Study on Amphipathic Modification and QSAR of Volatile Turpentine

Mar 29, 2016 - College of Plant Protection, Northwest Agriculture and Forestry University, Yangling, Shaanxi 712100, People's Republic of China ... In...
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Research Article pubs.acs.org/journal/ascecg

Study on Amphipathic Modification and QSAR of Volatile Turpentine Analogues as Value-Added Botanical Fungicides against CropThreatening Pathogenic Fungi Yanqing Gao,† Xiangrong Tian,† Jian Li,*,‡ Shibin Shang,§ Zhanqiang Song,§ and Minggui Shen§ †

College of Plant Protection, Northwest Agriculture and Forestry University, Yangling, Shaanxi 712100, People’s Republic of China College of Forestry, Northwest Agriculture and Forestry University, Yangling, Shaanxi 712100, People’s Republic of China § Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry, Nanjing, Jiangsu 210042, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: In view of potential agro activity of turpentine analogues, many studies have been conducted on the application of this biorenewable and abundant natural resource. Three series analogues of derivatives from volatile turpentine were prepared to study suitable amphipathic properties. The evaluation of fungicidal activity was carried out against three pathogenic microbials. In addition to the overall good effect, it was found that compounds 2hydroxyethyl carboxylate, 5k, and 2-(2-hydroxyethoxy)ethyl carboxylate, 5l, demonstrated extreme activity, with IC50 values of 6.013 and 6.610 μg/mL against Setosphaeria turcica, which were close to the control carbendazim. The preliminary structure−activity relationship (SAR) was analyzed, and compounds with appropriate amphipathic features and small steric hindrance displayed more desirable performances. Meanwhile, the quantitative structure−activity relationship (QSAR) model (R2 = 0.9548, F = 47.82, S2 = 0.0125) was built and indicated that the most two important structural features were the total hybridization composite of the molecular dipole and molecular volumn. Moreover, the study of SAR and QSAR indicated that the structural modification of the amphipathic group, which can regulate the permeabilization property of the molecules, was beneficial to the fungicidal activity. On the basis of this, a potential alternative approach may have been discovered to ensure food safety. KEYWORDS: Turpentine, β-Pinene, Fungicidal activity, SAR, QSAR

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INTRODUCTION With the development of organic agriculture, some cropthreatening pathogenic fungi cause large decreases in crop yields.1 Setosphaeria turcica, for instance, a serious and common pathogen, is the key causal agent of leaf blight in corn, which results in great deal of distress on corn and causes low output. In the process of crop protection against plant diseases, the abuse of chemical fungicide leads to considerable concern for food safety due to the potential negative effect of the chemical residue on living creatures.2−4 Thus, there are urgent needs for studies on efficient and ecofriendly fungicides. As an alternative means to classic agrochemicals, the utilization of more ecofriendly and biorenewable natural resources, such as biological metabolites, has become an attractive approach for integrated and ecological disease management.5,6 Some volatile secondary metabolites produced during the long-standing plants−circumstance interaction have the ability to protect plants from pest and disease attacks by induction of systemic acquired resistance, including phytoalexins.7 From secondary metabolites, some lead compounds, such as terpenoids, alkaloids, and avonoids, have been developed as © XXXX American Chemical Society

potential fungicides that are highly efficient and have less resistance and lower pollution.8 Resin canal secretions include a diverse array of resin acids and more complex volatile oils. In the periodic process of resin tapping, the resin can outflow repeatedly to resist external damage. As an abundant ingredient, turpentine has excellent performance and potential for use in agricultural fields.9,10 For instance, turpentine and its analogues have been explored as antifeedants,11 repellents,12,13 and fungicides to maintain food security in modern agriculture. Among these analogues, it is worth noting that in the pine tree body, β-pinene can transform spontaneously into cumic acid, which has good antimicrobial activity.14 There are some similar and alternative structural forms of cumic acid, among them dehydrocumic acid can be chemically prepared from volatile turpentine in high yield.15 The development of an ecofriendly fungicide from dehydrocuReceived: February 2, 2016 Revised: March 17, 2016

A

DOI: 10.1021/acssuschemeng.6b00236 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Scheme 1. Preparation of Title Compounds

above, QSAR, an efficient means for biological activity screening and mechanism of action, was also carried out through some quantification software packages. After justification of the essential structural features for the activity, the mechanism of action can be elaborated advantageously. The objective of this study was to expand the application of volatile turpentine in a high value-added field to increase food safety.

mic acid that can be applied in the control of crop pathogens has proved promising. However, during the design and modification of potential fungicides derived from dehydrocumic acid in integrated disease management, there are many constraints that need to be considered.16 On the one hand, as the dehydrocumic acid contains a free carboxyl group, one of the important constraints is the amphipathic characteristic, which is an essential factor due to the mechanism of contact and permeabilization between the fungicide and microbial cell membrane.17,18 In order to adjust the hydrophilic/lipophilic properties and increase the solubility both in organic and aqueous media, a series of ester and amide derivatives from volatile turpentine were prepared to obtain novel and stable natural product-based fungicides. In addition to monoesters, diesters and bisamides were prepared through the D−A addition reaction of β-pinene and maleic anhydride (an agent proved to have antimicrobial activity).19 The fungicidal activities of the title turpentine analogues against dangerous crop-threatening pathogens Setosphaeria turcica (S. turcica), Rhizoctonia solani (R. solani), and Fusarium graminearum (F. graminearum) were investigated. By analysis of the structures of these analogues and their activity, the preliminary relationship of the structure and activity can be proposed. On the other hand, as the activity screening of large candidates is a time-consuming and costly task, new technology for crop diseases control is needed currently.20 Given the facts described

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MATERIALS AND METHODS

Synthetic Procedures and Identification. β-Pinene (1) was obtained from a commercial source. All the other chemicals used in the synthesis were of reagent grade. A Nicolet IS10 spectrophotometer was used for the Fourier transform infrared (FT-IR) spectra. A Bruker AV-300 nuclear magnetic resonance spectrometer was used for the 1H NMR spectra with CDCl3 or DMSO-d6 as the solvent and TMS as an internal standard. An Agilent-5973 spectrophotometer was utilized for the MS spectra. A Bruker Q-TOF mass spectrometer equipped with an electrospray ionization source was applied for ESI mass spectral data. Thin-layer chromatography (TLC) was employed to trace all reactions, which was carried out using Merck silica gel 60 GF254 plates with eluent of petroleum ether/ethyl acetate (v/v = 8:1) and visualized under 254 nm UV light. The synthesis routine of the title compounds is shown in Scheme 1. The pathogenic fungi were provided by the Research and Development Center of Biorational Pesticide, Northwest A & F University, China. Synthesis of 4-Isopropylcyclohexa-1,3-dienecarboxamides (5a−n). From β-pinene, through alkaline oxidation, dehydration, isomerism, and acylation, 2-hydroxy-6,6-dimethylbicyclo[3.1.1]B

DOI: 10.1021/acssuschemeng.6b00236 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Table 1. Fungicidal Activity of Compounds against Setosphaeria turcica RIR (%) concentrations (μg/mL)

a

compound

256

128

64

32

16

3 5a 5b 5c 5d 5e 5f 5g 5h 5i 5j 5k 5l 5m 5n 7a 7b 7c 7d 8a 8b 8c 8d 8e carbendazim

85 100 100 100 100 100 100 100 100 100 90 100 100 85 85 100 100 100 100 100 100 100 100 100 100

52 70 75 88 80 91 90 90 90 77 69 97 97 55 56 93 95 96 95 97 97 98 97 98 99

25 47 53 66 59 75 70 67 73 51 44 85 87 26 28 79 80 84 82 83 87 86 87 87 85

17 33 36 44 42 53 46 41 50 34 29 70 71 15 17 55 56 61 59 60 67 67 70 70 73

8 24 21 30 25 33 32 30 32 28 22 56 55 7 8 35 38 40 39 43 43 50 45 50 60

IC50a 135.321 72.170 65.449 43.597 54.201 32.827 39.156 43.947 41.513 62.855 77.550 6.013 6.610 133.182 129.752 29.246 26.933 22.758 24.372 21.973 18.331 14.347 15.566 12.945 3.556

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

3.22a 1.21b 1.02b 0.65bc 0.51b 0.42bc 0.63bc 0.60bc 0.58bc 0.59b 0.68b 0.05d 0.05d 4.59a 3.24a 0.33c 0.29c 0.19c 0.20c 0.21c 0.13c 0.12c 0.16c 0.21c 0.01d

y = a + bx y y y y y y y y y y y y y y y y y y y y y y y y y

= = = = = = = = = = = = = = = = = = = = = = = = =

−2.193 + 0.016x −1.473 + 0.020x −1.462 + 0.022x −1.149 + 0.026x −1.232 + 0.023x −0.931 + 0.028x −1.085 + 0.028x −1.257 + 0.029x −1.154 + 0.028x −1.362 + 0.022x −1.644+ 0.021x −0.179 + 0.030x −0.208+ 0.031x −2.241+ 0.017x −2.110+ 0.016x −0.888+ 0.033x −0.888+ 0.033x −0.807+ 0.035x −0.807+ 0.033x −0.783+ 0.036x −0.697 + 0.038x −0.518+ 0.036x −0.572+ 0.037x −0.477+ 0.037x −0.115+ 0.032x

R2

log IC50

0.969 0.988 0.953 0.986 0.954 0.960 0.985 0.992 0.992 0.999 0.995 0.998 0.989 0.955 0.954 0.960 0.983 0.973 0.972 0.994 0.968 0.998 0.966 0.993 0.984

2.131 1.858 1.816 1.639 1.734 1.516 1.593 1.643 1.618 1.798 1.890 0.780 0.800 2.124 2.113 1.466 1.430 1.357 1.387 1.342 1.263 1.157 1.192 1.112 0.551

Values in columns followed by similar letters were not significantly different according to Fisher’s protected LSD test (P = 0.05).

heptane-2-carboxylic acid (2), dehydrocumic acid (3), and chloride (4) were prepared.21 Here, 100 mL of alcohol was added dropwise to the above chloride (4) and reacted at room temperature for 12 h. After removing the excess alcohol by vacuum distillation, the reactant was washed with deionized water and purified by vacuum distillation to give the 12 resulting compounds 5a−l. In addition, the above chloride (4) was added dropwise to the two alcohols (glycol and ethylene diglycol, 0.015 mol). After the same procedure discussed above, the other two esters, 5m and 5n, were prepared. Synthesis of Dialkyl 1-Isopropyl-4-methylbicyclo[2.2.2]oct5-ene-2,3-diamide (7a−d). Appropriate amounts of toluene and TsOH (0.2 g, 0.001 mol) were added to a solution of maleic anhydride (20.0 g, 0.200 mol) and alcohol (27.6 g, 0.600 mol). After refluxing for 4 h and removing the excess alcohol, the maleates were obtained through washing with alkaline and deionized water. A solution of βpinene (50.0 g, 0.370 mol), maleic anhydride (20.0 g, 0.200 mol), and metaphosphate (5.0 g, 0.063 mol) was catalyzed with small amounts of iodine. The reactant was heated to 140 °C for 6 h. The above resulting compounds were dissolved in dichloromethane, and excess amines were added at low temperature. The reactant was stirred at ambient temperature overnight,and washed with acid, alkaline, and water. The four resulting diamide derivatives 7a−d from turpentine were obtained. Synthesis of Dialkyl 1-Isopropyl-4-methylbicyclo[2.2.2]oct5-ene-2,3-dicarboxylate (8a−e). Appropriate amounts of toluene and TsOH (0.2 g) were added to a solution of maleic anhydride (20.0 g, 0.200 mol) and alcohol (27.6 g, 0.600 mol). After refluxing for 4 h and removing the excess alcohol, the maleates were obtained through washing with alkaline and deionized water. A solution of β-pinene (27.0 g, 0.200 mol) and metphosphate (2.7 g, 0.034 mol) was added to the above maleates (0.1 mol), and the reaction was catalyzed with trace iodine. The five resulting compounds 8a−e were obtained after purification by silica gel chromatography [ethyl acetate/petroleum ether (v/v = 1:10)]. Fungicidal Activity. The fungicidal effect was determined with reference to a drug-containing plate assay established by Quiroga.16

The test samples were dissovled in DMSO to prepare various concentrations: 2560, 1280, 640, 320, and 160 μg/mL. Two milliliter sample solutions were mixed well with 18 mL potato−dextrose agar (PDA) at 50 °C to give the needed series of concentrations (256, 128, 64, 32, and 16 μg/mL). Here, 15 mL of the above medium was poured into Petri dishes (with diameters of 90 mm), and drug-containing medium was prepared after condensation. Six millimeter diameter fungus-coated discs prepared with a hole puncher were inoculated upside down on the surface of the cooled agar. For the purpose of comparison, the commercial agricultural fungicide carbendazim was tested under the same test condition. The negative control was tested on Petri dishes with 10 mL of DMSO. All of the plates were incubated under routine conditions, and after 3 days, the mycelium radius was measured. The relative inhibition rates (RIR) of test chemicals were calculated by eq 1

RIR/(%) = (Tzone − Czone)/Tzone × 100

(1)

where Tzone and Czone were the inhibition zone and the negative control, respectively, which were calculated by the average expanded diameters of the measured colonies plus 6 (diameter of colonies). The standard deviations of the tested biological values were less than 5%. The IC50 values were computed by probit analysis with the SPSS statistics program, version 17.0,22 through which the regression equation y = a + bx of the inhibitory activity could also be obtained. Building and Validation of the QSAR Model. According to the common procedure, the QSAR model was built and validated. First, the most stable configurations of title compounds were optimized at the DFT/6-31G (d) level by a Gaussian 03W package of programs.23 In the geometry optimization and the lowest energy calculation of the entire compounds at the DFT using the 6-31G(d) level of a Gaussian 03W package of programs, the most stable configurations of the title compounds were generated. Meanwhile the corresponding “.log” and “.chk” files were obtained by a Gaussian calculation. We can check the calculated chemical parameters [highest occupied molecular orbital C

DOI: 10.1021/acssuschemeng.6b00236 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering (HOMO) and lowest unoccupied molecular orbital (LUMO), molecular reactivity, electron transport, etc.] that we need from the generated “.log” and “.chk” files. For example, with HOMO and LUMO, we can open the “.chk” file and edit MOs, visualize HOMO, LUMO, and HOMO−LUMO, click update, and HOMO, LUMO, and HOMO−LUMO values could be obtained. Second, to obtain a compatible structure form with CODESSA 2.7.15, the calculated results were changed using Ampac 9.1.3. Finally, using CODESSA 2.7.15, all the molecular descriptors were calculated. In order to determine the structural features of theessential significance for fungicidal activity against S. turcica, the heuristic analysis was closed for building the QSAR model. During the process of the model building, the satisfying R2 (squared correction coefficient), S2 (squared standard error of the estimates), and F (Fisher significance ratio) were used to illustrate the standards of statics. In the QSAR study, to get better linear regression, the tested IC50 values were transformed into the corresponding logarithms IC50 and used as dependent variables. To ensure a high quality of the final QSAR results, two verification methods, internal validation and the “leave-one-out” cross-validation, were used for the determination.

and 7d > 7c > 7b > 7a. In summary, for all the analogs, the best number of carbon atoms for the R2 substituent was four, and the induced electron withdrawing group with a low volume for the R2 substituent was beneficial for fungicidal activity. There were several postulations to elucidate about antimicrobial activity mechanisms.24 The most common features of fungicides were positively charged and amphipathic three-dimensional structures with hydrophilic and hydrophobic faces, which have been reported by many studies regarding membrane permeabilization.25,26 As the first step of fungicidal action is the fungicide adsorption at the membrane surface, the interaction between compounds with appropriate amphipathic features and the lipid bilayer is essential.27−29 On the basis of this membrane permeabilization mechanism, the analogues with the hydrophobic chain would penetrate fungi cell membranes and possess better fungicidal activity. The introduction of ester and amide groups into compound 3 increased their membrane permeabilization properties, which strengthened the contact and interaction between chemicals and fungi cell membranes and led to better fungicidal activity. So, the level of fungicidal activity was diesters (8a−e) > bisamides (7a−d) > monoesters (5a−n), which had good compliance with the mechanism described above. The identical series of monoesters derivatives from β-pinene, compounds 5e−h, with relatively suitable hydrophobic chains, increased the solubility in organic media and had better activity than other compounds. It needs explained that compounds 5k and 5l displayed outstanding fungicidal activity, which could be due to electrostatic interaction caused by the free hydroxyl.12 The above activity consequence was also conformed to the mechanism. Contrarily, compounds 5i, 5j, 5m, and 5n, with adverse steric effect, possessed unfavorable actions. In short, the fungicidal activity was affected by the permeabilization property, electrostatic effect, and steric effect. The activity consequences of turpentine analogues against R. solani and F. graminearum were tested, from which it was shown that the fungicidal activity of all test compounds was poor, and the death rate of most analogues was about 20%−40% at concentration of 256 μg/mL. QSAR Study on Fungicidal Activity against S. turcica. The screening of several significant molecular descriptors from a large number of descriptors was an essential procedure in QSAR study. All the descriptors can be categorized into six groups, and the most important factors in this work were quantum-chemical and geometrical descriptors. Some descriptors such as molecular volume, molecular surface area, dipole moment, total dipole of the molecule, etc. were involved. There are several regression approaches available to establish the relationship between activity and molecular descriptors. After many attempts, the heuristic regression was chosen to obtain a QSAR model with satisfactory values of R2, F, and S2. The other important step was to determine the number of descriptors, which can be accomplished by the “breaking point” rule. In the best multilinear regression, the R2 value trend was significantly different before the number 4 and after. From the t values in the t-test, the significance of the descriptors can be determined. Additionally, the number of molecular descriptors met the rule given by eq 2

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RESULTS AND DISCUSSION Synthesis of Turpentine Analogues. In the synthesis of derivatives from turpentine−maleic anhydride, there were two steps of D−A addition reaction and esterification/amidation. Taking into account the subsequent separation and purification, we have tried both orders of the two reactions. It was shown that the suitable process for ester compounds 8a−e was first esterification and then D−A addition reaction. However, the appropriate reaction scheme was converse for compounds 7a− d. The phenomenon was possibly due to steric hindrance of alcohols. In addition, there were many acid catalysts in esterification, such as toluenesulfonyl chloride, sulfuric acid, and p-toluenesulfonic. For a high yield of the title compounds, p-toluenesulfonic was selected as the acid catalyst of esterification. Fungicidal Activity and Structure−Activity Relationships (SAR). Table 1 shows the fungicidal activity of compounds 5a−n, 7a−d, and 8a−e against S. turcica. Compared with compound 3, the turpentine analogues displayed more significant fungicidal activity, which indicated that the substituted ester and amide were positive on the activity. In general, the level of fungicidal activity was diesters (8a−e) > bisamides (7a−d) > monoesters (5a−n). Remarkably, 5k and 5l displayed IC50 values of 6.013 and 6.610 μg/mL, respectively, which were similar to carbendazim, a benzimidazole fungicide with similar structures. These observations revealed that substitutions patterns on dehydrocumic acid moiety had an important influence. Although it seemed difficult to construct an obvious structure−activity relationship from the data shown in Table 1, some preliminary conclusions can be obtained. The monoester compounds 5a−n, with the R1-nitrosubstituent, showed a complicated tendency with fungicidal activity. For compounds 5a−e, the activity of the compounds was gradually increasing, but for compounds 5f−n, the activity of the compounds gradually decreased in general. It was indicated that the steric effect played a role with four carbon atoms for the R2 substituent. Among them, compounds 5k and 5l showed extremely good activity, which involved an electrostatic effect, an essential factor in antimicrobial activity mechanisms. With regard to the diester compounds 8a−e and diamide compounds 7a−d, the activity of the R1-oxgen substituent was higher than that of the R1-nirtogen substituent. With an increase in the carbon atoms of the R2 substituent, the activity of the compounds was higher: 8e > 8d > 8c > 8b > 8a

k ≤ (n/3) − 1

(2)

which means that the molecular descriptors k is no more than one-third of the sample number plus 1 [(n/3) − 1]. Finally, the four-descriptor QSAR model was determined as the best D

DOI: 10.1021/acssuschemeng.6b00236 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

descriptors. The results of the internal validation are listed in the Supporting Information. The difference between RTraining2 and RTest2 for the three sets was small enough to be ignored, and the average values of RTraining2 and RTest2 were essentially identical to the overall R2 value, which was the approving predictive power of the obtained model. In the “leave-one-out” method, every fourth compoundm 1,5,9, etc., was an external test set, and the others were the training set. The R2 value of the training set and test set were close, and the QSAR model obtained in this study was suitable. With regard to the molecular descriptors, the first important descriptor was the total hybridization composite of the molecular dipole (μh), which measured the hydrophilic/ lipophilic properties of the compounds.30,31 An appropriate μh value illustrated that the molecules can penetrate the fungi cell membrane or wall more smoothly as well as interact with action target. The structural modification of dehydrocumic acid through esterification and amidation can regulate the hydrophilic and lipophilic properties of the compounds. Among the turpentine analogues, diester demonstrated better fungicidal activity, which illustrated that the μh value was an essential factor in regulating the activity. The second important descriptor was molecule volume, which belonged to geometrical descriptors and reflected the space volume and three-dimensional size of the molecule. As shown in eq 3, the molecule volume had an adverse effect on the log IC50 values. Molecules with large volumes, such as compounds 5m and 5n, had relatively poor fungicidal activity, which can be attributed to steric hindrance and inferior permeabilization. The molecular electrostatic potential (MEP) of compounds 5m and 5n was shown in Figure 2, and the resulting surface simultaneously displayed molecular size and shape and the electrostatic potential value.32

model. In the Supporting Information, the four significant descriptors and their values are listed. The quantitative model had the following statistical characteristic: R2 = 0.9548, F = 47.82, and S2 = 0.0125. The four descriptors are shown in Table 2, which ranged in Table 2. Best Four-Descriptor Model descriptor no.

X

±ΔX

t-test

descriptor

0 1 2 3 4

3.3337 × 10 2.0986 × 10 −2.4454 × 10 5.5282 1.4532 × 10−2

2.8155 2.0552 1.8653 6.9131 × 10−1 2.4082 × 10−3

11.8407 10.2113 −13.1101 7.9966 6.0343

intercept μha MVb NOELc DMd

a Total hybridization composite of the molecular dipole. bMolecule volume. cNumber of occupied electronic levels. dDipole moment.

descending order according to their statistical significance. The experimental and predicted log IC50 values are listed in the Supporting Information, and the comparison between the experimental and predicted values is shown in Figure 1. The

Figure 1. Experimental log IC50 versus predicted log IC50.

equation of the four-descriptor QSAR model is described in the eq 3. In eq 3, a positive sign appears in the model and indicates that the descriptor value has a positive correlation with log IC50. Contrarily, a negative sign indicates a negative correlation.

Figure 2. MEP plot of compounds 5m and 5n.

log IC50 = 33.337 + 20.986 × μh − 24.454 × MV + 5.5282 × NOEL + 0.014532 × DM N = 24, R2 = 0.9548, F = 47.82, S2 = 0.0125

The third and fourth important descriptors obtained in the model were the number of occupied electronic levels and the dipole moment. These two descriptors both belonged to quantum-chemical descriptors and represented or depended directly on the quantum-chemically calculated charge distribution in the molecules and therefore described the polar interactions between molecules or their chemical reactivity. The frontier molecular orbitals (FMOs) of compounds 5k and 5l were shown as the HOMO and LUMO in Figure 3, and these popular quantum chemical parameters could determine the molecular reactivity and predict the excitation properties and the ability of electron transport.33,34 In Figure 3, the MEP was

(3)

The validation of the established QSAR model was carried out by two validation methods. The internal validation method, described as compounds 1,4,7, etc., was assigned to group A; compounds 2,5,8, etc. to group B; and other compounds to group C. Two of these three groups were selected randomly as the training set, and the other group was the test set. The values of the corresponding test sets were predicted by the obtained correlation equation from the training set using the identical E

DOI: 10.1021/acssuschemeng.6b00236 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

ACS Sustainable Chemistry & Engineering

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demonstrated simultaneously and indicated the occupied electronic levels of compounds 5k and 5l. Both the SAR and QSAR studies jointly revealed that the permeabilization properties of the turpentine analogues had an important effect on fungicidal activity. The compound with an adequate hydrophilic/lipophilic property and small molecular volume was beneficial to the activity. On the basis of these satisfying results, the expanded application of volatile turpentine for exploitation of ecofriendly fungicides can be achieved.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00236. IR, 1H NMR, MS, and elemental analysis data for the target compounds. The asymmetric unit and crystal packing of compound 2. The “breaking point” rule results. The substituted groups of β-pinene derivatives. Fungicidal activity of compounds against R. solani and F. graminearum. Fungicidal activity and structure descriptors of the title compounds. The difference between the experimental log IC50 and predicted log IC50. Internal validation of the QSAR model. (PDF)

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REFERENCES

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Figure 3. Ground state isodensity surface MEP plots of compounds 5k and 5l. The green parts represent positive molecular orbitals, and the red parts represent negative molecular orbitals.

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-29-87091977. Fax: +86-29-87082392. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Nature Science Foundation of China (Project Nos. 31401783, 31400509, and 3150048), Chinese Universities Scientific Fund (Project No. Z109021513), and Open Foundation of Key Laboratory of Biomass Energy and Materials of Jiangsu Province (Project No. JSBEM201608) provided financial support for this research. F

DOI: 10.1021/acssuschemeng.6b00236 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acssuschemeng.6b00236 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX