Structure-Based Discovery of Nonpeptide Allatostatin Analogues for

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Agricultural and Environmental Chemistry

Structure-based Discovery of Non-peptide Allatostatin Analogs for Pest Control Shan-shan Huang, Shan-shan Chen, Hong-ling Zhang, Han Yang, Hui-juan Yang, Yujie Ren, and Zhen-peng Kai J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b00197 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 23, 2018

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Structure-based Discovery of Non-peptide Allatostatin

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Analogs for Pest Control

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Shan-shan Huang †, Shan-shan Chen ‡, Hong-ling Zhang †, Han Yang †, Hui-juan Yang , Yu-jie Ren *,†, and Zhen-peng Kai *,†



5 6 7



8

Technology, Shanghai, 201418, P.R. China

9



10

School of Chemical and Environmental Engineering, Shanghai Institute of

Institute of Agro-food Standards and Testing Technologies, Shanghai Academy of agr

icultural Science, Shanghai, 201403, P.R. China

11 12 13 14

* Corresponding author

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Email: [email protected] and [email protected]

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ABSTRACT: :FGLamide allatostatins (ASTs) are regarded as possible insecticide

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candidates although the absence of in vivo effects, rapid degradation, poor water

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solubility, and high production costs, preclude their practical use in pest control. In

20

contrast to previous research, the C-terminal tripeptide (FGLa) was selected as the

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lead

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(2-amino-1-[3-oxo-3-(substituted-anilino)propyl]pyridinium nitrate derivatives) were

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designed based on the structure-activity relationship and docking results of FGLa. All

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the non-peptide analogs (S1-5) were more potent on juvenile hormone (JH)

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biosynthesis than the lead compound. They significantly inhibit the biosynthesis of JH

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in vivo following injection. The pest control application demonstrated that S1 and S3

27

have larvicidal effects following oral administration (the IC50 values were 0.020 mg/g

28

and 0.0016 mg/g, respectively). Good oral toxicities and excellent water solubility of

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S1 and S3 suggest that they have considerable potential as insecticides for pest

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management.

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KEYWORDS: Allatostatin; juvenile hormone; non-peptide analogs; oral toxicity;

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cockroach

compound

in

this

study.

Five

non-peptide

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AST

analogs

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■ INTRODUCTION

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Insect neuropeptides have been recognized as the safe insecticide candidates

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because they play a central role in insect metabolism, homeostasis, development,

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metamorphosis, reproduction and behaviour. However, the intrinsic properties of

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insect neuropeptides, such as poor absorption, transport and bioavailability, short

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biological half-lives, lack of effect in vivo and high production costs have limited their

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application in pest control.1 The rational design of neuropeptide analogs that can

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block degradation, improve the absorption through pest cuticle and the transport in

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hemolymph was indicated as a general strategy of insect neuropeptides for control of

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pests.1

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Allatostatins (ASTs) comprise a family of insect neuropeptides originally

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isolated from of the cockroach Diploptera punctata that inhibit the production of

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juvenile hormone (JH).2 More than 230 ASTs have been predicted from cDNA

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sequences in insects and they can be classified into three different peptide families:

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FGLamide ASTs, W(X)6Wamide ASTs

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appeared candidates on the development of pest control agents. Following the

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sequencing of FGLamide ASTs, the development of analogs quickly ensued. Previous

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structure-activity studies demonstrated that the C-terminal Y/FXFGL-NH2 is the

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‘active core’ region of the FGLamide ASTs.4 Leu8, Phe6 and Tyr4 in Dippu-AST 5

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were the crucial amino acid residues for JH biosynthesis inhibition with the Alanine

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scanning technique.5 The first, third or fifth residues of the C-terminal pentapeptide

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were substituted by sterically hindered amino acids (Aic, Cpa and Bzd) or

and PISCF ASTs.3 The FGLamide ASTs

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hydrocinnamic acid in several AST analogs. Their bioactivities showed that those

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analogs retained biological activity.6-8 Piulachs et al. synthesized ketomethylene and

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methyleneamino pseudopeptide analogs of ASTs, thereby reducing the susceptibility

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of the bond to hydrolysis.9 Garside et al. found several FGLamide AST

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peptidomimetics inhibited JH biosynthesis and oocyte growth significantly in vivo

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although the effects of injection were monitored only at one specific age.10

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In the previous studies, approximate 200 FGLamide AST analogs were

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synthesized with the C-terminal pentapeptide as the lead compound by Yang’s

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group.11-18 Their results suggested that an aromatic group, an appropriate length of

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linker, and a FGLa moiety should appear in the bioactive AST analogs. The

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subsequent study showed an alteration of the peptide backbone. This approach

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involved a replacement of the C-terminal GL region with succinic acid and

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conservation of the benzene ring.14 The highly substituted analogs show similar

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bioactivity to the lead compound.

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In contrast to previous research, the C-terminal tripeptide (FGLa) was selected as

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the lead compound in the present study. We first quantitated the biological activities

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of a series of alanine-replacement FGLa analogs to determine the residues most

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critical for the inhibition of JH biosynthesis. Based on the structure-activity

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relationship (SAR) study with alanine scanning, nine analogs that mimicked amino

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acids of the tripeptide were designed with the peptidomimetic approach by replacing

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portions of the peptides with unnatural structures. Finally, non-peptide AST analogs

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(2-amino-1-[3-oxo-3-(substituted-anilino)propyl]pyridinium nitrate derivatives) were

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synthesized on the basis of the peptidomimetic approach. Figure 1 shows the design

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strategy. The possible applicability of these non-peptide compounds for pest control

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was demonstrated in this paper.

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■ MATERIALS AND METHODS

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Chemicals and Instruments. All the chemicals and reagents were commercially

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available. Rink Amide-AM resin (0.53 mmol/g substitution), Wang resin (1.5 mmol/g

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substitution), Fmoc-protected amino acids, Benzyloxycarbonylglycine (Z-Gly-OH),

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1-hydroxybenzotriazole anhydrate (HBTU),

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O-benzotriazole-N,N,N',N'-tetramethyluronium hexafluorophosphate (HOBt),

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trifluoroacetic acid (TFA), 4-dimethylaminopyridine (DMAP) and

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N,N'-diisopropylethylamine (DIEA) were purchased from GL Biochem, Ltd.

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(Shanghai, China). Thioanisole, benzaldehyde, dithioglycol, phenol,

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phenylmethanamine, (E)-cinnamic acid, hydrocinnamic acid, butanedioic anhydride,

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furan-2,5-dione, fumaric acid, butyraldehyde, 3-methylbutanal, NaBH4,

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2-aminopyridine, 2-amino-3-methylpyridine, ethyl acrylate, aniline, 2-methylaniline,

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citronellol, HPLC grade n-hexane, N,N-dimethyl-formamide (DMF), dichloromethane

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(CH2Cl2) and acetonitrile were purchased from Sigma-Aldrich (St. Louis, MO).

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Juvenile hormone III was purchased from Toronto Research Chemicals (Toronto, ON,

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Canada). Melting points were determined using a WRS-2A melting point apparatus

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(Shanghai ShenGuang Instrument Co., Ltd., Shanghai, China). Chromatographic

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separations were performed on silica gel flash columns. 1H NMR and 13C NMR

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spectra were recorded on an AVANCE III or on an Avance DMX500 spectrometer

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(Bruker, Fällanden, Switzerland) in DMSO-d using TMS as an internal standard.

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HRMS were recorded on a solariX 70 FT-MS spectrometer (Bruker) using

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methanol/water (1:1, v/v) as solvent. LC-mass spectra were recorded on a

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LCMS-2020 spectrometer from Shimadzu Corporation (Kyoto, Japan). The structures,

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purity, and MS data of all target compounds are shown in Figure 1, Table 1 and 2.

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Synthesis of peptides and peptidomimetics. Five tripeptides (FGLa, AGLa,

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FALa, FGAa and FGL) were synthesized from Rink Amide-AM resin (189 mg, 0.1

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mmol) or Wang resin (67 mg, 0.1 mmol) using the standard Fmoc/tBu chemistry and

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HBTU/HOBt protocol.12 Compounds of series A, B and C were syntheiszed with the

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methods previously reported.13,14 The structures of peptides were confirmed by the

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molecular ions (Table 2). All of the crude peptides and peptidomimetics were purified

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on a C18 column with a flow rate of 10 mL/min using acetonitrile/water (50:50)

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containing 0.06% TFA as an ion-pairing reagent.

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Synthesis of 2-amino-1-[3-oxo-3-(substituted-anilino)propyl]pyridinium

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nitrate derivatives (General Procedure for designed compounds ).

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2-Aminopyridine or 2-amino-3-methylpyridine 1 (10 mmol) and ethyl acrylate 2 (15

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mmol) were mixed in a dry round-bottom flask and five drops nitric acid was added.

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The mixture was heated to 100 oC and stirred for 2 h under a nitrogen atmosphere.

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After reaction completion, ethyl acetate/petroleum ether (2:3, v/v) was added. The

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unreacted ethyl acrylate and solvent were removed under vacuum, and a light yellow

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solid was obtained. Substituted anilines 4 (10 mmol) and ethanol were then added to

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the round-bottom flask. The mixture was heated to 95 oC and stirred for 24 h under a

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nitrogen atmosphere. After reaction completion, the mixture was adjusted to pH 7.0

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with NaOH solution. The solvent was removed under vacuum. The products were

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purified by column chromatography using a mixture of CH2Cl2/MeOH (20:1).19All

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the structure elucidation data are presented in Table 1 and 2.

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Insect. Animals fed Lab Chow and water ad libitum were kept at 27±0.5 °C,

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50±5% RH. Newly emerged (Day 0) mated female D. punctata were isolated, placed

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in containers and provided with food and water.

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Bioassays for JH III in vitro. A pair of corpora allata from day 7 mated

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D. punctata were incubated for 3 h at 30 °C in medium 199 (GIBCO, 100 µL) with

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Hanks’ salts, L-glutamine, 25 mM HEPES buffer (pH 7.2) and 2% Ficoll in the dark

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with gentle shaking. After incubation, 20 ng of citronellol was added to the medium as

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an internal standard. 200 µL n-hexane was mixed with the incubation medium and the

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mixture was centrifuged for 5 min, then the n-hexane phases were transferred to new

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analytical vials. The measurement of JH III was determined using gas

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chromatography tandem mass spectrometry as described previously.20 The retention

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time of JH III was 10.36 min. The quantification transition for JH II was 85.1→59.1

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(collsion energy: 10 eV). The confirmation transitions were 81→79.1 (collsion energy:

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5 eV), 94.9→67.1 (collsion energy: 10 eV), 120.9→93 (collsion energy: 10 eV) and

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120.9→105.1 (collsion energy: 15 eV), respectively.

141 142

Bioassays for JH III in vivo. Females were injected with 5 µL of FGLa, designed analogs (1 µM) using a 10 µL syringe on day 1, and the hemolymph JH III

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concentration determined on day 3. Control insects were similarly injected, but with 5

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µL of double distilled water. For the hemolymph collection, a volume of 50 µL

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hemolymph was immediately transferred to a glass centrifuge tube containing 50 µL

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acetonitrile, 50 µL 0.9% sodium chloride solution and 20 ng of citronellol. The

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sample was extracted twice with 100 µL hexane. The organic phase was transferred to

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a new glass vial. The quantity of JH III was determined as above.

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Assays for impact of feeding on D. punctata mortality. Three groups of D.

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punctata were used for feeding assays. The initial populations of each group were

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made up of 100 newly hatched larvae. These larvae were fed with standard laboratory

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cockroach food that had been treated with designed compounds (S1-5) (> 95% pure,

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the concentration was 0.1 mg/g, 0.01 mg/g, 0.001 mg/g and 0.0001 mg/g,

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respectively). Treated food was made by adding 1 mL of a stock solution containing

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designed compounds dissolved in water, to 1 g of food. The excess solvent was

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evaporated while the food was continuously stirred to ensure the even distribution of

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AI in the bait.21 The experimental units were maintained in a room at 27±0.5 °C, 50±5%

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RH and 12/12 h light/dark photoperiod. Larval mortality were recorded every day

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after treatment.

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■ RESULTS AND DISCUSSION

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Design and bioactivities of analogs in vitro. In this study, a tripeptide, FGLa,

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was synthesized as the lead compound. Treatment with FGLa showed a significant

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effect on JH biosynthesis in vitro (the IC50 value was 2.06 µM). It demonstrates that

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the C-terminal tripeptide could be the lead. The alanine scan is a common method to

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define the crucial residues and has proven useful in characterizing the SARs of several

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insect neuropeptide including allatostatin.5,22 We determined the amino acids of the

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lead compound FGLa that are most critical for bioactivity by replacing alanine for

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each residue. Their potencies in the in vitro assay are shown in Table2. AGLa ([Ala1]-

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FGLa) and FGAa ([Ala3]-FGLa) were completely inactive, even at a concentration of

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10 µM. Assay of FALa ([Ala2]-FGLa) showed that this analog had an effect on JH

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biosynthesis (IC50: 37.58 µM), but was 18-fold less potent compared with FGLa. We

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also synthesized FGL, which shares the same sequence with FGLa but without the

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C-terminal amide to validate the function of the C-terminal amide. Analog FGL did

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not show any bioactivity on JH biosynthesis. It suggested that the side chain of Phe1,

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Leu3 and the C-terminal amide of lead compound are crucial for JH biosynthesis.

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We built the three-dimensional structure of AST receptor23 of D. punctata by

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homology modeling using the crystal structure of the nociceptin/orphanin FQ receptor

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as the template and identified the ligand-binding pocket using blind docking

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calculations. Figure 2A shows the C-terminal amide of FGLa forms hydrogen bond

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with His336 of receptor. The benzene ring of the residue Phe of FGLa has a π-π

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interaction with the benzene ring of Tyr253 (Figure 2A). The hydrophobicity of the

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binding pocket was shown in Figure 2B. It indicated the Phe and Leu of FGLa had

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strong hydrophobic interactions with the receptor. The docking result was consistent

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with the results of our biological assays. It also can explain why the biological activity

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is lost during the replacement.

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Based on the Ala-replacement and docking results, analogs of series A, B and C

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that mimicked residues of the lead compound were designed by replacing portions of

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the peptides with unnatural amino acids.

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In series A, aromatic acids were used as mimics of the residue Phe. In

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comparison to the lead, these analogs showed similar inhibition of JH biosynthesis

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(Table 2). In the structures of B1, 3-(benzylcarbamoyl)propanoic acid was used to

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mimic the FG sequence. Compared with the lead compound tripeptide, B1 improved

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the effect on JH inhibition (IC50 value: 0.48 µM). Structural studies on ASTs showed

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flexible conformations in aqueous solution.24,25 There is considerable debate on the

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actual active conformation: turn or linear. Two trans-cis isomers (a

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restricted-linear-conformation mimic B2 and a restricted-turn-conformation mimic B3)

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were synthesized with the purpose of probing the potential active conformations

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(Figure 1). The trans-isomer of tripeptide mimic (B2) can inhibit JH biosynthesis in

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vitro, whereas the cis-isomer one (B3) has no effect on inhibition (Table 2). The

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bioassay results of the two cis-trans isomers support the linear conformation model.

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The docking results (Figure 2) also showed that the FGLa was docked into the AST

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receptor model in the linear conformation. The effect of B2 is 9-fold less than that of

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FGLa, as the restricted plane in B2 may affect the position of the important

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pharmacophore (benzene ring and L-leucine) and debase its bioactivity. Appropriate

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conformation flexibility should be considered in the further analog design. In series C,

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substitution of Leu with benzylamine, n-butylamine and isopentylamine demonstrated

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little inhibition of JH biosynthesis by C1 (19-fold) relative to FGLa, and C2 and C3

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showed no effect on JH biosynthesis (Table 2). The C terminus (CONH2) was ignored

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in series C. The bioactivity of series C compounds suggesting that the hydrogen bond

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of the C terminus with the receptor is important to AST activity. Thus, in the potent

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FGLa analog, the benzene ring of Phe region should be conserved, the side chain of

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Leu region can be replaced by aromatic groups, and the C-terminal NH2 cannot

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readily be reduced.

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According to the above results, the five analogs

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(2-amino-1-[3-oxo-3-(substituted-anilino)propyl]pyridinium nitrate derivatives) were

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designed (Figure 1). In designed compounds, the points of synthetic modification

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included (1) conservation of the benzene ring of Phe and the C-terminal amide, (2)

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replacement of Leu with pyridinium, (3) connection of all the three effective groups

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with a flexible linker propanamide segment. On the basis of the IC50 values, all the

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designed compounds are more potent than the lead compound FGLa (Table 2).

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Particularly, the compound S1 had the same IC50 value as some natural ASTs, such as

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Dippu-AST 3 and Dippu-AST 13 (IC50:0.018 µM and 0.020 µM , respectively).26

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Figure 3 shows the compound S1 has the similar interactions as FGLa with the AST

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receptor. In addition, the pyridinium ring of designed analogs , a π-π interaction with

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Trp307, which can explain the higher bioactivity of these compounds (Figure 3). The

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water solubility of these analogs is better than the lead compound FGLa (a peptide of

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3 hydrophobic amino acids), because they are the pyridinium nitrate derivatives.

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Effect of designed analogs on JH biosynthesis in vivo. Following injection of the

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designed analogs into newly emerged female D. punctata, JH biosynthesis was

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assayed after 3 d with significant inhibitory effects apparent (Figure 4), whereas the

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lead peptide FGLa was inactive in vivo. Assuming a hemolymph volume of 50 µL for

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day 1 adult female D. punctata,27 the final concentrations of the injected analog in the

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hemolymph were approximately 100 nM. The inhibition of compounds S1, S2, S3, S4

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and S5 was 81.1 ± 1.8%, 76.8 ± 4.4%, 71.0 ± 6.0%, 71.3 ± 4.1% and 43.9 ± 6.6%,

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respectively.

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Pest control application. Insect neuropeptides offer potential candidates for the

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development of novel eco-friendly insecticides. However, they have a number of

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characteristics that make them rather unsuitable for pest control, such as unstable in

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the environment, poor solubility and rapid degradation in the digestive system of

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insects. ASTs only have effects (reducing the growth of oocytes or JH in the

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haemolymph in certain cockroaches) by injection at very high and sometimes in

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repeated doses.28,29 We have synthesized an AST mimic, H17, which shows a highly

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significant inhibition of JH production in topical cuticular assays in vivo.11 There is no

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insecticidal AST mimic mainly absorbed in stomach up to now. Oral toxicity is

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crucial for the pest control application. Hence an oral toxicity test with designed

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compounds was performed in this study. The results of the experiments in which the

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larval mortality of D. punctata were exposed to designed compounds treatments are

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shown in Figure 5. S1 and S3 have significant larvicidal effects following oral

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administration with high mortality (81.0 ± 5.6% and 100%, respectively) at the

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concentration of 0.1 mg/g, whereas compounds S2, S4 and S5 are inactive in this

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experiment. The larval mortality of JH mimic fenoxycarb and anti-JH compound

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pitavastatin30 at the same concentration were 79.3 ± 3.7% and 32.7 ± 2.0%,

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respectively. The dead larvae appeared seven days after the treatment with molting

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disturbances. In the oral toxicity assay, the IC50 values of S1 and S3 were 0.020 mg/g

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and 0.0016 mg/g, respectively. Structural modifications to the phenyl group indicate

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that the electron-donating groups maintained the bioactivity. However, the

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electron-withdrawing group completely eliminated the insecticidal activity. The

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bioassay results for the compound S3 indicated that the introduction of a meta-

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electron-donating group in the phenyl group enhanced the oral toxicity against the

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cockroach.

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In

conclusion,

novel

non-peptide

AST

analogs

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(2-amino-1-[3-oxo-3-(substituted-anilino)propyl]pyridinium nitrate derivatives) were

263

synthesized with the C-terminal tripeptide of the FGLamide ASTs as the lead

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compound. All the designed compounds have significant effects on JH biosynthesis

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both in vitro and in vivo. Good oral toxicities of compounds S1 and S3 suggest that

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they have considerable potential as insect growth regulators for pest control.

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Furthermore, good water solubility of these designed compounds makes them easier

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to formulate as insecticides.

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AUTHOR INFORMATION

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Corresponding Authors

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* Tel.: +86-13671951027. Fax: +86-21-60877220. E-mail: [email protected] (Z.P.K).

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* Tel.: +86-2160877231. Fax: +86-2160877231. Email: [email protected] (Y.J.R).

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Notes

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The authors declare no competing financial interest.

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Funding Sources

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This work was supported by grants from the National Key Research and Development

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Plan of China (No. 2017YFD0200504) and Shanghai Municipal Science and

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Technology Commission (17142201300).

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Supporting Information

281

Synthesis of peptides, series A, B and C peptidomimetics. Homology modeling of

282

Dippu-AstR, docking calculations and molecular dynamics simulations of FGLa and

283

analogs.

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(10) Garside, C. S.; Nachman, R. J.; Tobe, S. S. Injection of Dip-allatostatin or

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Figure 1. Design of FGLa analogs.

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Figure 2. Binding sites of FGLa in D. punctata AST receptor. (A) The hydrogen bond

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and π-π interactions of FGLa with receptor. (B) The hydrophobicity of the binding

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pocket. Hydrogen bond is shown with dotted yellow line.

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Figure 3. The same hydrogen bond and π-π interactions of FGLa and the compound

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S1 with AST receptor. FGLa is shown with purple stick and the compound S1 with

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orange stick. Hydrogen bond is shown with dotted yellow line.

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Figure 4. JH biosynthesis following injection of FGLa and compounds S1-5. Females

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were injected with 5 µL of compound (1 µM) using a 10 µL syringe on day 1, and

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hemolymph JH III titer determined on day 3. Values represent mean ± s.e.m., ****p