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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 8  |  Issue : 2  |  Page : 195-203

Synthesis, antioxidant, and anti-tyrosinase activity of some aromatic oximes: an experimental and theoretical study


1 Université Mohamed Cherif Messaadia, Souk Ahras, Algeria; Laboratoire de Chimie des Matériaux Constantine, Université Frères Mentouri Constantine 1, Constantine, Algeria
2 Laboratoire de Chimie des Matériaux Constantine, Université Frères Mentouri Constantine 1, Constantine, Algeria
3 Centre de Recherche en Biotechnologie, Constantine, Algeria

Date of Web Publication30-Oct-2019

Correspondence Address:
Prof. Mustapha Bencharif
Laboratoire de Chimie des Matériaux Constantine, Université Frères Mentouri Constantine 1, Constantine.
Algeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jrptps.JRPTPS_46_18

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  Abstract 

In this study, five oximes were synthesized, characterized, and their antioxidant activity investigated experimentally and theoretically. Labeled OX1–OX5, our oximes were subjected to three antioxidant activity assays (1,1-diphenyl-2-picrylhydrazyl, cupric reducing antioxidant capacity test [CUPRAC], and ferric reducing antioxidant power test). The related results were very fulfilling presenting certain antioxidant properties for the majority of the studied compounds, especially with CUPRAC test where OX1 and OX2 presented a better result than the used standard antioxidants. Besides, the thermodynamic molecular descriptors (bond dissociation enthalpy, ionization potential, and proton affinity) of our molecules were calculated with the density functional theory to elucidate the experimental results. The resulting values were in a good agreement with the experimental results. The second part of this work consisted of an anti-tyrosinase activity evaluation of the five oximes where three of them displayed an inhibitory effect against this enzyme. To analyze this result, a molecular docking was conducted and the obtained values were in accordance with the experience. Absorption, distribution, metabolism, and excretion properties were also assessed via Molinspiration software and all our molecules satisfied the related conditions. Thus, the conducted theoretical and experimental studies were in excellent harmony, together supporting the antioxidant power of our compounds, which could permit their use in cosmetic, pharmaceutical, or food industry domains, after further clinical tests.

Keywords: Antioxidant activity, anti-tyrosinase activity, density functional theory calculations, molecular docking, oximes


How to cite this article:
Bensegueni R, Guergouri M, Bensouici C, Bencharif M. Synthesis, antioxidant, and anti-tyrosinase activity of some aromatic oximes: an experimental and theoretical study. J Rep Pharma Sci 2019;8:195-203

How to cite this URL:
Bensegueni R, Guergouri M, Bensouici C, Bencharif M. Synthesis, antioxidant, and anti-tyrosinase activity of some aromatic oximes: an experimental and theoretical study. J Rep Pharma Sci [serial online] 2019 [cited 2019 Nov 15];8:195-203. Available from: http://www.jrpsjournal.com/text.asp?2019/8/2/195/269941




  Key messages: Top


  • The majority of our oximes showed a good antioxidant activity.


  • OX3, OX4, and OX1 revealed an anti-tyrosinase activity.


  • The same result is obtained theoretically.



  •   Introduction Top


    The existence of free radicals in the human body is essential for the immune defense. However, the excess of these can be harmful to health. Hence, the pharmacological search for new and more efficient antioxidant agents remains essential. Oximes are compounds characterized by the general structure R’R’’C = N-OH, which have been widely studied. Their antioxidant activity was investigated and shown in various studies[1],[2],[3] such as in the case of naringenin-oxime where the addition of the oxime group has increased the antioxidant activity of pure naringenin.[4] They are also used in other therapeutic applications as in the antidotal treatment of poisoning by organophosphorus nerve agents.[5] However, the mechanism of antioxidant action of oximes has not been investigated enough theoretically. In this work, a set of five oximes, labeled OX1–OX5, were synthesized [Figure 1], characterized with proton nuclear magnetic resonance (1H NMR), and then subjected to three antioxidant activity tests: 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging activity test, cupric reducing antioxidant capacity test (CUPRAC), and ferric reducing antioxidant power test (FRAP).
    Figure 1: Chemical structures of the studied compounds

    Click here to view


    In the context of the density functional theory (DFT), three thermodynamic molecular descriptors were calculated for the tested molecules, to propose the most favorable mechanism for the free radicals scavenging activity. The three possible mechanisms that may coexist are hydrogen atom transfer (HAT), single electron transfer–proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET).[6],[7] These mechanisms are defined by the next equations:

    [INLINE 1]

    [INLINE 2]

    [INLINE 3]

    Each mechanism is characterized by one or more descriptors. Thus, HAT mechanism is related to the O–H bond dissociation enthalpy (BDE), when the first step of the SET–PT mechanism is controlled by the ionization potential (IP) and the second one is governed by the proton dissociation enthalpy (PDE) of the corresponding cation radical. For the SPLET mechanism, the proton affinity (PA) of the anion RX- and the electron transfer enthalpy (ETE) are the enthalpies that manage the two steps of the mechanism, respectively. Therefore, BDE, IP, and PA are the main molecular descriptors, which determine the preferred antioxidant mechanism. The lower their values, the higher the scavenging activity.[8]

    For CUPRAC and FRAP tests, the mechanism consists of an electron transfer from the studied molecule to the copper (II) complex and the iron (III) complex,[9] respectively. That is why the corresponding activities are related to the IP values of the tested compound, the higher is the IP value, the lower is the antioxidant activity.

    Besides, the anti-tyrosinase activity of our oximes was evaluated experimentally. Afterward, the results thus obtained are analyzed and explained theoretically using a molecular docking calculation. Tyrosinase is a three copper enzyme (EC 1.14.18.1) found in different organisms from bacteria to animals.[10] It is implicated in the biosynthesis of melanin,[11] a defense pigment present in skin. Tyrosinase inhibitors, such as kojic acid conjugates, are commonly used in cosmetic industry against skin disease.[12] It should be noted that an anti-tyrosinase potency for some oximes has been shown.[13],[14]


      Materials and Methods Top


    Materials

    For synthesis: salicylaldehyde, 2-hydroxy-1-naphthaldehyde, 2-thiophenecarbaldehyde, 1H-indole-3-carbaldehyde, 3,4-ethylenedioxythiophene-2-carbaldehyde, 4-methoxyacetophenone, phenylhydrazine hydrochloride, pyridine, and absolute ethanol were obtained from Sigma-Aldrich (Sternheim, Germany). All products were used without further purification.

    For antioxidant tests: 1,1-diphenyl-2-picrylhydrazyl (DPPH), butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), neocuproine, trichloroacetic acid (TCA), potassium ferricyanide (K3Fe(CN)6), tyrosinase from mushroom (EC 1.14.18.1, ≥1000 unit/mg solid), l-DOPA (3,4-dihydroxy-l-phenylalanine), and kojic acid were obtained from Sigma-Aldrich, iron (III) chloride (FeCl3), copper (II) chloride (CuCl2), and ammonium acetate (ACNH4) were obtained from Biochem Chemopharma, France. All other chemicals and solvents were of analytical grade.

    All calculations were performed with Gaussian 9.0 software,[15] whereas GaussView 5.0.9 (Gaussian, Inc. Wallingford, CT 06492 USA)[16] was used for result visualization and analysis.

    1H NMR spectra were recorded on a Bruker Avance DPX 250 spectrometer with tetramethylsilane (TMS) as internal reference. Measurements and calculations of the antioxidant activity were carried out on a 96-well microplate reader, PerkinElmer, France, Multimode Plate Reader EnSpire.

    Synthesis

    Preparation of oximes: Our compounds were synthesized by a condensation between an aldehyde (or a chalcone in the case of OX5) and an hydroxylamine. An equimolar mixture (0.04mol) of hydroxylamine hydrochloride (NH2OH.HCl), pyridine, and the carbonyl compound are dissolved in 20mL of ethanol and refluxed for 3h. The reaction is followed by thin layer chromatography. After evaporation of ethanol, the reaction mixture is taken up in ether and washed with a solution of HCl (10%) to remove the residual base. After washing with water until neutral pH, the organic phase is dried and evaporated in a vacuum and then recrystallized in ethanol to get the oxime [Scheme 1].[17]
    Scheme 1: Synthesis scheme of the studied oximes

    Click here to view


    Identification of 1H-indole-3-carbaldehyde oxime (OX1)

    White fine needles, yield: 90%. Melting point (Mp): 195°C. 1H NMR (CDCl3, 250 MHz): 11.00 (s, 1H, N-H); 10.70 (s, 1H, NO-H); 7.85 (s, 1H, CH=N); 7.52 (s, 1H, H2); 7.31(d, 1H, H4, 3J = 8.52 Hz); 7.03 (d, 1H, H7, 3J = 7.9 Hz); 6.55 (t, 2H, H5, H6, 3J = 7.52–7.61 Hz). IR (cm-1): 3380 (νN-H), 3047 (νNO-H), 3157–2745 (νC-H), 1635 (νC = N), 1456 (νN-O), 1412 (δC-C in-the-plan), 1335 (νC-N), 1229 (νN-O), 1096 (νC-N), and 741 (γortho-disubstituted aromatic out of-the-plan).

    Identification of 2-hydroxynaphthaldehyde oxime (OX2)

    Light green powder, yield: 92%. Melting point (Mp): 157°C. 1H NMR (DMSO-d6, 250 MHz): 11.6 (s, 1H, NO-H); 11.17 (s, 1H, O-H); 9.12 (s, 1H, CH=N); 8.50 (d, 1H, H4, 3J = 8.55 Hz); 7.83 (d, 2H, H5, H8, 3J = 9.15 Hz); 7.52 (t, 1H, H7, 3J = 7.49 Hz);7.40 (t, 1H, H6, 3J = 7.35 Hz); 7.24 (d, 1H, H3, 3J = 8.9 Hz). IR (cm-1): 3322 (νO-H), 3025 (νNO-H), 3010–2800 (νC-H), 1631 (νC = N), 1589 (νC = C), 1465 (νN-O), 1238 (νC-O), and 937 (δC = C out of-the-plan).

    Identification of 2-thienylcarbaldehyde oxime (OX3)

    White powder, yield: 82%. Melting point (Mp): 129°C. 1H NMR (CDCl3, 250 MHz): 8.95 (s, 1H, NO-H); 7.75 (s, 1H, CH=N); 7.60 (d, 1H, H4, 3J = 4.99 Hz); 7.43 (d, 1H, H2, 3J = 2.94 Hz); 7.13 (t, 1H, H3, 3J = 3.81–3.85 Hz). IR (cm-1): 3157 (νC-Hα), 3058 (νNO-H), 3050–2746 (νC-H), 1630 (νC = N), 1428 (δC- Hα in-the-plan), 1412 (δC-C in-the-plan), 1228 (νN-O), 1047 (νC-S), and 935 (δC = C out of-the-plan).

    Identification of 2-salicylaldehyde oxime (OX4)

    Pale brown powder, yield: 69%. Melting point (Mp): 52–56°C. 1H NMR (DMSO-d6, 250 MHz): 11.42 (s, 1H, NO-H); 10.20 (s, 1H, O-H); 8.40 (s, 1H, CH=N); 7.5(d, 1H, H6, 3J = 7.63 Hz); 7.25 (t, 1H, H4, 3J = 8.36 Hz); 6.90 (t, 1H, H5, 3J = 7.98 Hz); 6.86 (d, 1H, H3, 3J = 7.39 Hz). IR (cm-1): 3379 (νO-H), 3076 (νNO-H), 3020–2800 (νC-H), 1620 (νC = N), 1587 (νC = C), 1496 (νN-O), 1257 (νC-O), 991 (δC = C out of-the-plan), and 740 (γortho-disubstituted aromatic out of-the-plan).

    Identification of (E)-3-(3,4-ethylenedioxythienyl)-1-(4-methoxyphenyl)prop-2-en-1-one oxime (OX5)

    Yellow powder, yield: 34%. Melting point (Mp): 126°C–127°C. 1H NMR (CDCl3, 250 MHz): 8.8 (s, 1H, NO-H); 7.82 (d, 1H, CH=C, J = 16.75 Hz); 7.77 (m, 2H, H2, H6); 7.37 (m, 2H, H3, H5); 7.08 (d, 1H, CH=C, J = 15.78 Hz); 7.01 (s, 1H, Hα); 4.40 (m, 4H, CH2); 4.02 (s, 3H, CH3). IR (cm-1): 3167 (νC-Hα), 3051 (νNO-H), 3025–2736 (νC-Harom.), 2660–2646 (νC-Halkanes), 1896 (γpara-disubstituted aromatic out of-the-plan), 1576 (νC = N), 1468 (νN-O), 1187 (νC-O), 991 (δC = C out of-the-plan), and 705 (νC-S).

    Preparation of the precursor chalcone: In a round-bottomed flask, 3,4-ethylenedioxythiophene-2-carbaldehyde (5 mmol) and 4-methoxyacetophenone (5 mmol) were dissolved in 6mL of ethanol. A total of 7mL of 10% of NaOH were added, and the mixture was stirred at 60°C for 20min. The mixture was cooled and poured in 50g of ice water. The precipitate was filtered, washed with cold water, and dried [Scheme 2].
    Scheme 2: Synthesis scheme of the studied chalcone.

    Click here to view


    Identification of (E)-3-(3,4-ethylenedioxythenyl)-1-(4-methoxyphenyl)prop-2-en-1-one

    Yellow powder, yield: 75%. Melting point (Mp): 130°C. 1H NMR (CDCl3, 250 MHz): 8.03 (d, 2H, H2, H6, 3J = 8.8 Hz); 7.85 (d, 1H, CH=C, J = 15.31 Hz); 7.26 (d, 1H, CH=C, J = 15.33 Hz); 6.99 (d, 2H, H3, H5, 3J = 8.83 Hz); 6.49 (s, 1H, Hα); 4.30 (m, 4H, CH2); 3.91 (s, 3H, CH3).

    Antioxidant activity assessment

    The antioxidant activity of the five oximes was tested using DPPH scavenging activity, CUPRAC, and FRAP power assays. BHA and BHT were used as positive standards.

    1,1-Diphenyl-2-picrylhydrazyl scavenging activity method

    The free radical-scavenging activity was determined by the DPPH assay.[18] Briefly, 40 µL of each sample solution prepared in ethanol at different concentrations was added to 160 µL of a DPPH ethanolic solution (0.1mM). The mixture was left in darkness for 30min. The absorbance was measured at 517 nm using microplate reader. BHT and BHA were used as antioxidant standards. The results were given as percentage of inhibition (or scavenging) of the initial DPPH at 100 μg/mL of the sample concentration. The percentage of inhibition was calculated by using the formula:

    [INLINE 4]

    Cupric reducing antioxidant capacity method

    The cupric reducing antioxidant capacity was determined according to the method by Apak et al.[19] In each well, the reaction mixture containing 40 µL of our sample, 50 μL of copper (II) chloride solution, 50 μL of neocuproine alcoholic solution, and 60 μL of ammonium acetate aqueous buffer at pH 7. This mixture was left 30min before measuring absorbance at 450 nm. The results were expressed as A0.50 values (µg/mL), corresponding to the sample concentration indicating 0.5 absorbance.

    Ferric reducing antioxidant power method

    The method as described by Oyaizu[20] was used to estimate the reducing power activity of our compounds. A total of 10 μL of each sample solution at various concentrations were added to 40 μL of 0.2 M phosphate buffer (pH 6.6) and 50 μL of K3Fe(CN)6 (1%). Everything was incubated for 20min at 50°C. A total of 50 μL of TCA (10%) and 10 μL of ferric chloride (0.1%) was added to the mixture and completed with 40 μL of distilled water. The ferric reducing antioxidant power was followed spectrophotometrically at 700 nm, and the A0.50 values (µg/mL) were calculated.

    All data on antioxidant activity test were the average of triplicate analyses. Data were recorded as the mean ± standard deviation. Significant differences between mean values were determined using Student’s t test; P values <0.05 were regarded as significant.

    Density functional theory calculations

    Molecular modeling and geometry optimization

    Molecular structures of the five molecules were optimized by DFT using B3LYP functional with the 6-311G(d,p) basis set. No geometrical constraints were applied. Frequency calculations were carried out at the same level of theory. Solvent effects were considered via the integral equation formalism variant of the polarizable continuum model (IEFPCM) salvation model.[21]

    Descriptors calculation

    Molecular descriptors are calculated using –() at 298.15K and 1 atmosphere. For the gas phase, the enthalpy values of hydrogen atom (H.), proton (H+) and electron (e-) are equal to −1309.408 kJ/mol, 6.197 kJ/mol, and 3.145 kJ/mol, respectively.[22],[23] In ethanol, the value of −1307.479 kJ/mol was used for the hydrogen atom enthalpy,[24] whereas those of proton and electron, H(H+sol) and H(e-sol), were obtained from the corresponding solvation enthalpies[25] and were equal to −1012.303 kJ/mol and −102.154 kJ/mol, respectively.

    [INLINE 5]

    [INLINE 6]

    [INLINE 7]

    [INLINE 8]

    [INLINE 9]

    Calculation of vibrational frequencies, using the unrestricted open shell level UB3LYP/6-311G(d,p), was conducted for the generated radicals RX° and RX-H. It should be noted that no significant change was observed in results obtained after geometry optimization for the said radicals.

    Anti-tyrosinase activity

    Tyrosinase inhibitory activity of Mushroom species was investigated spectrophotometrically as given by Masuda et al.[26] with minor changes. l-DOPA was used as substrate, whereas kojic acid was used as standard. Thus, 150 μL of 100mM sodium phosphate buffer (pH 6.8), 10 μL of sample solution dissolved in methanol at different concentrations, and 20 μL tyrosinase enzyme solution in buffer were mixed and incubated for 10min at 37°C, and then 20 μL of l-DOPA was added. The mixture was incubated a second time for 10min at 37°C before reading its absorbance at 475 nm in a 96-well microplate.

    [INLINE 10]

    Molecular docking

    Arguslab 4.0.1 software[27] was used to estimate the affinity between tyrosinase and the tested molecules and to analyze the intermolecular interactions that are involved. Thus, a molecular docking calculation was carried out considering the flexibility of ligands. The stability of the enzyme–ligand complex is inversely related to its Gibbs free energy (ΔGbinding) calculated by the Ascore function, which is implemented in Arguslab and is given by:

    [INLINE 11]

    where, ΔGVdW represents the van der Waals interaction between the protein and the ligand, ΔGhydrophobic represents the hydrophobic effect, ΔGH-bond and ΔGH-bond(chg) are assigned to the hydrogen bonding and the hydrogen bonding involving charged donor and/or acceptor groups between the ligand and the protein, respectively, the deformation effect is given by ΔGdeformation, and finally ΔG°, which corresponds to the effects of the translational and rotational entropy loss during complexation.

    Already modeled and optimized at the B3LYP/6-311G(d,p) level of the DFT theory, the ligands will be directly used. As for the three-dimensional structure of the enzyme, it is obtained from the Research Collaboratory for Structural Bioinformatics protein data bank (http://www.rcsb.org/). Therefore, the entry 1wx2, which meets the selection criteria of a protein data bank file, was proposed by Warren et al.[28] and which was used in many similar studies,[29],[30] was selected. Water molecules and the caddie protein were removed. Hydrogen atoms were added and docking parameters were adapted as the binding site bounding box dimensions, which were equal to: 20 × 20 × 15 Å3. The docking engine ArgusDock was chosen and the default value of grid resolution (0.4 Å) was kept. The intermolecular interactions analysis was perfected using the discovery studio 4.0 software.[31]

    Lipinski’s rule of five

    Given by Lipinski et al.[32] in 1997, the rule of five is based on certain criteria to estimate druglikeness of a molecule having a pharmacological activity. These criteria are log P lower than 5, number of hydrogen bond donors less than 5, number of acceptors of hydrogen bonds less than 10, and molecular weight not exceeding 500Da. The rule is used in drug design to preselect molecules presenting good absorption, distribution, metabolism, and excretion (ADME) properties that must have a medicament in the organism. We have used Molinspiration property calculator (http://www.molinspiration.com) to calculate the four parameters of Lipinski’s rule in addition to the number of rotatable bonds that have to be inferior to 10 to have a good oral bioavailability.[33]


      Results and Discussion Top


    Antioxidant activity assessment

    Results of the three used methods are present in [Table 1].
    Table 1: Antioxidant assays (1,1-diphenyl-2-picrylhydrazyl scavenging activity, ferric reducing antioxidant power test, and cupric reducing antioxidant capacity test) results. The corresponding antioxidant standard values are reported

    Click here to view


    1,1-Diphenyl-2-picrylhydrazyl scavenging activity method

    Among the tested compounds, OX2 and OX1 showed the highest activity, with a percentage of inhibition of 74.63 ± 2.94 and 34.50 ± 1.56, respectively. The first value is quite close to those of BHA and BHT.

    Cupric reducing antioxidant capacity method

    The obtained results from the CUPRAC assay showed that OX1 and OX2 presented a better antioxidant activity than the tested standards, with an A0.50 of 2.60 ± 0.16 µg/mL and 3.62 ± 0.31 µg/mL, respectively. In a global way, the CUPRAC antioxidant power order obtained for the studied oximes was: OX1 > OX2 > OX4 > OX3 > OX5.

    Ferric reducing antioxidant power method

    FRAP results showed that OX2, once again have the best reducing power activity among the tested molecules with an A0.5 of 6.89 ± 0.23 µg/mL, a comparable value to those of the used standards. If OX5 presented a relatively weak FRAP activity, no significant activity was observed for OX4, OX1, and OX3.

    Molecular descriptors calculation

    1. Comparison with DPPH scavenging assay results: For all molecules in vacuum, the following sequence: BDE > IP > PA, is respected, which favors a HAT mechanism, whereas the SPLET mechanism is preferred for the five molecules in ethanol, as the three descriptors are ordered in the flowing manner: PA > BDE > IP [Table 2]. This is in concordance with literature where HAT mechanism is favored in nonpolar media and SET-PT or SPLET mechanisms are preferred in polar ones because of the charged species creation.[34] Notice that three of our molecules (OX1, OX2, and OX4) present each two transferable hydrogen atoms. In the case of OX4 and OX2, these atoms will be noted Ha and Hb, as shown in [Figure 1]. To determine which atom transfer is favorable in each case, we have to compare the corresponding PA values. Remember that PA was found to be the determinant descriptor in ethanol phase. Thus, for OX1, the results showed that the N-H bond proton departure is more favorable compared to that of the O-H bond one. This may be due to the fact that the departure of the proton from the N-H bond creates a stabilizing resonance at the two indole rings. Concerning the two other oximes, the PA value of the Hb proton is inferior to the Ha one for OX4, whereas the opposite situation is observed for OX2. This indicates that the most labile proton is Hb for OX4 and Ha for OX2. Overall, PA values’ order is: OX2-Ha< OX4-Hb < OX1-N < OX3 < OX5. This is in perfect concordance with the experience [Table 1]. On the contrary, calculations conducted in gas phase showed the following sequence of BDE values (determinant descriptor in the gas phase): OX1-N < OX3 < OX5 < OX2-Ha< OX4-Ha.
      Table 2: Molecular descriptors (bond dissociation enthalpy, ionization potential, and protein affinity) values (kJ/mol) calculated in vacuum and in solvent (ethanol), for the tested molecules and standards

      Click here to view


    2. Comparison with CUPRAC assay results: As aforementioned for this electron transfer based test, the antioxidant activity is inversely proportional to the IP values of molecules. In ethanol, IP of our oximes are ordered as: OX1 < OX2 < OX3 < OX4 < OX5. This constitutes a nearly perfect accord between the theoretical approach and the experimental one.


    Anti-tyrosinase activity

    Three of the tested oximes showed an anti-tyrosinase activity. OX4 revealed the highest activity among these compounds with 51.89% ± 6.04% of the enzyme amount inhibited at 200 µg/mL of sample’s concentration. At the same concentration, OX3 and OX1 showed less activity with an inhibition percentage of 32.95 ± 11.07 and 10.33 ± 7.17, respectively [Table 3]. No inhibition effect was observed in the case of OX2 and OX5.
    Table 3: Percentage of inhibition (at 200 µg/mL of sample) and ΔGbinding values of the tested oximes

    Click here to view


    Molecular docking

    From the molecular docking calculation results [Table 3], all the tested molecules show an importance with respect to the used standard (kojic acid). This means that they may constitute a thermodynamically favorable complex.

    As well, the enzyme–ligand interaction analysis showed that the three molecules presenting an inhibitory effect were docked in enzyme’s active site pocket [Figure 2], which is located around the dicopper center (CuA and CuB).[35] OX4 presented a hydrogen bond with Glu182 and three hydrophobic interactions, two of them with Trp184 and a third with Ile42, whereas OX3 formed a hydrophobic interaction with Trp184. Then, the binding mode of OX1 consisted of a three hydrogen bonds with Asn191, Ser206, and Thr203, respectively, and two hydrophobic interactions with His194 and His54 [Table 4].
    Figure 2: Tyrosinase interactions with OX4 (A), OX3 (B), and OX1 (C). Ligands are represented as ball and stick, whereas all other residues are displayed as lines except those interacting with the ligand. The ochre balls correspond to the copper ions

    Click here to view
    ,
    Table 4: Favorable intermolecular interactions between the three active oximes (OX4, OX3, and OX1) and the enzyme (tyrosinase)

    Click here to view


    Lipinski’s rule of five

    All our molecules fulfil the Lipinski’s rule criteria and the extra condition on the number of rotatable bonds [Table 5], suggesting their good pharmacokinetic permeability and their oral bioavailability, which may allow them to constitute lead compounds for oxidative stress diseases therapies.
    Table 5: Evaluation of the druglikeness properties of the five molecules by Molinspiration Property Calculator

    Click here to view



      Conclusion Top


    The obtained results from the used experimental tests illustrate that the majority of our oximes showed a good antioxidant activity, even better than the customary used antioxidant standards, for OX2 and OX1. The most thermodynamically favorable DPPH scavenging mechanism was investigated theoretically and the SPLET mechanism was designated for the tested compounds, in ethanol phase, and HAT mechanism for the gas phase. The obtained values of PA, which is the determinant descriptor for SPLET mechanism, are ordered in perfect agreement with the experience. On the contrary, for CUPRAC test, results were in harmony with the calculated IP. Moreover, OX3, OX4, and OX1 revealed an anti-tyrosinase activity. Nevertheless, no inhibition effect against this enzyme was observed for OX2 and OX5. This same result is obtained theoretically through a molecular docking calculation. The submission of our molecules to Molinspiration software showed that they fulfill the Lipinski’s rule of five. From all these results, we can say that our compounds could be used as antioxidant agents even in pharmacological, cosmetic, or food industries. Nevertheless, further clinical tests are needed to fully validate this conclusion.

    Financial support and sponsorship

    This study was supported by the Directorate General of Scientific Research and Technological Development of the Algerian Ministry of Higher Education and Scientific Research (DGRSDT-MESRS, Algeria).

    Conflicts of interest

    There are no conflicts of interest.



     
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        Figures

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        Tables

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