Potential Antioxidant Anthraquinones Isolated from Rheum emodi Showing Nematicidal Activity against Meloidogyne incognita

1 Division of Agricultural Chemicals, Indian Agricultural Research Institute, New Delhi 110012, India 2Department of Science and Technology, New Delhi 110016, India 3 Department of Chemistry, Institute of Home Economics, Hauz Khas, New Delhi 110016, India 4Department of Chemistry, University of Delhi, New Delhi 110 007, India 5 Council of Scientific and Industrial Research-National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India 6Division of Nematology, Indian Agricultural Research Institute, New Delhi 110012, India 7 Plasma Bioscience Research Center and Department of Electrical and Biological Physics, Kwangwoon University, Seoul 139-701, Republic of Korea


Introduction
Rheum emodi (family: Polygonaceae), commonly known as Indian or Himalayan rhubarb, is a perennial herb and a potential source of bioactive molecules against pests and diseases.It is distributed in the temperate and subtropical alpine region of the Himalayas.Ethnomedical uses of Rheum emodi have been recorded from India, China, Nepal, and Pakistan for 57 different types of ailments [1,2].Rheum emodi contains different secondary metabolites that are categorized as anthraquinones, stilbenes, anthrones, chromones, oxantrone ethers and esters, flavonoids, carbohydrates, lignans, phenols, and sterols.The major constituents of this herb are hydroxyanthraquinones and their glycosides which are responsible for various physiological activities.Some of the bioactive compounds identified in these plants extracts include rhein, emodin, aloe-emodin, chrysophanol, physcion, and their glycosides [3].Other complex compounds have also been identified as torachrysone-8-O--d-glucopyranoside, sulphated emodin glucoside, and acetylated chrysophanol glucoside 2 Journal of Chemistry [4].Revandchinone-1, 3, and 4 exhibited significant antifungal activity against Aspergillus niger and Rhizopus oryzae and antibacterial activity against gram positive (Bacillus subtilis, Bacillus sphaericus, and Staphylococcus aureus) and gram negative (Klebsiella aerogenes, Chromobacterium violaceum, and Pseudomonas aeruginosa) bacteria [5].Most of the compounds have been evaluated for their medicinal activities.Hydroxyl anthraquinones and their glycosides have been found effective against animal pathogens, namely, Candida albicans, Aspergillus fumigatus, Cryptococcus neoformans, and Trichophyton mentagrophytes [6,7].The petroleum ether and chloroform extracts of the rhizomes have been reported to exhibit moderate antifungal and antibacterial activities; the benzene extract of R. emodi inhibited the growth of H. pylori [8].
However, only few reports are available on the agricultural pest control use of R. emodi extracts including nematicidal activity which is also far and fewer.So, the present study is undertaken to explore the possibility of using R. emodi extracts and isolated bioactive molecules in nematode control and antioxidant potential evaluation by DPPH radical scavenging activity.In the literature several methods have been reported about efficient extractions and HPLC analysis of R. emodi [9].But, we have standardized accelerated solvent extraction (ASE) method for efficient extraction of bioactive constituents of R. emodi and compared it with Soxhlet extraction method.This was followed by separation on flash chromatography and purity check by analytical reverse phase high pressure liquid chromatography (HPLC).LC-MS/MS method in ESI mode has been employed in the direct infusion mode for the detection and confirmation of the structure of isolated, purified products which were further characterized by their spectroscopic details using infrared (IR) and 1 H NMR spectroscopy.

Chemicals and Plant Materials.
Laboratory and analytical grade chemicals, reagents, and solvents were locally procured.Quercetin, gallic acid, and 1,1-diphenyl-2-picrylhydrazyl (DPPH) were procured from Merck India Ltd.Authentic samples of the rhizomes of Rheum emodi were procured, Delhi [10].The nematode Meloidogyne incognita (root-knot nematode) cultures were obtained, cultured, and identified from the farms of the Division of Nematology, IARI, New Delhi.

Accelerated Solvent Extraction.
The extraction was carried out using a Dionex ASE 300 (Sunnyvale, CA, USA) accelerated solvent extractor.A 50 g portion of the root powder of R. emodi was packed tightly in a 100 mL stainless steel vessel and extracted with methanol at different temperatures (40, 50, and 60 ∘ C).The extractions were performed under pressure at 10.34 Mpa, with 5 min equilibration, 5 min static time, 60% purge volume, and a 60 sec purge for a total of three cycles.About 80-100 mL of solvent was collected in each extraction.The solvent was removed under reduced pressure at 40 ∘ C in a rotary evaporator (Heidolph, Germany) to obtain dark yellow coloured viscous residue.All the operations were repeated thrice and the extracts were kept at −20 ∘ C until analysis.

Soxhlet Extraction. R. emodi rhizome powder (150 gm)
was Soxhlet extracted with methanol (500 mL) for 8 hours.It was concentrated under reduced pressure and then partitioned with chloroform and ethyl acetate.The chloroform extract gave positive test for anthraquinones and showed four bands on the TLC plate.Four bands were scrapped and eluted with methanol.The purity of individual bands was checked by HPLC.

Thin Layer Chromatography (TLC)
. Activated silica gel coated plates (6 cm × 20 cm) of 0.25 mm thickness were used.Preparative TLC was performed on silica gel coated glass plates (20 cm × 20 cm) of 0.25 mm thickness.The preparative plates were developed in different solvent combinations like (i) petroleum ether : ethyl acetate : formic acid (2 : 8 : 0.1) and (ii) hexane : chloroform : acetic acid (9 : 1 : 0.1).Plates were air-dried and visualized with iodine vapor.The individual bands were scraped and extracted with methanol and the solvent evaporated under vacuo to obtain the compounds.

High Performance Liquid Chromatography (HPLC).
Analytical reverse phase HPLC (WATERS e2695) fitted with quaternary gradient pump, autosampler, controller Empower 2 software, waters 2998 PDA detector, and waters spherisorb R 5 m ODS2 (4.6 × 250 mm) analytical column was used for all the analysis.Anthraquinones were separated under gradient elution at a flow rate of 1 mL min −1 using a mobile phase of ACN : methanol (95 : 5) and water (0.2% CH 3 COOH v/v).A 20 L volume of sample was injected each time and peaks were detected at  max of 290.The retention time (  ) for each compound was recorded.Water used in HPLC analysis was obtained from Millipore water purifier system with resistivity of 18.2 MΩcm.

Fourier Transform Infrared Spectroscopy (FT-IR).
FT-IR was performed by Bruker Alpha making KBr palates, using Hydraulic press and the data processed using Opus 65 software.instrument.Samples were dissolved in deuterated chloroform (CDCl 3 ), dimethyl sulfoxide (DMSO), or carbon tetrachloride (CCl 4 ) as per the requirement.Tetramethylsilane (TMS) was used as an internal standard.Chemical shifts were recorded in  (ppm) relative to TMS. [11][12][13][14][15].It facilitates rapid and accurate identification of chemical compounds, especially when a pure standard is not available.The technique has been extensively used in the past for the analysis of phenolic compounds [16].The most active extract of R. emodi was analyzed for its active constituents and their structures confirmed by electrospray mass spectroscopy (ESI-MS) in the direct infusion mode.

Electrospray Mass Spectroscopy (ESI-MS). Liquid chromatography coupled with mass spectrometry (LC-ESI-MS) is a powerful technique to analyze complex botanical extracts
Mass spectroscopy was carried out using Thermo Electron spectrometer (Thermo Electron Corporation, USA).Detection of mass was done by electron-spray ionization (ESI) source with Finnigan LCQ tune plus program fitted with MAX-detector.Xcalibur software was used for the purpose of identification, quantification, and fragmentation of required masses.The MS parameters optimized in direct infusion mode were spray voltage 3.5 to 5 kV, sheath gas flow rate 10 mL min −1 , auxiliary gas flow rate 5 mL min −1 , spray current 0.5, capillary temperature 225 ∘ C, capillary voltage 20-35 V, and tube lens offset 40-65.The mass spectra were recorded in negative ion mode (Figure 1).

Characterization of R. emodi Anthraquinones.
The preparative thin layer chromatography of the chloroform extracts afforded four compounds: chrysophanol, physcion, emodin, and aloe-emodin.The bands on the TLC were scrapped and eluted with methanol.The purity of the individual bands was checked by TLC followed by their analysis by HPLC (Figure 2).The spectral characteristics of pure anthraquinones are as follows.

Extraction of Nematodes from Plant Root. M. incognita
infected plant material was collected from the glass house of Division of Nematology, I.A.R.I., New Delhi.The infected roots of brinjal (cv.Pusa Purple Long) were washed thoroughly with tap water to remove soil adhering to the roots.Meloidogyne egg mass was collected from the brinjal under stereoscopic microscope.The egg masses were kept on double layered tissue paper supported by wire mesh and kept 2-3 days in incubator at 25-30 ∘ C and 70% RH for hatching.The number of freshly hatched second stage (J 2 s) juveniles was counted per 1 mL aliquot of distilled water.
Fresh nematode suspension (1 mL) was taken in a square counting dish (marked with 24 equal squares) under stereoscopic binocular microscope.The total number of nematodes in one mL was determined by counting the root-knot juveniles in each square.This process was repeated for five times and the average was taken.Suspension of the J 2 s was diluted further to get approximately 25 J 2 s per mL.
2.6.Test Procedure 2.6.1.Antioxidant Activity.The antioxidant activity of test extracts such as CE1, CE2, CE3, and standard compounds gallic acid and quercetin was assessed by radical scavenging effect of 1,1-diphenyl-2-picrylhydrazyl (DPPH) [17][18][19].The diluted working solution of the test extracts and the standards (1-100 mg/L) were prepared in methanol.1 mL of DPPH (0.002%) solution in methanol was mixed separately with 1 mL of the sample solution and the standard solution.Different concentrations (50, 100, 500, and 1000 mg/L) of test extracts and the standards were pipetted to the test tubes and volume adjusted to 3 mL with methanol.1 mL of alcoholic DPPH (0.002%) solution was added to each sample and the samples were vortexed and then incubated in dark at room temperature for 30 min.Methanol (1 mL) with DPPH solution (0.002%, 1 mL) was used as blank.The spectrophotometric measurements at 517 nm against blank samples were made with a pair of matched quartz cuvettes using Analytik Jena UV-Vis spectrophotometer (SPCORD 200).The optical density was recorded and radical scavenging activity was expressed as percentage inhibition of DPPH radical and was calculated by the following equation: Extract concentration providing 50% inhibition (IC 50 ) was calculated from the graph plotted between inhibition percentages against extract concentration with the help of statistical package (GW BASIC).
2.6.2.Nematicidal Activity.Root-knot nematode suspension (2 mL) containing 50 juveniles was taken in petri plates; an equal volume of the test solutions was added separately to obtain the desired test concentrations of 200, 100, 50, and 25 mgL −1 , respectively.Each treatment was replicated thrice separately for 24, 48, and 72 hours.For each treatment a control was taken by adding 2 mL of nematode suspension with equal volume of ethanol-Tween 80 emulsified water at 25-30 ∘ C.After 24 h of exposure, the petri plates for each treatment were observed under stereoscopic binocular microscope for determining mortality.A revival test was performed for each treatment by decanting the test solvents and adding distilled water to the petri plates.After 48 h of exposure readings were taken again for each treatment.The revived juveniles were counted and deducted from the number of immobile juveniles obtained in the previous reading.Similar process was followed after 72 hr exposure time.The nematodes were considered dead if appeared motionless, when probed with a fine needle.The percent mortality in treatment as well as in control for each compound was calculated and the corrected percent mortality was calculated by using the following Abbot's formula (1925) [20]: where  = total mortality in treatment and  = total mortality in control.

Antioxidant Activity.
The extracts of R. emodi were analyzed for antioxidant activity by using DPPH free radical scavenging activity.The anthraquinone derivatives, such as aloe-emodin, emodin, rhein, chrysophanol, and physcion, are reported to possess antiangiogenic activity, by preventing blood vessel formation in zebra-fish embryos [24].The anticancer effect of aloe-emodin has been established in two human cancer cell lines, Hep G2 and Hep 3B.Aloeemodin inhibited cell proliferation and induced apoptosis in both examined cell lines by different antiproliferative mechanisms [25].In our study all the extracts showed a concentration dependent scavenging of DPPH radicals (Figure 5).CE1 with (IC 50 18.28 mg/mL) was most active, followed by ethyl CE2 (IC 50 19.37mg/mL) and CE3 (IC 50 20.27mg/mL) (Table 1).All the extracts were comparatively less active than both quercetin hydrate (QH) (4.95 mg/L) and gallic acid (GA) (11.26 mg/L) in quenching DPPH radical than standard antioxidants.The extracts contained a high number of phenolic compounds, which were found to have significant positive correlation with free radicals (DPPH and OH) scavenging efficacies.The results were in agreement with those of [26] who found the methanolic extract to be a more active radical scavenger than aqueous extract.
The HPLC analysis had shown the methanolic extract of R. emodi rhizomes to be rich in the major anthraquinones  and their glucosides, namely, emodin, aloe-emodin, chrysophanol, physcion, emodin glucoside, chrysophanol glucoside, and revandchinone, whereas ethyl acetate and chloroform extract was found to be low in aloe-emodin and other glucosides.The lower IC 50 value for methanolic extract in free radical scavenging activity of DPPH is attributed to the fact that the extract is rich in bioactive constituents including anthraquinones.However, our results are in contrast to the study [4] where the compounds like marsupsin and maesopsin obtained from the rhizome/root extracts of R. emodi are found to possess antioxidant activity, whereas chrysophanol, physcion, and emodin as well as their 8-Oglucosides were found to be inactive.and physcion (ED 50 102.61mg/L) were equally active followed by emodin (139.95mg/L) and aloe-emodin (148.50 mg/L).The CE1 has also outperformed the positive control (Aza 20%) (112.70 mg/L) (Table 2).The results revealed that crude methanolic extract (CE1) was rich in anthraquinones, glucosides, and other phenolics contained in R. emodi which were very effective in controlling M. incognita menace.The pure compounds were, however, comparatively less lethal than crude fractions.The observations substantiate the fact that bioactive constituents collectively showed symbiotic effect as the positive control azadirachtin (20%) was less active than the crude extract (CE1).The revelation opens a new horizon that the CE1 could be formulated as potent nematicidal and the same can be evaluated in the field conditions.

Conclusion
The present study conclusively demonstrated that the anthraquinones and other phenolics of R. emodi especially in the methanolic extract (CE1) have great potential and can be suitably utilized into environmentally benign pest control strategies.The potential antioxidant activity of the R. emodi constituents as revealed by the DPPH assay strengthens the importance of anthraquinones in ethno-medical use as well as in contemporary medicine.Further LC-MS analysis revealed that the activity of CE1 was mainly due to the synergistic combinations of its anthraquinone and their glucosides along with other phenolics.The constituents can suitably become one of the lead molecules to be further exploited in various spheres of pest and disease control mechanisms after suitable clinical testing.

3. 1 .
Extraction and Characterization of Anthraquinones.The mass spectra of four hydroxyanthraquinones identified in

Figure 5 :
Figure 5: Free radical scavenging activity of the extracts of Rheum emodi and standard antioxidants.

Table 2 :
Antinemic activity (ED 50 ) of isolated anthraquinones and solvent extractives of Rheum emodi against j 2 s of Meloidogyne incognita after 72 hrs.