New chalcone derivatives as potential antimicrobial and antioxidant agent

Seven chalcone derivatives were synthesized by the Claisen-Schmidt condensation. The structures of the compounds were confirmed by spectral data (Ultraviolet/visible, infrared, nuclear magnetic resonance and mass spectroscopy). The compounds were tested for their in silico and in vitro antimicrobial and antioxidant activities. The molecular docking assessments showed that all the compounds exhibited good binding affinity with the target microorganism proteins but, compounds 6e and 6g showed better binding affinity compared with the standards. The antimicrobial test revealed that all the compounds screened were active against Staphylococcus aureus and Bacillus subtilis and had minimum inhibitory concentrations (MIC) between 0.4 and 0.6 mg/mL. Compounds 6a, 6c and 6d had moderate activities on Salmonella typhi. Compounds 6b and 6c had moderate activity on Escherichia coli. Compound 6c had moderate activity on Aspergillus niger while compounds 6a and 6e had poor activity. All the compounds except compound 6e had no inhibition against Pseudomonas aeruginosa. The in-vitro antioxidant activity was assessed using ethylenediaminetetraacetate (EDTA) as the standard. Compounds 6c, 6e and 6g gave excellent inhibitory activity better than the standard. Compound 6a gave good activity at 500 μg/mL and 1000 μg/mL concentrations but, below the standard at 250 μg/mL and no inhibition at 125 μg/mL. Compound 6d had good inhibition at 500 μg/mL and 1000 μg/mL but, no inhibition at 125 μg/mL and 250 μg/mL. Compound 6b was found to be inactive in all the concentrations. Absorption, distribution, metabolism and excretion properties of the compounds were assessed using SwissADME. The results of lead likeness showed that compound 6e is a lead-like molecule.

The immediate response of antigen is the Reactive-Oxygen Species (ROS) appearance in the body during microbial invaders. Free oxygen radicals are very toxic to pathogen and are used as agent to prevent attack of tissues by microorganism 31 .
Oxidative stress is seen as the main factor concerned with the development of chronic diseases and it happens when oxygen radical production and levels are higher than those of the antioxidant 32 . The importance of ROS for immune function could be used by the microbes to reduce defense mechanism of the host to survive. One of the important characteristic of plants accountable for antioxidant activity is the presence of derivatives of phenol and the power to hunt free radicals which can act as immunity against the harmful action of ROS. So antagonizing the production of ROS and free radical by addition of antioxidants can play important function in averting these free radical related diseases 33 .
Chalcones have preventive effects against many microorganisms. The antimicrobial effects are due to reactions between these compounds and the cell membrane of the target microorganism, their ability to attach with outer cell, absorbable proteins and the cell walls 34 . It is therefore possible to think that chalcones could inhibit the microbes through their antioxidant properties.
Glucosamine-6-phosphate synthase is responsible for the metabolism of hexosamine, an important process in the biosynthesis of amino sugars needed for cell growth and development. UDP-3-O-((R)-3-hydroxymyristoyl)-N-acetylglucosamine deacetylase is involved in the biosynthesis of lipid A, a phosphorylated glycolipid that anchors the lipopolysaccharide to the outer membrane of the cell 35 . DNA gyrase is involved in the control of topological transition of DNA, thereby promoting replication and transcription 36 . Urate oxidase catalyse the oxidation of uric acid to allantoin and the inhibition of this enzyme leads to accumulation of toxic uric acid in the microorganisms. Dihydrofolate reductase is an important enzyme in the conversion of pteridine to folic acid required by all cells for growth and development. Given the roles of these enzymes in the growth and development of organisms, their successful inhibition have been characterized as therapeutic target for drug development research.
This work was designed based on the reported pharmacological application of chalcones and the need for an antimicrobial host to have some antioxidant abilities to attack the reactive oxygen species produced by microorganism.
We herein report the synthesis of some new chalcones with good antimicrobial and antioxidant activities.

Materials and methods
The chemicals used for experimental were of analytical grade purchased from Sigma-Aldrich and used it without purification. Melting points were determined using Fischer John's melting point apparatus and were not corrected. UV-Visible spectra were recorded on UV5800PC series spectrophotometer using matched 1 cm quartz cells. The IR spectra were recorded on Buck Scientific m910 FTIR U S A using KBr discs. Jeol 400 MHz was used for NMR whereas Waters Q-TOF premier HAB213 was used for Mass spectroscopy.

Experimental
Synthesis of chalcone derivatives. Acetophenone (4) (0.01 mol) and substituted benzaldehyde (5) (0L.01 mol) were mixed in a round bottom flask. Ethanol (30 mL) was annexed and then 40% potassium hydroxide (15 mL). The mixture was stirred for 30 min at room temperature, then left to stand for 24 h. The mixture was poured into a beaker containing crushed ice to quench the reaction and then neutralized with 10% HCl. The precipitates formed were filtered, washed with distilled water and dried. They were then recrystallized from absolute ethanol to obtain the desired products (6)(7)(8)(9)(10)(11)(12).  www.nature.com/scientificreports/ Dock tools 1.5.4 was used to determine the grid box size for the potential binding site. The structure of the compounds was optimized with Gaussian 09 37 . The determined dimension was X = 26, Y = 26, Z = 26 with 1.00 Å as the grid spacing. Lamarckian genetic algorithm method was applied to obtain the optimum binding site for the ligand 38 . Gasteiger charges were computed using Auto-Dock tools graphical user interface supplied by MGL tools 39 . We however used optimal interactions and the best Auto-Dock socre for the interpretation of the best conformation.
In silico prediction. The physicochemical properties, lipophilicity, water solubility, pharmacokinetics, Druglikeness, and medicinal chemistry properties of the synthesized compounds were assessed using SwissADME online software.

Biological studies
Antimicrobial activity of the synthesized chemical compounds. Methodology  Experimental. The modified methods of Cowan 34 was adopted in the antimicrobial assay. Different concentrations of the synthesized molecules were transferred into sterilized Petri dish, and 16 mL of sterile molten agar was added and allowed to gel. Using a permanent marker, seven equal parts were made on the plates and then the test microorganisms were added on the segments, and labeled. The culture plates were incubated for 24 h at 37 °C for bacterial and 48 h at 25 °C for fungi. After incubation, the plates were observed for sensitivity and further incubated for 24 h at 37 °C, and 48 h at 25 °C to evaluate the bactericidal and fungicidal activity respectively.

Antioxidant activity.
Ferrous ion chelating activity. The chelating of ferrous ions by the synthesized compounds were evaluated employing the method of Singh and Rajini 40 . Different concentrations of the compounds were added to 100 μL of 2 mM ferrous sulphate solution and 300 μL of 5 mM ferrozine and mixed. The mixture was incubated at room temperature for 10 min. The absorbance of the solution was recorded at 562 nm. Ethylene diamine tetracetate (EDTA) was used as standard.
The tests were carried out in triplicate and the percentages inhibition were calculated using :

Spectral characterization
In the FTIR, the band at 3167 cm −1 and 3478 cm −1 in compounds 6e and 6g respectively are due to OH, band at 3403 cm −1 in compound 6f is due to C-NH of the amine group. The bands between 1808-1878 cm −1 in all the compounds are due to C=O stretch conjugated with olefinic bond while the bands at 1601-1637 cm −1 are due to the C=C stretch that conjugated with a carbonyl group of ketone. The band at 821 cm −1 in compound 6b is due to C-Cl stretch. These bands indicates successful formation of the desired chalcones. www.nature.com/scientificreports/ In the 1 HNMR, the peak at 9.83 ppm in compound 6e is assigned to the OH of the phenolic group. The appearance of doublet at 7.91-8.133 ppm and multiplets at 7.18-8.51 ppm were indicative of the successful formation of the desired chalcones. The appearance of singlets at 3.94 and 3.81 ppm in compound 6c confirmed the presence of dimethyl group, while the appearance of singlet at 3.93 ppm in compound 6e indicates the presence of a methyl group. All these indicate successful formation of the target products.

Percentage of inhibition
The peaks at 189-198 ppm in the 13 CNMR indicates the presence of C=O of ketone. The peaks at 190 ppm in compounds 6c, 6d and 6e are due to C-O. All the aromatic and aliphatic peaks were accounted for in the carbon-13 NMR. The carbon-13 NMR showed all the peaks expected of successful coupled products.
The high resolution mass spectrometer (HRMS) peak of the derivatives appeared as molecular ions (M + ). The results corresponded to three decimals with the calculated values. The spectra used for the characterisation of the new compounds are available as supporting documents.
Absorption, distribution, metabolism and excretion prediction. One of the procedures in drug development processes is the ability of the drug target to be orally bioP available 41 . Lipinski rule proposed some rules that drug target must have; the molecular weight ≤ 500, hydrogen bond acceptor (HBA) ≤ 10, hydrogen bond donor (HBD) ≤ 5, lipophilicity (logP) ≤ 5. The rule highlights feasible bioavailability problem if more than two tests are breached 42 . All the synthesized compounds obeyed the rule except compound 6d which has logP of 5.71 as shown in Table 1.
The values of the consensus LogP o/w is an indication that the compounds are highly lipophilic and they will be distributed in the lipid regions of the body to a greater extent. The results ( Table 2) indicates that the compounds will also have good absorption, distribution, metabolism and excretion characteristics since the LogP o/w is > 2 and < 6 43 .  www.nature.com/scientificreports/ The water solubility of the compounds (Table 3) as predicted by SwissADME software shows that compound 6e, 6f and 6g will have more absorption and distribution in the aqueous region of the body than compound 6a, 6b and 6c while compound 6d will have poor absorption and distribution in the aqueous region of the body.
The results of pharmacokinetics prediction is presented in Table 4. The high GI adsorption is an indication that the compounds will be broken down and digested very quickly in the body and it will be largely absorbed by the small intestine. The BBB (blood brain barrier) regulates an external surface interaction between the blood and the brain, the BBB result above, it shows that the compounds can have access to the central nervous system (CNS), so it can be used to treat any infection of the central nervous system. The negative value of Log K p shows that it is likely to have a low skin permeability.
Permeability glycoprotein (P-gp) major role is to protect the central nervous system from xenobioties. The synthesized compound is not a good P-gp substrate and so it is not a good inhibitor of xenobiotics. The drug is an inhibitor of CPY450 enzymes, so it blocks the metabolic activities of one or more CYP450 enzymes.
The compounds obeyed Lipinski, Ghose, Veber, Egan and muegge rule of five (Table 5) therefore, they will be orally bioactive in systematic circulation and the bioavailability score of 0.55 shows that the compounds can act as good oral drugs.
The pan-assay interference compounds (PAINS) and Brenk alert allows the identification of potentially problematic fragments in the studied molecules, from the Table 6, the compounds are said to contain problematic   6a, 6b, 6c, 6d, 6f and 6g has no leadlikeness therefore, they cannot be subjected to chemical modifications 44 . The synthetic accessibility score for the compounds are far less than 5 therefore, the compounds can easily be synthesized. Looking at their binding affinities compared with ciprofloxacin and fluconazole as the standards, the compound all had good binding affinity with the target microorganism proteins but, compounds 6e and 6g showed better binding affinity when compared with the standard as shown in Table 7. The docking protocol was validated using 3P3E as shown in Fig. 1.
Closer studies on compounds 6e and 6g were carried out with the receptor 2VF5 and 1WS3 as shown in Figs. 2, 3, 4, 5 and 6 to gain more insight about the compound interaction with the proteins of the microorganisms. Figure 2 Fig. 6 and Table 8. Figure 3 showed the binding pose of compound 6g in the cavity of 2VF5, an indication of outstanding hydrogen bonding interaction between the compound and the amino residues making the compound to have high binding affinity with the organisms. Table 9 also showed the interaction between the amino residue and the compound 6g; the proteins involved the types of hydrogen  www.nature.com/scientificreports/  www.nature.com/scientificreports/ interactions, bond lengths and bonding energies. Because of these interactions, compounds 6e and 6g can be said to have drug target since they show good antagonist on the biochemical processes of the receptors. We observed that the standard drugs and co-crystalized ligands had better antibacterial activity than the reported derivatives, however, the synthesized derivatives showed better antifungal activity in the in silico experiment.
In vitro antimicrobial activities. The results of the antimicrobial assays (Tables 10 and 11   www.nature.com/scientificreports/ on Pseudomonas aeruginosa. All the compounds except compound 6e had no inhibition activity against Pseudomonas aeruginosa. Only compound 6d had activity on Pseudomonas aeruginosa. All the compounds screened had poor activity on Aspergillus niger except compound 6c that showed moderate activity. None of the reported compounds had antimicrobial activity comparable with the standard drugs. We therefore would consider further optimization of the active derivatives. We also noticed that in spite of the good in silico results against C. albicans protein, the compounds were inactive against C. albicans suggesting that the concentration used for the assay   www.nature.com/scientificreports/ is below the active concentration. However, the in silico results is in agreement with the results obtained from the invitro experiment which strongly suggest the inhibition of the tested enzymes in the in vitro experiments. Antioxidant activity. The compounds were assessed for their in-vitro antioxidant activities using ethylenediamine tetracetate (EDTA) as the standard. Compounds 6c, 6e and 6g gave excellent inhibitory activities above those of the standard. Compound 6a gave good activity at 500 μg/mL and 100 μg/mL concentrations but, below the standard at 250 μg/ml and no inhibition at 125 μg/mL. Compound 6d had good inhibition at 500 μg/ mL and 1000 μg/mL but, no inhibition at 125 μg/mL and 250 μg/mL. Compound 6b was found to be inactive in all the concentrations. The IC50 values showed that only compounds 6c and 6f had better antioxidant activity.

Conclusions
In this paper, we have described a versatile approach to obtain chalcone derivatives. All the compounds were evaluated for their antimicrobial and antioxidant activities. Compound 6b was the most active against E. Coli, compound 6d was the most potent against S. typhi, compounds 6a, 6c and 6d had the same activity (MIC 0.4 mg/ mL) againt S. aureus, compounds 6c and 6e were the most active (MIC 0.4 mg/mL) against B. subtilis only compound 6d showed activity (MIC 0.8 mg/mL) against C. albicans, only compound 6e showed activity (MIC 0.8 mg/mL) against P. aeruginosa while all the compounds were active against A. niger. Compound 6c had highest antioxidant activity. SwissADME was used to predict the absorption, distribution, metabolism and excretion properties of the compounds and the results showed that the reported derivatives have druggable properties. In particular, compound 6e was reported to be drug-like.