Biofunctional Constituent Isolated from Citrullus colocynthis Fruits and

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Biofunctional Constituent Isolated from Citrullus colocynthis Fruits and Structure−Activity Relationships of Its Analogues Show Acaricidal and Insecticidal Efficacy Ju-Hyun Jeon and Hoi-Seon Lee* Department of Bioenvironmental Chemistry and Institute of Agricultural Science and Technology, College of Agriculture and Life Sciences, Chonbuk National University, Jeonju 561-756, Republic of Korea ABSTRACT: The acaricidal and insecticidal potential of the active constituent isolated from Citrullus colocynthis fruits and its structurally related analogues was evaluated by performing leaf disk, contact toxicity, and fumigant toxicity bioassays against Tetranychus urticae, Sitophilus oryzae, and Sitophilus zeamais adults. The active constituent of C. colocynthis fruits was isolated by chromatographic techniques and was identified as 4-methylquinoline on the basis of spectroscopic analyses. To investigate the structure−activity relationships, 4-methylquinoline and its structural analogues were tested against mites and two insect pests. On the basis of the LC50 values, 7,8-benzoquinoline was the most effective against T. urticae. Quinoline, 8-hydroxyquinoline, 2methylquinoline, 4-methylquinoline, 6-methylquinoline, 8-methylquinoline, and 7,8-benzoquinoline showed high insecticidal activities against S. oryzae and S. zeamais regardless of the application method. These results indicate that introduction of a functional group into the quinoline skeleton and changing the position of the group have an important influence on the acaricidal and insecticidal activities. Furthermore, 4-methylquinoline isolated from C. colocynthis fruits, along with its structural analogues, could be effective natural pesticides for managing spider mites and stored grain weevils. KEYWORDS: Citrullus colocynthis, Sitophilus oryzae, Sitophilus zeamais, structure−activity relationship, Tetranychus urticae

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INTRODUCTION

to investigate the acaricidal and insecticidal properties of C. colocynthis fruits against the mites and two insect pests and to determine their structure−activity relationships.

Both spider mites and weevils are pests of various plants, vegetables, and postharvest grains in a wide range of agricultural environments.1−3 Populations of these insect pests cause serious economic losses and a decline in the grain quality of agricultural products worldwide.1,4 The management of these insect pests has principally been carried out using synthetic chemical applications such as contact, fumigant, or spraying treatments with organophosphate and insect-growth regulators.3,5 Although these substances are useful for controlling insect pests, their repeated use could pose a meaningful risk to human health and cause environmental pollution, as well as unwanted toxicity on nontarget organisms.3,4,6 For these reasons, various studies have investigated the effects of phytochemicals as useful alternatives to insect pest management agents. Botanical pesticides have long been used as an alternative source to synthetic chemicals for pest management because plant secondary metabolites show a broad range of insecticidal effects against agricultural, hygienic, and stored product insect pests, such as aphids, mites, mosquitoes, and weevils.1,3,7,8 Moreover, because plant-derived materials have multiple novel target points of action, the chance of developing a resistant individual is very low.9 Citrullus colocynthis, a member of the Cucurbitaceae family, is a medicinal plant distributed throughout Asia, which has been used in material for traditional medicines for arthritis, diabetes, inflammatory disorders, and stomachache.10−13 However, the acaricidal and insecticidal activities of the active constituent isolated from C. colocynthis fruits and its structurally related analogues against Tetranychus urticae, Sitophilus oryzae, and Sitophilus zeamais adults have not yet been reported. Therefore, the aim of the current study was © XXXX American Chemical Society

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MATERIALS AND METHODS

Chemicals and Plant Material. Quinoline (98%), 2-hydroxyquinoline (98%), 4-hydroxyquinoline (98%), 6-hydroxyquinoline (95%), 8-hydroxyquinoline (99%), 2-methylquinoline (98%), 6-methylquinoline (98%), 8-methylquinoline (97%), and 7,8-benzoquinoline (97%) were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA) (Figure 1). Abamectin and dichlorvos, which were used as positive control, were obtained from Fluka Chemical Co. (Buchs, Switzerland). All other chemical materials used in the study were of reagent grade and are commercially available. The fruits of C. colocynthis were supplied from the International Biological Material Research Center in Daejeon, South Korea. A voucher specimen was authenticated by Prof. Jeongmoon Kim and deposited in the herbarium at the Department of Landscape Architecture, Chonbuk National University. Plant Preparation. C. colocynthis fruits were air-dried and ground to a powder. The dried C. colocynthis fruits (100 g) were extracted twice with methanol (10 L) in a shaking incubator at 25 ± 2 °C for 36 h and filtered (filter paper No. 2, Toyo Roshi, Tokyo, Japan) under vacuum. The methanol extracts were concentrated and combined under a vacuum at 45 °C to yield a crude extract (12.7%, based on the dry weight of the fruits). The methanol extract (10 g) was sequentially partitioned into hexane (1.93 g), chloroform (2.88 g), ethyl acetate (1.52 g), butanol (1.81 g), and water-soluble (1.56 g) fractions for Received: May 28, 2014 Revised: August 6, 2014 Accepted: August 11, 2014

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dx.doi.org/10.1021/jf502536e | J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Journal of Agricultural and Food Chemistry

Article

insecticidal constituent in fraction CF432, further isolation was performed using preparative HPLC (recycling preparative HPLC; Japan Analytical Industry Co., Ltd.). A JAI gel W series column (W 253 500 mm + W 252 500 mm) was used with chloroform (100%) as the mobile phase at a flow rate of 3.5 mL/min. During this step, three subfractions (CF4321−CF4323) were obtained and were then bioassayed. The active CF4322 fraction was subjected to further chromatography using a JAI gel GS series column (GS 310 500 mm + GS 310 300 mm) with chloroform (100%) at a flow rate of 3.0 mL/ min. Finally, the potent active principle (CF43221) was isolated to a single peak. The structure of CF43221 was determined by spectroscopic analyses. Electron ionization (EI) mass spectra were obtained on a GSX 400 mass spectrometer (JEOL Ltd., Tokyo, Japan). 1 H and 13C NMR spectra were analyzed in CD3OD, using a JNMECA600 spectrometer, at 600 and 150 MHz (with tetramethylsilane as an internal standard), respectively, with the chemical shifts expressed in δ (ppm). Mite and Insect Rearing Conditions. Respective cultures of T. urticae, S. oryzae, and S. zeamais used for the bioassay were obtained from the National Academy of Agricultural Science, Rural Development Administration (Suwon, Korea). T. urticae were reared in the laboratory on kidney bean (Phaseolus vulgaris L.) seedlings, and S. oryzae and S. zeamais were reared on rice grains in plastic containers (30 × 30 × 30 cm). They were maintained in the laboratory at 25 ± 1 °C, 65 ± 5% relative humidity (RH), and a 16 h light/8 h dark photoperiod. No mites and insects were exposed to any known acaricides or insecticides during maintenance. Bioassay. The acaricidal and insecticidal activities against the three insect species were evaluated by leaf disk, contact, and fumigant bioassays using C. colocynthis extract, the active constituent, and its structural analogues, using the bioassays presented by Akhtar et al.1 and Jeon et al.3 The initial concentrations were chosen in a previous screening. Adults of mixed sex were used for each bioassay. To evaluate the residual toxicity against T. urticae, leaf disks (2.5 cm diameter) cut from kidney bean were dipped in solutions (2.0, 1.0, 0.5, 0.1, 0.05, 0.025, 0.010, and 0.005 μg/mL) of the test materials for 15 s, and the disks were then evaporated for 20 min in a hood. Acetone was utilized as a carrier solvent. Twenty T. urticae adults were introduced onto the leaf disk supported by a cotton pad in a Petri dish (60 × 15 mm), which was then covered with a lid. To evaluate the contact toxicity of the test materials against S. oryzae and S. zeamais adults, various amounts (2.10, 1.05, 0.52, 0.26, 0.13, 0.06, 0.03, 0.01, 0.005, 0.001, and 0.0005 mg/cm2) of each material were liquefied in ethanol and applied to filter paper (5 cm

Figure 1. Quinoline structural analogues: (1) quinoline; (2) 7,8benzoquinoline; (3) 2-hydroxyquinoline; (4) 4-hydroxyquinoline; (5) 6-hydroxyquinoline; (6) 8-hydroxyquinoline; (7) 2-methylquinoline; (8) 4-methylquinoline; (9) 6-methylquinoline; (10) 8-methylquinoline. subsequent bioassays. The fractions were concentrated using rotary vacuum evaporation and were refrigerated prior to use at 4 °C. Isolation and Identification. To isolate the active compound of the chloroform fraction derived from the methanol extract of C. colocynthis fruits, the chloroform fraction was chromatographed on a silica gel column (Merck 70−230 mesh, 12 × 15 cm; Rahway, NJ, USA) and eluted with a gradient of chloroform/ethyl acetate (from 10:0 to 0:10, v/v). The separated fractions were collected and analyzed by thin-layer chromatography (TLC; chloroform/ethyl acetate = 8:1, v/v), and similar fractions based on their TLC pattern were combined to yield subfractions designated CF1−CF5. The active CF4 fraction was chromatographed on a silica gel column and was eluted with chloroform/ethyl acetate (9:1, 8:2, 7:3, and 5:5, v/v). The active CF43 fraction was also chromatographed on a silica gel column and eluted with chloroform/ethyl acetate (7:3, v/v), and the fractions obtained were bioassayed against the target insects. To determine the

Table 1. Acaricidal and Insecticidal Activities of Methanol Extract and Solvent Fractions Derived from Citrullus colocynthis Fruits mortality (%, mean ± SE)a species

method

dose

methanol extract

T. urticae

leaf disk

5.0 mg/mL

5.3 ± 0.4 b

S. oryzae

contact

2.1 1.0 0.5 1.4 0.7 0.3

mg/cm2 mg/cm2 mg/cm2 mg/cm3 mg/cm3 mg/cm3

100.0 62.1 13.8 100.0 41.8 11.4

± ± ± ± ± ±

0.0 1.5 0.7 0.0 1.2 0.6

a bc d a c d

18.3 ± 1.7 a 0b − 14.1 ± 2.2 a 0b −

100.0 100.0 73.5 100.0 91.3 47.6

± ± ± ± ± ±

0.0 0.0 2.1 0.0 1.4 2.3

a a b a a c

0a −b − 0a − −

2.1 1.0 0.5 1.4 0.7 0.3

mg/cm2 mg/cm2 mg/cm2 mg/cm3 mg/cm3 mg/cm3

100.0 79.2 18.9 100.0 43.5 15.3

± ± ± ± ± ±

0.0 1.7 2.2 0.0 1.8 0.9

a ab d a c d

16.8 ± 1.8 a 0b − 17.5 ± 0.9 a 0b −

100.0 100.0 61.8 100.0 94.6 41.1

± ± ± ± ± ±

0.0 0.0 1.9 0.0 0.7 2.2

a a bc a a c

0a − − 0a − −

fumigant

S. zeamais

contact

fumigant

a

hexane fraction 0b

chloroform fraction

control (solvent only)

7.1 ± 1.2 b

0a

Means within a column followed by the same letter are not significantly different (P = 0.05; Scheffe’s test). b−, not tested. B



Article

diameter) (Whatman Co., Maidstone, UK), with an ethanol-only negative control. Each filter paper was placed in the bottom of a Petri dish (5.5 cm diameter × 1.5 cm) after being dried in a fume hood for 10 min. Groups of 20 insects were separately placed in the Petri dish, and the lid was replaced. To evaluate the fumigant toxicity against S. oryzae and S. zeamais, filter papers were impregnated with ethanol solutions (100 μL) of different concentrations (1.40, 0.70, 0.35, 0.18, 0.09, 0.05, 0.01, 0.005, 0.001, and 0.0005 mg/cm3) of each test material. Filter paper treated with ethanol (100 μL) was used as a negative control. After drying in a hood for 10 min, each filter paper was placed in the lid of a Petri dish (5.5 cm diameter × 1.5 cm). Twenty adult weevils were then placed in the Petri dish, which was covered with a 60 mesh cloth to avoid direct contact with the samples tested, and the lid was replaced to assess the fumigant action. T. urticae, S. oryzae, and S. zeamais adults placed in treated Petri dishes were kept under the same conditions as for rearing. All treatments were replicated three times, and mortalities were recorded after 48 h of treatment. Statistical Analysis. LC50 values were determined and transformed to arcsine square root values for analysis of variance (ANOVA).14 The effects of each treatment were determined to be significantly different at P < 0.05.

The acaricidal and insecticidal activities of 4-methylquinoline isolated from C. colocynthis fruits and structurally related analogues against three insect pests were evaluated using the leaf disk, contact toxicity, and fumigant toxicity bioassays. Acaricidal activities of 4-methylquinoline and its structural analogues against T. urticae were examined by the leaf disk method (Figure 2). On the basis of the 48 h mortality rates, the

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RESULTS AND DISCUSSION The acaricidal and insecticidal toxicities of the methanol extract and the five fractions (hexane, chloroform, ethyl acetate, butanol, and water-soluble) obtained from C. colocynthis fruits are shown in Table 1, from evaluations using the leaf disk, contact toxicity, and fumigant toxicity applications against T. urticae, S. oryzae, and S. zeamais. Significant differences were observed in the acaricidal and insecticidal toxicities against the different insects. Upon examination of the methanol extract of the C. colocynthis fruits, a clear dose−response relationship was observed against S. oryzae and S. zeamais. Moreover, the chloroform and hexane fractions produced strong and weak insecticidal toxicities against Sitophilus spp., respectively. However, no activity was observed from the ethyl acetate, butanol, and water-soluble fractions (data not shown). When tested against T. urticae, the methanol extract of C. colocynthis fruits and its fractions showed a dramatic decrease in the acaricidal toxicity during the leaf disk bioassay. No mortality was observed for the negative controls (solvent only). These results indicate that the acaricidal and insecticidal toxicities of plant-derived materials can have different potencies depending on the species of insect investigated. Due to the strong insecticidal toxicities observed from the chloroform fraction of the methanol extract of C. colocynthis fruits, the active constituent was isolated from this fraction by silica gel column chromatography and preparative HPLC. Structural determination of the isolated constituent was performed by spectroscopic analyses, including EI/MS, 1H NMR, 13C NMR, COSY, DEPT, and HMQC NMR and then by direct comparison with an authentic reference compound. The active compound was characterized as 4-methylquinoline. It was identified on the basis of the following evidence: 4methylquinoline (C10H9N); EI/MS (70 eV) m/z M+ 143 (100, base peak), 135 (40), 105 (39), 107 (40), 79 (46), 51 (13); 1H NMR (CD3OD, 600 MHz) δ 2.61 (s), 7.26−7.27 (d, J = 6.7 Hz), 7.42−7.43 (t, J = 6.9 Hz), 7.63−7.75 (t, J = 83.1 Hz), 8.00−8.05 (d, J = 35.7 Hz), 8.09−8.11 (d, J = 7.3 Hz), 8.42− 8.43 (d, J = 1.6 Hz); 13C NMR (CD3OD, 150 MHz) δ 150.6, 148.2, 135.5, 130.0, 128.5, 128.1, 126.5, 124.4, 122.9, 19.6. The spectroscopic data of authentic 4-methylquinoline were found to be similar to those of the previous data.10,15

Figure 2. Responses of T. urticae adults to 4-methylquinoline and its structural analogues, determined by a leaf disk assay at 1.0 mg/mL, at 48 h. Twenty adults per replicate; 3 replicates per treatment; n = 60. Mean values corresponding to each treatment with the same letters are not significantly different (P = 0.05; Scheffe’s test).

analogue 7,8-benzoquinoline demonstrated the highest mortality (100% at 1.0 mg/mL). However, other compounds (quinolone, 2-hydroxyquinoline, 4-hydroxyquinoline, 6-hydroxyquinoline, 8-hydroxyquinoline, 2-methylquinoline, 4-methylquinoline, 6-methylquinoline, and 8-methylquinoline) were found to have no acaricidal activity against T. urticae. The excellent acaricidal activity of 7,8-benzoquinoline against T. urticae was compared with that of a synthetic acaricide (abamectin) (Table 2). On the basis of the LC50 values to T. urticae calculated from the leaf disk method, 7,8-benzoquinoline (0.017 μg/mL) was determined to be 31.06 times more toxic than abamectin (0.528 μg/mL). These results indicate that 7,8benzoquinoline was the most effective for control of T. urticae, due to the low concentration required to produce strong activity. The insecticidal activities of 4-methylquinoline and its structural analogues against S. oryzae and S. zeamais were tested on the basis of their active radicals (R1, R2, R3, and R4) by comparing the LC50 values using contact toxicity and a fumigant toxicity bioassay (Tables 3 and 4). From the 48 h LC50 values obtained with the contact toxicity bioassay, the most toxic compound against S. ozryae was 8-methylquinoline (0.001 mg/cm2), followed by quinoline (0.071 mg/cm2), 7,8benzoquinoline (0.077 mg/cm2), 2-methylquinoline (0.094 mg/cm2), 4-methylquinoline (0.132 mg/cm2), 8-hydroxyquinoline (0.163 mg/cm2), and 6-methylquinoline (0.166 mg/cm2). 8-Methylquinoline (0.001 mg/cm3) was also the most toxic compound identified from the fumigant toxicity bioassay against S. oryzae, followed by 2-methylquinoline (0.034 mg/ cm3), 7,8-benzoquinoline (0.045 mg/cm3), quinoline (0.064 mg/cm3), 6-methylquinoline (0.081 mg/cm3), 4-methylquinoline (0.096 mg/cm3), and 8-hydroxyquinoline (0.109 mg/cm3). Among the 4-methylquinoline analogues, 8-methylquinoline (which has a methyl group at the R4 position in the quinoline C



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Table 2. Comparison of Acaricidal Activity of 7,8-Benzoquinoline and Synthetic Acaricide against T. urticae sample

% mortality (mean ± SE)

concn (μg/mL)

95% confidence limit

RTa

7,8-benzoquinoline

0.050 0.025 0.010 0.005

100.0 69.4 27.2 11.8

± ± ± ±

0.0 1.9 1.6 1.1

0.017

0.014−0.019

31.06

abamectin

1.500 1.000 0.500 0.250

85.4 70.3 49.5 39.6

± ± ± ±

1.4 1.8 0.7 1.3

0.528

0.511−0.543

1.00

control (solvent only) a

LC50 (μg/mL)

0

Relative toxicity = LC50 value of abamectin/LC50 value of 7,8-benzoquinoline.

Table 3. Contact Toxicity of 4-Methylquinoline and Its Structural Analogues against Sitophilus spp. Adults, 48 h Sitophilus oryzae

Sitophilus zeamais

compound

LC50 (mg/cm )

slope ± SE


LC50 (mg/cm )

slope ± SE


quinoline 2-hydroxyquinoline 4-hydroxyquinoline 6-hydroxyquinoline 8-hydroxyquinoline 2-methylquinoline 4-methylquinoline 6-methylquinoline 8-methylquinoline 7,8-benzoquinoline dichlorvos (positive control) control (solvent only)

0.071 >0.210 >0.210 >0.210 0.163 0.094 0.132 0.166 0.001 0.077 0.00043

2.1 ± 0.61

0.0515−0.0771

2.8 ± 0.74

0.0465−0.0701

± ± ± ± ± ± ±

0.1552−0.1807 0.0811−0.1095 0.1255−0.1508 0.1538−0.1793 0.0008−0.0017 0.0624−0.0855 0.0003−0.0005

0.057 >0.210 >0.210 >0.210 0.159 0.078 0.135 0.150 0.001 0.076 0.00039

± ± ± ± ± ± ±

0.1438−0.1702 0.0771−0.0893 0.1214−0.1431 0.1414−0.1715 0.0009−0.0016 0.0599−0.0820 0.0003−0.0004

2

2.3 3.1 2.1 4.7 1.4 2.9 3.5

0.47 1.05 0.55 1.14 0.74 0.84 1.24

2

2.4 2.5 2.9 3.7 1.9 2.7 2.8

0.89 0.56 0.67 1.14 0.83 0.96 1.08

Table 4. Fumigant Toxicity of 4-Methylquinoline and Its Structural Analogues against Sitophilus spp. Adults, 48 h Sitophilus oryzae

Sitophilus zeamais

compound

LC50 (mg/cm3)

slope ± SE


LC50 (mg/cm3)

slope ± SE


quinoline 2-hydroxyquinoline 4-hydroxyquinoline 6-hydroxyquinoline 8-hydroxyquinoline 2-methylquinoline 4-methylquinoline 6-methylquinoline 8-methylquinoline 7,8-benzoquinoline dichlorvos (positive control) control (solvent only)

0.064 >0.140 >0.140 >0.140 0.109 0.034 0.096 0.081 0.001 0.045 0.00032

2.1 ± 1.05

0.0448−0.0652

2.8 ± 0.74

0.0381−0.0533

± ± ± ± ± ± ±

0.0899−0.1204 0.0244−0.0428 0.0835−0.1087 0.0734−0.0907 0.0009−0.0016 0.0368−0.0554 0.0002−0.0004

0.042 >0.140 >0.140 >0.140 0.094 0.039 0.084 0.063 0.001 0.038 0.00027

± ± ± ± ± ± ±

0.0825−0.1053 0.0223−0.0551 0.0638−0.0980 0.0511−0.0808 0.0009−0.0016 0.0255−0.0497 0.0002−0.0004

3.3 2.5 3.1 4.5 2.5 3.3 3.8

1.08 0.98 1.09 1.47 0.88 0.97 0.74

2.8 3.1 3.3 4.1 2.3 2.8 3.3

1.01 1.21 1.08 1.36 0.53 1.36 0.96

toxicities against Sitophilus spp. Furthermore, 7,8-benzoquinoline (which has an arene group in the quinoline’s benzene ring) showed higher potential toxicity than the other quinoline analogues against T. ureticae. In our earlier study, we reported that the biological activity depends on the presence of various functional groups in the parent structure (benzene, naphthoquinone, anthraquinone, or quinoline).16−19 Taken together, introducing or changing the position of the functional groups in the quinoline skeleton had potent acaricidal and insecticidal activities against the three insect species. Our study is the first report on the acaricidal and insecticidal activity of the constituent isolated from C. colocynthis fruits against T. urticae, S. oryzae, and S. zeamais.

skeleton) showed the most toxicity against Sitophilus spp. and was approximately 57.0−71.0 times more effective than quinoline (the parent structure). However, whereas the introduction of a hydroxyl group in the R1, R2, and R3 positions of quinoline showed no activities, introduction at the R4 position caused an increase in insecticidal activities. The insecticidal activities of 4-methylquinoline analogues against S. zeamais were similar to those against S. oryzae. These results demonstrate that introducing various functional groups (hydroxyl, methyl, and arene) into the quinoline structure can change the insecticdal activity of the compound. In addition, the placement of hydroxyl or methyl groups in the R4 position seems to be very important for increasing insecticidal D



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Recent studies have sought to develop newer and safer control agents to replace older pesticides due to toxicity, pest resistance, and environmental damage.20 Diwan et al.21 reported that the acute lethal doses of the methanol extract and water-soluble fraction derived from C. colocynthis were 200 and 2000 mg/kg, respectively. According to the safety database, 22 the oral LD 50 values in mammals of 8hydroxyquinoline (1200 mg/kg), 2-methylquinoline (1230 mg/kg), and 6-methylquinoline (800 mg/kg) indicate low and moderate acute toxicities. Sakthivadivel and Daniel23 reported larvicidal activity of the petroleum ether extract of C. colocynthis against Culex quinquefasciatus, Anopheles stephensi, and Aedes aegypti. Moreover, quinoline analogues have been widely used as components in the food industry and in traditional medicines for many years.19,24,25 On the basis of our results, 4-methylquinoline isolated from natural sources and its structurally relative analogues would be good replacements for synthetic pesticides. For this reason, further studies should be conducted to evaluate the effects of the 4-methylquinoline analogues on a wide range of insect pests, as well as to develop formulations to increase the potency and stability.

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AUTHOR INFORMATION

Corresponding Author

*(H.-S.L.) Phone: +82-63-270-2544. Fax: +82-63-270-2550. Email: [email protected]. Funding

This research was carried out with the support of the Cooperative Research Program for Agriculture Science and Technology Development (Project PJ01004502, Development of integrated pest management techniques using natural products in the grain storage), Rural Development Administration, Republic of Korea. Notes

The authors declare no competing financial interest.

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REFERENCES

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Biofunctional Constituent Isolated from Citrullus colocynthis Fruits and

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