 7.12 Hz, 2 x -O-CH2-), 5.17 (1H, s, >CH-), 5.80 (1H, singlet, NH), 6.93 (1H, fine d, J = 1.3 Hz, H-2), 7.00 (1H, d, J = 3.3 Hz, H-4), 7.13 (1H, dd, J= 1.3 Hz and 3.3 Hz, H-5);13C NMR (d, ppm, CDCl3) 14.34 -H2C-CH3), 18.40 and 19.33 (=C-CH3), 34.61(>C-H-),59.61 and 59.81 (-O-CH2-), 102.86 and 103.28 (=C-CO2Et), 120.29 (C5), 124.60 (C-4), 127.60 (C-2), 144.68 (C-3), 147.94 (>C-NH-), 167.76 (>C=O); High resolution EIMS (M+) found (calculated): 335.1370 (335.1191).

Diethyl 2,6-dimethyl-4-(quinolin-4-yl)-1,4-dihydropyridine-3,5- dicarboxylate (6a) was obtained as grey solid (59% yield); m.p.193-201°C; Rf = 0.09 (1:2, ethyl acetate: heptane); FTIR (solid) n (cm-1) 3266.05 (NH), 1698.05 (ester >C=O); 1H NMR:(d, ppm, CDCl3) 0.95 (6H, triplet, J=7.12 Hz, 2 x -OCH2-CH3), 2.40 (6H, s, 2 x -CH3), 3.95 (4H, q, J= 7.12 Hz, 2 x -O-CH2-), 5.85 (1H, s, >CH-), 6.04 (1H, s, >NH), 7.43 (1H, d, 4.60 Hz, H-3), 7.57 (1H, t, J = 7.80 Hz, H-5), 7.67 (1H, t, J = 7.80 Hz, H-6), 8.02 (1H, d, 7.80 Hz, H-5), 8.60 (1H, d, J = 7.80 Hz, H-8), 8.80 (1H, d, J = 4.60 Hz, H-2); 13CNMR ((d, ppm,CDCl3): 13.97(CH3), 21.57(CH3), 41.03(>CH-), 61.22 (OCH2), 118.55(C-3), 122.68 (C-5), 125.11(C-6), 129.90(C-10),129.97(C-7), 136.32(C-8), 146.19(C-4), 148.20(C-9), 150.10(C-2), 1167.60 (>C=O); High resolution EIMS (M+) found (calculated): 380.1670 (380.1736).

Diethyl 4-(7-chloroquinolin-4-yl)-2,6-dimethyl--1,4-dihydropyridine-3,5-dicarboxylate (6b) was obtained as a white solid (72% yield); 198-2050C; Rf = 0.06 (1:2, ethyl acetate: heptane); FTIR (solid) n (cm-1 ) 3265.51 (NH), 1697.77 (ester >C=O); 1 H NMR:(d, ppm, CDCl3) 1.00 (6H, triplet, J=7.12 Hz, 2 x -OCH2 -CH3), 2.40 (6H, s, 2 x -CH3), 3.92 (4H, q, J= 7.12 Hz, 2 x -O-CH2-), 5.80 (1H, s, >CH-), 5.90 (1H, s, >NH), 7.40 (1H, d, 4.60 Hz, H-3), 7.56 (1H, t, J = 7.80 Hz, H-5), 7.57 (1H, t, J = 7.80 Hz, H-6), 8.05 (1H, d, 7.80 Hz, H-5), 8.60 (1H, d, J = 7.80 Hz, H-8), 8.80 (1H, d, J = 4.60 Hz, H-2); 13CNMR (d, ppm, CDCl3): 13.97(CH3), 21.57(CH3), 41.03(>CH-), 61.22(OCH2), 119.38(C-3), 124.55(C5), 125.19(C-10), 127.69(C-6), 129.00(C-8), 135.08(C-7), 146.53(C-4), 148.84(C-9), 151.27(C-2), 167.10 (>C=O); High resolution EIMS (M+) found (calculated): 414.1276 (380.1346).

Diethyl 4-(8-fluoroquinolin-4-yl)-2,6-dimethyl--1,4-dihydropyridine-3,5-dicarboxylate (6c) was obtained as a white solid (82% yield); 198-2050C; Rf = 0.10 (1:2, ethyl acetate: heptane); FTIR (solid) n (cm-1) 3262.41 (NH), 1695.87 (ester >C=O); 1H NMR: (d, ppm, CDCl3) 1.00 (6H, triplet, J=7.12 Hz, 2 x -OCH2-CH3), 2.40 (6H, s, 2 x -CH3), 3.90 (4H, q, J= 7.12 Hz, 2 x -O-CH2-), 5.75 (1H, s, >NH), 5.82 (1H, s, >CH-), 7.35 (1H, d, 7.90 Hz, H-7), 7.40 (1H, d, J = 4.50 Hz, H-3), 7.57 (1H, t, J = 7.90 Hz, H-6), 8.40 (1H, d, J = 7.90 Hz, H-5), 8.80 (1H, d, J = 4.50 Hz, H-2); 13CNMR (d, ppm,CDCl3): 13.97(CH3), 21.57(CH3), 41.03(>CH-), 61.22(OCH2), 113.25(C-7), 118.83(C-5), 120.14(C-3), 126.48(C6), 128.44(C-8), 138.85(C-7), 146.38(C-4), 150.34(C-9), 158.53(C-2), 167.10 (>C=O); High resolution EIMS (M+) found (calculated): 398.1856 (398.1642).

Diethyl 2,6-dimethyl-4-(2,4,5-trimethoxyphenyl)-1,4-dihydropyridine-3,5-dicarboxylate (6d) as a creamy coloured solid (85.9% yield); m.p.178-180°C; Rf = 0.12 (1:2, ethyl acetate: heptane); FTIR (solid) n (cm-1 ) 3347.13 (NH), 1690.02 (ester >C=O), 1202.22 (CO-C); 1H NMR: (d, ppm, CDCl3 ) 1.30 (6H, triplet, J=7.12 Hz, 2 x -OCH2 -CH3), 2.37 (6H, s, 2 x -CH3), 3.85 (6H, s, 2 x OCH3), 3.92 (3H, s, OCH3), 4.15 (4H, q, J= 7.12 Hz, 2 x -O-CH2-), 5.20 (1H,s, >CH-), 5.92 (1H, s, >NH), 6.54 (1H, s, H-3), 6.88 (1H, s, H-6); 13CNMR (d, ppm,CDCl3): 14.34 (CH3),19.29 (CH3), 36.16 (>CH-), 55.95 (-OCH3), 56.53 (OCH3) 59.43 (OCH2), 98.05(C-3), 102.58 (CO-C<), 115.36(C-6), 127.52 (C-1), 142.29(C-5), 143.78(C-4), 148.06(C-2), 152.31(NH-C<), 168.71 (>C=O); High resolution EIMS (M+) found (calculated): 419.2085 (419.1944).

Diethyl 4-(4-(benzyloxy)phenyl)-1,4-dihydro-2,6-dimethyl pyridine-3,5-dicarboxylate (6f) white solid (53.6% yield); m.p.170°C; Rf = 0.31 (1:2, ethyl acetate: heptane); FTIR (solid) n (cm-1) 3356.57 (NH), 1692.10 (ester >C=O), 1197.57 (C-O-C); 1H NMR: (d, ppm, CDCl3) 1.35 (6H, triplet, J=7.12 Hz, 2 x -OCH2 -CH3), 2.44 (6H, s, 2 x -CH3), 4.23 (4H, q, J= 7.12 Hz, 2 x -O-CH2-), 5.11 (1H, s, >CH-), 5.13 (2H, s, OCH3Ph), 6.00 (1H, s, >NH), 6.94 (2H, d, AB system J = 7.8 Hz, H-3 and H-5), 7.14 (2H, d, AB system J = 7.8 Hz, H-2 and H-6) 7.27 (3H, m, Ph) 7.34 (2H, m, Ph); 13CNMR (d, ppm,CDCl3): 14.31(CH3),19.51(CH3), 38.79 (>CH-), 59.73 (2 x -OCH2), 69.99 (OCH2Ph), 104.27(C-3), 102.58(COC<), 114.14(C-3), 127.55, 127.87, 128.54, 129.00, 137.30, 140.67, 143.77, 157.22 (Ar =C-O-), 167.79 (>C=O); High resolution EIMS (M+) found (calculated): 435.2070 (435.2046).

Diethyl 2,6-dimethyl-4-(thiophen-2-yl)-1,4-dihydropyridine-3,5- dicarboxalate (6g) brown coloured solid (89.2% yield); m.p.162.4- 162.6°C; Rf = 0.34 (1:2, ethyl acetate: heptane); FTIR (solid) n (cm-1) 3343.40 (NH), 1692.27 (ester >C=O), 1649.13 (alkene, C=C). 1H NMR: (d, ppm, CDCl3) δ1.27 (6H, triplet, J=7.12 Hz, 2 x -OCH2-CH3), 2.33 (6H, s, 2 x -CH3), 4.17 (4H, q, J= 7.12 Hz, 2 x -O-CH2-), 5.34 (1H, s, >CH-), 6.03 (1H, singlet, NH), 6.77 (1H, d, J = 3.3 Hz, H-2), 6.83 (1H, dd, J = 3.3 Hz, H-3), 7.03 (1H, d, J = 3.3 Hz, H-4); 13C NMR (d, ppm, CDCl3) 14.33 -H2C-CH3), 18.42 and 19.38 (=C-CH3), 34.41(>C-H-),58.40 and 59.92 (-O-CH2-), 103.43 (=C-CO2Et), 123.10, 123.14, 126.34 (C-2), 144.74, 151.63 (>C-NH-), 167.46 (>C=O); High resolution EIMS (M+) found (calculated): 335.1407 (335.1191).

Diethyl 2,6-dimethyl-4-(5-methylfuran-2-yl)-1-4-dihydropyridine-3,5-dicarboxylate (6h) dark brown solid (55.8% yield); m.p.145-147°C; Rf = 0.35 (1:2, ethyl acetate: heptane); FTIR (solid) n (cm-1) 3342.35 (NH), 1697.09 (ester >C=O), 1204.99.13 (-CO-C-). 1H NMR: (d, ppm, CDCl3) 1.32 (6H, triplet, J=7.12 Hz, 2 x -OCH2-CH3), 2.24 (3H, s, Ar-CH3), 2.37 (6H, s, 2 x -CH3), 4.23 (4H, q, J = 7.12 Hz, 2 x -O-CH2-), 5.19 (1H, s, >CH-), 5.83 (2H, s, Ar-H), 6.02 (1H, s, > NH); 13C NMR (d, ppm, CDCl3) 13.69 (CH3-Ar), 14.34 (-H2C-CH3), 19.36 (=C-CH3), 33.35(>C-H-),58.37 and 59.73 (-O-CH2-), 100.77 (=C-CO2 Et), 105.01, 105.85, 145.07, 150.22, 157.05 (>C-NH-), 167.71 (>C=O); High resolution EIMS (M+) found (calculated): 333.1732 (333.1576).

Diethyl 2,6-dimethyl-4-(ferrocenyl)-1-4-dihydropyridine-3,5- dicarboxylate (6i) golden yellow solid, (57% yield); m.p. 226- 228°C; Rf = 0.38(1:2, ethyl acetate: heptane); FTIR (solid) n (cm-1) 3339.7 (NH), 1695.8 (ester >C=O), 1204.99.13 (-C-O-C-). 1H NMR: (d, ppm, CDCl3) 1.36 (6H, triplet, J=7.12 Hz, 2 x -OCH2-CH3), 2.37 (6H, s, 2 x -CH3), 3.96 (4H, d, J= 5.1 Hz, ferrocene H), 4.06 (5H, s, ferrocene H), 4.27 (4H, q, J=7.12, 2 x -O-CH2-), 4.85 (1H, s, >CH), 5.67 (1H, broad s, >NH); 13C NMR (d, ppm, CDCl2) 14.53 (-H2C-CH3), 19.65 (=C-CH3), 32.07(>C-H-), 59.90 (-OCH2-), 69.95 (ferrocene), 76.71 (ferrocene), 103.79 (=C-CO2Et), 143.77 ( >C-NH- ), 167.95 (>C=O); High resolution EIMS (M+) found (calculated): 437.1410 (437.1290).

DPPH assay

This assay spectrophotometrically measures the colour decay of the stable free radical diphenylpicrylhydrazyl (DPPH) by interaction with an antioxidant [9]. Fifty mL of various concentrations of methanolic solution of the sample was added to 5 mL of a 101 µmol methanolic solution of DPPH. After a 30 min incubation period at room temperature, the absorbance was read against a blank at λ517 nm. Inhibition of free radical DPPH in percent (I%) was calculated in following way:

I %= ( Ablank – Asample / Ablank) x 100

Where Ablank is the absorbance of the control reaction (containing all reagents except the test compound), and Asample is the absorbance of the test compound. Concentration providing 50% inhibition (IC50) was calculated from the graph by plotting inhibition percentage against sample concentration. Assays were carried out in triplicate. Synthetic antioxidant Butylated Hydroxy Toluene (BHT) was used as positive control.

ß Carotene-linoleic acid assay

In this assay, antioxidant capacity of the compound is determined by measuring the conjugated dienes produced from linoleic acid oxidation. A stock solution of b-carotene-linoleic acid mixture was prepared as following: 0.5 mg b-carotene was dissolved in 1 mL of chloroform (HPLC grade), 25 mL linoleic acid and 200 mg Tween 40 was added. The chloroform was completely evaporated using a vacuum evaporator. Distilled water (100 mL) saturated with oxygen (30 min, 100 mL/min.) was added with vigorous shaking. 2.5 mL of this mixture was added to three test tubes and ethanolic solution (350 mL) of the test compound (concentration 2 mg/mL) was added and the emulsion thus produced was incubated for up for 24 hours at room temperature. The same procedure was repeated with positive control BHT and a blank. After completion of the incubation period absorbance of the mixture was taken at λ 490 nm. Antioxidant capacities of the synthetic curcuminoids were compared with BHT and blank run under identical conditions.

Results and Discussion

Limited methods exist for the synthesis of unsymmetric 1,4- DHPs [2]. However, for the synthesis of symmetrical 1,4-DHPs the Hantzsch synthesis is the method of choice and we employed this method to produce our compounds 6a-i. The aldehydes 5 a-i were heated with the β-dicarbonyl compound ethyl acetoacetate 4 and ammonia (Figure 3). The 1,4-DHPs 6a-i were isolated, purified by flash column chromatography and characterised spectroscopically by FTIR, 1H NMR, 13C NMR and MS. Characteristic features of the 1,4-DHPs were the absorbance of the >NH functional group in 6a-i as a single peak in the FTIR spectra at around 3300 cm-1 and its resonance as a singlet at about 6ppm in the 1HNMR spectra. The other characteristic feature in the 1HNMR spectra was the resonance of the tertiary SP3 proton at position-4 of 1,4-DHP as a singlet at around 4.9 ppm. The molecules 6a-i all gave the correct molecular ions in the mass spectra.

Two complementary assays were employed for screening the antioxidative properties of the synthetic compounds 6a-i. One of the assays measured the free radical scavenging activity-using 2,2-Diphenylhydroxyl Stable Free Radical (DPPH) and a second assay involved the inhibition of the lipid oxidation to determine antioxidant capacity of the samples. The inhibition of linoleic acid oxidation was determined by employing a modified b-carotene/linoleic acid assay. The principle of the β-carotene bleaching assay for evaluating antioxidant activity is based on the discoloration of yellowish colour of a β-carotene solution due to the breaking of π-conjugation by addition reaction of lipid or lipid peroxyl radical (L∙ or LOO∙ ) to a C=C double bond of β-carotene. The radical species is generated from the autoxidation of linoleic acid by heating under air atmosphere. In the presence of antioxidants, oxidation of b-carotene is scavenged, preventing bleaching the colour of b-carotene.

The antioxidant activities of the 1,4-DHPs 6a-i using the two assays, DPPH and b-carotene/linoleic acid are summarised in Table 1 and Figure 4. Both compounds 6b and 6f showed a small concentration dependence profile in DPPH assay but had low antioxidant activity in both DPPH and b-carotene/linoleic assays. The remaining 1,4-DHPs did not demonstrate any substantial antioxidant properties with DPPH assay and no concentration dependency as shown in Figures 4 And 5.

However, in b-carotene/linoleic assay compounds 6a, 6d and 6e showed remarkably good Relative Antioxidant Activity (RAA) of 71%, 80% and 78% respectively (Figure 6). Compound 6h showed 45% RAA comparable with that for ascorbic acid of 49% RAA in b-carotene/linoleic assay. The three 1,4-DHPs with highest RAA values are 6a, 6c and 6d. All these compounds have electron donating groups on the aromatic ring component. However, whilst 6d with 78% RAA has strongly donating three methoxy groups on the aromatic ring the other two 6a with 71% RAA and 6c with 80% RAA have a chlorine and fluorine atoms on quinoline rings respectively. Halogen atoms on the aromatic ring are classed as weakly electron donating groups but the pyridine component of the quinoline ring is strongly electron withdrawing. On the other hand, compound 6g with 45% RAA has a 2-thiophenyl ring attached which although aromatic is regarded as weakly electron donating as a result of the sulphur atom in the ring. That said the isomeric compound 6f being a 3-thiophenyl derivative actually gives a much lower 20% RAA compared with the previously mentioned 2-thiophenyl derivative 6g with 45% RAA. More structural activity research is needed to evaluate the effect of fluoro and methoxy groups in other positions of the aromatic ring for RAA activity. Generally, it appears that electron releasing groups on aromatic rings facilitate the RAA properties of the compounds.

The mechanism for the quenching of DPPH can occur by both electron and hydrogen atom transfer in different sequences is greatly influenced by solvent polarity and pH. Findings suggest that hydrogen-bonding solvents repress hydrogen atom transfer and favour electron transfer. This implies that compounds which are strongly active in hydrogen atom transfer appear to be slower reacting in protic solvents such as methanol and ethanol that are commonly used for the assay [10,11]. In the DPPH assay, an odd electron displays a strong absorption band at a wavelength of 519 nm, which loses absorption once the odd electron is paired off by a hydrogen atom of electron-donating antioxidant (Figure 7). The limited viable data obtained for our 1,4-DHPs by the DPPH assay method may be associated with the comparable nature of the two nitrogen radicals involved in the reaction mechanism, the protonic solvents used and molecular steric factors (Figure 7).

Conclusion

Using the DPPH assay did not yield viable results except for two compounds 6b and 6f albeit in low RAA showing a small concentration dependant profile. Aromatic rings with methoxy, fluoro and chloro groups gave high % RAA values in the b-carotene/linoleic acid assay. Compounds 6a, 6c, 6d and 6g possessed the highest antioxidant activity of 71%, 80%, 78% and 45% respectively compared with AA of RAA 49%. Therefore, in general 1,4-DHPs with electron donating groups on the aromatic ring gave much higher RAA values compared with that for L-ascorbic acid as a reference. Further structural activity relationship work is needed to determine and substantiate the relative RAA values for isomeric compounds containing Cl, F and OCH3 substituents on the aromatic rings.

Acknowledgement

The work was supported by the Sheffield Hallam University undergraduate research projects funding. There is no conflict of interest amongst the authors.


Figure 1: Structures of three commonly used drugs as calcium channel blockers.


Figure 2: Structures of dihydropyridines synthesised by the Hantzsch synthesis.


Figure 3: The Hantzsch synthesis for making 1,4-dihydropyridine derivatives 6a-i.


Figure 4: Antioxidative results by DPPH assay test for 1,4-DHP 6a-d.


Figure 5: Antioxidative results DPPH assay test for 1,4-DHP 6e-i.


Figure 6: Antioxidant activity of 1,4-DHPs 6a-i and ascorbic acid (AA) (control) with the b-carotene-linoleic acid antioxidant activity test compared to a blank after a 24 h period incubation.


Figure 7: Reaction of DPPH radical with antioxidant molecules (1,4-DHPs).

Absorbance Average

          (l- nm)

Concentration

(μg/ml)

6a

6b

6c

6d

6e

6f

6g

6h

6i

2

1.1

1.137

1.093

1.127

1.066

1.261

0.748

0.936

1.162

4

1.041

1.19

1.04

1.093

1.065

1.213

0.79

0.929

1.169

8

1.139

1.104

1.027

1.088

1.062

1.129

0.815

0.926

1.173

12

1.087

1.105

1.069

1.089

1.062

1.209

0.839

0.93

1.183

16

1.123

1.095

1.052

1.097

1.077

1.14

0.822

0.928

1.17

20

1.073

1.081

1.048

1.095

1.076

1.142

0.817

0.928

1.169

22

1.057

1.071

1.04

1.114

1.078

1.138

0.813

0.928

1.163

30

1.054

1.064

1.034

1.105

1.098

1.135

0.836

0.941

1.159

35

1.057

1.04

1.052

1.128

1.14

1.089

0.852

1.134

1.158

40

1.062

1.026

1.05

1.139

1.35

1.052

0.87

0.986

1.158

45

1.062

1.015

1.083

1.139

1.474

1.051

0.862

0.946

1.159

50

1.069

0.99

1.061

1.135

1.161

1.05

0.88

0.946

1.164


Table 1: The results from the DPPH assay test of the dihydropyridines 6a-i. Results were compared against a blank at 1230 nm.

References

  1. Safak C, Simsek R (2006) Fused 1,4-dihydropyridines as potential calcium modulatory compounds. Mini Rev Med Chem 6:747-755.
  2. Dhinakarani I, Padmini V, Bhuvanesh N (2015) One-pot synthesis of N-aryl 1,4-dihydropyridine derivatives and their biological activities. J Chem Sci 127: 2201-2209.
  3. Edraki N, Mehdipour AR, Khoshneviszadeh M, Miri R (2009) Drug Discovery Today 14:1058-1066.
  4. Vijesha AM, Isloor AM, Peethambar SK, Shivananda KN, Arulmoli T, et al (2011) Hantzsch reaction: Synthesis and characterization of some new 1,4-dihydropyridine derivatives as potent antimicrobial and antioxidant agents. European Journal of Medicinal Chemistry 46: 5591-5597.
  5. Velena A, Zarkovic N, Troselj KG, Bisenieks E, Krauze A, et al. (2016) 1,4-Dihydropyridine Derivatives: Dihydronicotinamide Analogues-Model Compounds Targeting Oxidative Stress. Oxidative Medicine and Cellular Longevity 1892412:1- 35.
  6. Olejnikova P, Svorc L, Olsovska D, Panáková A, Vihonská Z, et al. (2014) Antimicrobial Activity of Novel C2-Substituted 1,4-Dihydropyridine Analogues. Scientia Pharmaceutica 82: 221-232.
  7. Gutteridge JMC, Halliwell B (2000) Free radicals and antioxidants in the year 2000-a historical look to the future. Ann N Y Acad Sci 899:136-147.
  8. Sugamura K, Keaney JF Jr (2011) Reactive oxygen species in cardiovascular disease. Free Rad Biol Med 51: 978-992.
  9. Cuendet M, Hostettmann K, Potterat O, Dyatmiko W (2004) Iridoid glucosides with free radical scavenging properties from Fagraea blumei. Helv Chim Acta 80:1144-1152.
  10. Ri Bang Wu, Cui Ling Wu, Dan Liu, Xing Hao Yang, Jia Feng Huang, et al. (2015) Overview of Antioxidant Peptides Derived from Marine Resources: The Sources, Characteristic, Purification, and Evaluation Methods. Appl Biochem Biotechnol 176:1815-1833.
  11. Xie J, Schaich KM (2014) Re-evaluation of the 2,2-Diphenyl-1-picrylhydrazyl Free Radical (DPPH) Assay for Antioxidant Activity. J Agric Food Chem 62: 4251-4260.

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