Heterogeneous electron transfer of pesticides Current

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electrochemical methods for analysis of these compounds in the environmental samples and .... Further one-electron reduction yields a neutral form. 0. MV. MV e.
Transworld Research Network 37/661 (2), Fort P.O., Trivandrum-695 023, Kerala, India

New Trends in Analytical, Environmental and Cultural Heritage Chemistry, 2008: ISBN: 978-81-7895-343-4 Editors: Maria Perla Colombini and Lorenzo Tassi

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Heterogeneous electron transfer of pesticides Current trends and applications Romana Sokolová#, Magdaléna Hromadová* and Lubomír Pospíšil J. Heyrovský Institute of Physical Chemistry of AS CR, v.v.i., Dolejškova 3 CZ-18223 Prague, Czech Republic. #E-mail: [email protected] * E-mail: [email protected]

Abstract This review summarizes different aspects of biological activity of pesticides. The redox active groups involved in the electron transfer mechanism of action often participate in a concerted way. Therefore a simple classification is not easy. Surface activity of many compounds contributes significantly to the efficiency of the herbicidic activity. However, adsorption and also the inclusion phenomena may cause undesirable contamination problems. More sofisticated characterization of interfacial properties on basis of monolayer structures and their phase-transitions may avoid contamination problems in the future. Correspondence/Reprint request: Dr. Lubomír Pospíšil, J. Heyrovský Institute of Physical Chemistry of AS CR, v.v.i., Dolejškova 3, CZ-18223 Prague, Czech Republic. E-mail: [email protected]

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Electroanalytical methods are currently used in numerous procedures for the pollution controll. Despite of their specificities they offer a simple and economic solution to analytical laboratories. High selectivity of electrochemical methods increases the popularity of these techniques. An important field of research is the development of new sensors and immunosensors. This overview omitted the applications of redox processes for decontamination. Almost unexplored field is the development of herbicidic formulas for a controlled release of active substances.

Introduction Electron transfer reactions are key reactions in the biological activity of pesticides. They cause a selective interruption of the photosynthesis leading to agricultural elimination of weeds. Several review articles have summarized in the past the current state of research and knowledge of the electrochemical behavior of pesticides [1-5]. This review will highlight the pesticides with different redox–active groups, which determine the applicability of the electrochemical methods for analysis of these compounds in the environmental samples and the subsequent clean–up procedures. Adsorption properties and other non–covalent interactions of pesticides are being discussed in the light of their non–negligible role in the environmental pollution. The literature cited here covers publications till spring 2007.

1. Electron transfer reactions in bio-activity of pesticides Electron transfer plays a crucial role in the mode of action of many pesticides. Understanding of pesticidic redox properties helps the clarification of degradation processes and metabolic pathways encountered in Nature. Many of the degradation products of pesticides, which originate from the redox processes of parent molecules were found in ground water and in soil. This chapter gives a brief overview of the modes of action of herbicides, fungicides and insecticides in the targeted biological systems with respect to the electron transfer reactions.

1.1. Herbicides The classification of herbicides can differ according to their target sites, chemical classes, modes of action which include cell metabolism or growth, inhibition of light processes and cell division. Specific target sites involve many processes like the inhibition of cell wall synthesis (nitriles) and lipid synthesis (thiocarbamates), the inhibition of auxin transport (chlorophenoxy herbicides), photosynthetic inhibition (triazines, phenylureas), inhibition of cell

Heterogeneous electron transfer of pesticides

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division (chloroacetamides), changes in microtubule assembly (dinitroanilines, diphenylethers) and inhibition of mitosis/microtubule polymerization (pyridazinones). The other target sites include the inhibition of acetyl CoA carboxylase (aryloxyphenoxypropionates, cyclohexanediones), acetolactate synthase (chlorsulfuron), glutamine synthetase, protoporphyrinogen oxidase (oxyfluorfen, diphenylethers), carotenoid biosynthesis (triazoles, ureas, isoxazolidiones), EPSP synthese (glyphosate) etc. [6,7]. Diphenylether, isourazole and N-phenylphtalimide herbicides act like cell membrane disruptor by inhibition of plant protoporphyrinogen oxidase. The inhibition of protoporphyrinogen oxidase results in accumulation of protoporphyrin IX in the cells. Protoporphyrinogen molecules diffuse out of their site of synthesis in chloroplast where they should be normaly metabolized. Their subsequent nonenzymic oxidation results in peroxidative degradation of cellular constituents, especially membrane lipids [8-11]. The photosynthetic inhibitors can be divided into two distinct groups, the inhibitors of Photosystem I (PSI) and inhibitors of Photosystem II (PSII). Both of this group work in the energy production step of photosynthesis or in the light reactions. The Photosystem I accept electrons after the reduction of plastocyanin. PSI reduces NADP+ to NADPH. Typical examples are bipyridinium herbicides (paraquat), which take electrons from PSI and therefore inhibit the reduction of NADP+ [12]. A number of commercially important herbicides have been shown to inhibit PSII electron transport in higher plants and algae. They are represented by several herbicide groups including triazines (atrazine), substituted ureas (diuron, linuron, chlorotoluron), pyridazinones, phenyl carbamates, nitriles (ioxynil, bromoxynil), benzothiadiazoles, acid amides. Most of Photosystem II herbicides have two and more sites and also modes of action. On the contrary Buschmann et al. [13] concluded that herbicides DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea), bentazon (3-isopropyl1H-2,1,3-benzothiadiazin-4(3H)-one-2,2-dioxide), amitrole, and SAN 6706 (4-chloro-5 (dimethyl-amino)-2- (α,α,α-trifluoro-m-totyl)-3 (2H)-pyridazinone)) inhibiting PSII or producing chlorosis partly affect, but do not block, carotenoid and chlorophyll biosynthesis in dark-grown plants. Herbicides binding specifically to the chloroplast D1 protein with subsequent interruption of the electron and proton flow through PSII are most often used pesticides in agrochemistry. Photosystem II is a multi-enzymatic chlorophyll-protein complex located in the thylakoid membrane of algae, cyanobacteria and higher plants. This complex is made up of the two homologues proteins D1 and D2 carrying the main functional groups of PSII. The reaction centre of PS II catalyses the light-induced transfer of electrons from water to plastoquinone in a process that evolves oxygen. Due to this fact this protein complex is often called water-plastoquinone oxidoreductase.

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The reaction centre contains chlorophyll P680 (it was named along to absorption maximum of excitation of 680 nm). After the excitation of P680 to P680* the reduction of pheophytin follows. The other electron acceptor is plastoquinone QA, then electron is flown from QA- to QB on the D1 protein [12, 14, 15]. In the presence of a PSII herbicide such as diuron or DCMU bound to the QB binding site, QA cannot be oxidized by QB, thus photosynthetic electron flow is blocked. There are many investigations concerning the relationship between inhibitor binding by chloroplasts and inhibition of photosynthetic electron transport [16,17]. Diuron competes with atrazine for the herbicide-binding site in plants Spinacia oleracea (spinach) and Senecio vulgaris L. Most photosynthetic cyanobacteria, or blue-green algae, are sensitive to PSII inhibitors. Allen et al. [18] found that the binding properties of the herbicides diuron and atrazine to cyanobacteria Aphanocapsa 6308 membranes and spinach thylakoids are comparable. In addition, it appears that diuron and atrazine bind competitively to the same site in Aphanocapsa 6308 membranes, as has been shown to be true in spinach thylakoids and those of other higher plants. The radiolabelled azidoatrazine (2-azido- 4 -ethylamino- 6 -isopropylamino-s- triazine) was used as a label to identify the herbicide receptor protein. It inhibits photosynthetic electron transport at a site identical to that affected by atrazine [19]. The Clchannel blocker NPPB (5-nitro-2-(3-phenylpropylamino) benzoic acid) inhibits photosynthetic oxygen evolution of isolated thylakoid membranes. The binding of NPPB to the QB binding site of PS II is similar to the herbicide DCMU [20]. The ecotoxicological effects of PSII herbicides on the coral were studied by Jones et al. [15]. The plastohydroquinone molecule built in PSII is reoxidized by complex cytochrome b6f which carries electrons through plastocyanine to PSI. The herbicide dibromothymoquinone (DBMIB) inhibits the oxidation of complex cyt.b6f and interrupts the flow of electrons to plastocyanine. Many applications use the fact that the PSII complex binds several groups of herbicides to their advantage. The preparation of a biosensor for the detection of polluting compounds is based on the specific characteristics of PSII. The photosynthetic membrane isolated from higher plants and photosynthetic microorganisms, immobilised and stabilised, serves as a mediator of the biosensor [21-24].

1.2. Fungicides Compounds bioactive such as fungicides include many different chemical structures. Non-systemic fungicides are represented by chlorophenols, dithiocarbamates (thiram, chlorothalonil) and phtalimides. They act as multiple site agents. Systemic fungicides differ in mode of action. Organophosphate esters disrupt the membrane function by inhibition of phospholipide

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biosynthesis, triazoles disrupt the sterol biosynthesis. Metalaxyl inhibits mycelial growth and spore formation. Iprodione blocks oxidative phosphorilation and acts as a respiration inhibitor. Benzimidazoles inhibit tubulin biosynthesis. Residual fungicides remain on surfaces of leaves and provide protection [2528]. Among systemic benzanilide fungicides flutolanil specifically inhibits the succinate dehydrogenase complex of Basidiomycetes, without having any influence on fungi of other classes. Succinate dehydrogenase is an iron–sulfur protein that is an integral part of the inner mitochondrial membrane and a key element in the electron transport chain of mammals [29]. Many degradation products of fungicides are found in the food samples which poses health hazard to the population [30,31]. For dicarboximides such as vinclozoline, procymidone and iprodione, a likely hydroxylated derivative of vinclozoline, 3,5-dichloroaniline, are the main degradation products found in young wines [32].

1.3. Insecticides The most frequently encountered insecticides are derived from organophosphorus, acetamides, carbamates, pyrethroids and chlorinated compounds. Inhibition of cholinesterase activity is the basis for major toxic effects of organophosphorus insecticides. Derivatives of dibasic phosphonic acid (ethephon, 2-chloroethylphosphonic acid) do not behave like typical organophosphorus compounds towards cholinesterase enzymes. However, the phosphonic acid dianion phosphorylates serine residues in the active site of cholinesterases. Plasma cholinesterase is more susceptible than acetylcholinesterase to the effects of ethephon [29]. Chloro-nicotinyl pesticides (imidacloprid, 1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine) act as an agonist at postsynaptic nicotinic acetylcholine receptors of insects. Organochlorine compounds (lindane, hexachlorocyclohexane) are used against a wide range of soil-dwelling and plant-eating insects. They are commonly used on numerous crops for seed treatment, in warehouses and to control insect-borne diseases. Lindane undergoes extensive metabolism in mammals, proceeding through a pathway involving stepwise dehydrogenation, dechlorination and dehydrochlorination, which may be followed by conjugation with sulfate or glucuronide. Lindane induces a number of metabolizing enzymes, including the cytochrome P450 system, glutathione-S-transferase and UDP-glucuronosyl transferase. Carbamate insecticides (oxamyl, N,N-dimethyl2-methylcarbamoyloxyimino- 2-(methylthio) acetamide) act by inhibiting acetylcholinesterase activity. There are two major pathways of metabolism of oxamyl in animals: non-enzymatic hydrolysis to the oxime and enzymatic conversion to dimethyloxamic acid via dimethylcyanoformamide. Metabolism of oxamyl in plant tissues includes hydrolysis of the methylcarbamoyl group yielding oxamyl oxime, N-demethyloxime and gives its glucose conjugate.

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2. Redox- active substituents of industrial pesticides One of the first and probably the most frequently electrochemically studied pesticides are compounds derived from bipyridinium structure. Early studies concluded that an efficient herbicide should be a compound having a reversibly reduced redox center and should adsorb at interfaces. Since the early studies a large amount of organic compounds have been synthesized, tested for pesticidic properties and commercially applied. Here we give an overview of redox active substituents, which yield such a desired bio-activity. Reversible organic redox systems are not numerous. This class of compounds is listed first followed by compounds characterized by an irreversible electrochemical charge transfer, which is more frequently encountered and results in a more complicated pattern of products. The activity of pesticides proceeds in natural aqueous environment. However, protonation reactions hide the important initial redox steps, which could be observed only in aprotic media. The following sections often refer to redox reactions in aprotic solvents like acetonitrile, N,Ndimethylformamide, dimethylsuldoxide and similar media enabling to detect reactive radical intermediates and assess the reaction pathways.

2.1. Bipyridinium salts

A typical representative compound is methyl viologen (MV2+) or paraquat, N,N'-dimethyl-4,4'-bipyridinium dication (MV2+). It is easily reduced by a oneelectron reversible step to a reactive cation radical [33,34]. MV 2 +

+ e−

⇔ MV +•

(1)

Further one-electron reduction yields a neutral form MV +•

+ e−

⇔ MV 0

(2)

The redox processes are coupled with the cation radical dimerization [35,36] MV +•

+ MV +•

⇔ MV − MV 2 +

(3)

and disproportionation of the fully reduced neutral form MV 0

+ MV 2 +

⇔ MV +•

+ MV +•

(4)

This reaction sequence is applicable to aprotic media, whereas in water the fast protonation shifts the second reaction toward more positive potentials. As a result, a single two-electron reduction is observed. The electron transfer is indeed very fast, which makes paraquat an attractive electron transfer mediator. The other derivatives differing by the substituent on the nitrogen atom have slightly modified redox potentials, however the common features are the same.

Heterogeneous electron transfer of pesticides

CH3

N

N

7

CH3 N

paraquat

N

diquat

1

2

The 3,3'-ethynyl derivative has a trivial name diquat. A series of derivatives with longer alkyl subsitutents are used for research purposes often as electron transfer mediators [37-39].

2.2. Aromatic nitro compounds The nitro-group on an aromatic ring (Ar) is another structure that yields a reversible one-electron redox step. The resulting anion radical is relatively stable [40]. Its further reduction is rather complicated and the mechanism depends on pH of the medium [41]. Reduction in acidic solutions consumes six electrons and gives aniline as the final product. Ar-NO 2

+ 6e −

+ 6H +

→ Ar − NH 2

(5)

At higher pH the electron transfer reactions coupled to chemical steps can produce a large variety of products like nitrosobenzene, phenylhydroxyamine, phenylhydrazine, azoxybenzene, azobenzene or hydrazobenzene. It is beyond the scope of this chapter to present a detailed account on complicated reaction pathways. Compounds used as pesticides contain besides the nitro group also other substituents.

HO

R1 O2N

NO2

p-nitrophenol

3

CH3 R2

C R3

R1

R2

COOCH3 C2H5 OH CH3

R3 H CH3

dinoseb dinoterb

4 5

NO2

Nitrophenols, nitrodiphenylethers and analogous compounds have redox properties dominated by the presence of the reversibly reduced nitro group [42-45].

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2.3. Halogenated compounds Halogen substituents on the aromatic ring are reductively cleaved yielding halide anions, which itself can be electrochemically detected.

Ar-X + 2e −

+ H+

→ Ar + X −

(6)

Pesticides containing an aromatic halogen always have other substituents on the same aromatic ring. Hence there is not an unambiguous assignment of the electroactivity due to the carbon-halogen bond cleavage. Typically halogen function is used together with nitro-, amino-, cyano- or hydroxyl groups and the number of halogen substituents varies from one to five. R1 Cl

Cl

Cl

Cl

R1

R2

OH NO2 NO2

Cl Cl H

pentachlorophenol quintozene tecnazen

6 7 8

R2

The reductive dehalogenation is a common process in natural degration of pesticides [46-49]. Compounds with several and possibly different halogen substituents cleave the C-X bond through stepwise processes.

2.4. Organophosphates Phosphoric acid itself is redox inactive. Redox active derivatives of phosphoric acid are only those, which contain some other reducible group. In most cases it is a nitro group or a multiple bond. Their redox chemistry will be analogous to reactions described for nitroaromatics. R1

O O P

O

NO2

O R1 R2

R1 C2H5 CH3

R2 H CH3

paraoxon fenitrooxon

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Similar sulphur-containing pesticides are derived from thiophosphoric acid.

Heterogeneous electron transfer of pesticides R1

9

S O P

O

NO2

O R1 R2

R1 C2H5 CH3 CH3 CH3

R2 H H CH3 Cl

parathion methyl-parathion fenitrothion chlorthion

11 12 13 14

Other organophosphates bear either a poly-halogenated aromatic ring or hetero-ring with several nitrogen atoms (pyridine, pyrimidine, benzpyrazine, benzpyrane, pyrazole). These derivatives do not yield reversible redox systems.

2.5. Carbamates and thiocarbamates Carbamates do not yield faradaic currents on conventional electrodes. They are often strongly adsorbed on electrochemical interfaces, which can be used for a non-specific determination. O O

C NH

CH3

O N

C

S

C2H5

C2H5

carbaryl 15

cycloate

16

On contrary, thiocarbamates interact with mercury cations forming an insoluble product. This yields and anodic one-electron wave of mercury dissolution enhanced by the formation of mercurous thiocarbamates or dithiocarbamates [50,51]. The metabolic pathway for carbaryl in plants includes methyl and ring hydroxylation, carbamate ester hydrolysis, Ndemethylation, followed by conjugation to form water-soluble glycosides [52].

2.6. Urea derivatives Phenyl urea herbicides (like diuron) undergo oxidation in aqueous and also in aprotic solvents [53,54].

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NH

R1 H H Cl

CH3

C

N CH3

R2 H Cl Cl

fenuron monouron diuron

17 18 19

The oxidation involves one-electron step yielding a nitrogen radical, which readily forms an N-N dimer [55]. Sulfonylurea derivatives (sulfuron) can be reduced in two not well separated steps influenced by adsorption [56].

2.7. N-heterocyclic compounds This class of compounds is by large represented by symmetrical (1,3,5-) and asymmetrical (1,2,4-) triazines. Y

N

R1HN

Y

R1

Cl Cl Cl Cl SCH3 SCH3 SCH3 SCH3

N

N

NHR2

R2

C2H5 C2H5 CH(CH3)2 C2H5 C2H5 CH(CH3)2 C2H5 CH3

CH(CH3)2 C2H5 CH(CH3)2 C(CH3)3 CH(CH3)2 CH(CH3)2 C(CH3)3 CH(CH3)2

atrazine simazine propazine terbutylazine ametryne prometryne terbutryn desmetryne

O

(H3C)3C

O

N

NH2

N

SCH3 N

metribuzin

20 21 22 23 24 25 26 27

N N

NH2 CH3

N

28

metamitron 29

The hetero-ring of 1,2,4-triazine can be reduced to dihydro-derivative. The two separated bielectronic reduction of metamitron and also metribuzin yield firstly the hydrogenation of the 1,6-azomethin bond. The 2,3-position is reduced at more negative potentials. Both closely located redox centra

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evidently do not exert any electronic communication [57,58]. The reduction of the symmetrical 1,3,5-triazine leads to dimerization. However, these processes are of minor importance. Pesticides based on s-triazines are derivatives with multiple substituents, which undergo a stepwise cleavage and thus affect the major redox properties. Pyridine derivatives NH2

Cl

Cl

Cl

COOH N

picloram 30

can be reduced by processes involving four electrons and four protons. Raction pathways depend on the acidity of the medium. The mechanistic aspects of substituted N-heterocyclic pesticides have been treated in numerous publications [46-48, 59-70]. Dicarboxiimide-type pesticides O

Cl O

Cl

CH3

N

N

N

O Cl

Cl

O

vinclozoline

O

C

NH

CH(CH3)2

O

iprodione 32

31

O

Cl

CH3

N

Cl

O

CH3

procymidone 33

contain a five-member heterocycle, which is reductively oppened yielding chlorinated anilines as decomposition products.

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2.8. Phenoxy herbicides Derivatives of phenoxyalkanoic acids are used in the largest quantities. They are growth regulators. Many derivatives were developed after discovery of auxin (4-(indol-3-yl)acetic acid), a growth regulator produced by organisms. This class of herbicides is redox inactive by itself. Cl

O

CH

COOH

CH3 Cl

dichlorprop, 2,4-DP

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Indirect oxidation [71-73] is used for removal of these massively used compounds from contaminated water. Electrochemical methods, which were developed, are preceded by nitration.

2.9. Nitriles Several pesticides contain the CN group either on an aromatic ring or on alkylsubstituents of triazines. The redox mechanism [74] of aliphatic and aromatic nitriles depends on the pH. In acidic medium the reduction yields amines R-CN + 4e −

+ 4H +

→ R-CH 2 NH 2

(7)

whereas at low proton availability the C-CN bond is cleaved and the cyanide anion is released. R-CN + 2e −

+ 2H +

→ RH + CN -

(8)

The primary reduction product of aromatic nitriles is a relatively stable anion radical, which can mediate the electron transfer to compounds more difficult to reduce, even to carbon dioxide. However, other substituents usually undergo the reductive cleavage OH X

X

X Cl Br I

chloroxynil bromoxynil ioxynil

35 36 37

CN

and the above mentioned reactions apply to secondary products. As an example, may serve ioxynil and related derivatives.

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2.10. Pyrhenoid insecticides Leaves of Chrysantheum-family flower has been used for centuries as insecticides. The active compound pyrethrin inspired synthesis of similar derivatives like dimethrin and tetramethrin.

O

O N O

O

tetramethrin

38

Redox activity used for analysis derives from aromatic aldehydes formed by fast alcohololysis [75].

3. Surface activity and accumulation of pesticides Accumulation of toxic compounds at the interfaces plays an important role in the contamination of environment. Many pesticides are strong surfactants, which makes the problem of interfacial adsorption an emminent contamination problem. The transport and fate of pesticides in the environment involves complex phenomena, which are influenced by many processes. Chemical compounds in aquatic environment are distributed between three phases: the aqueous solution, the dissolved organic carbon materials or colloids and the sediment solids. The dissolved organic carbon exhibits a significant effect on pesticide adsorption/desorption behaviour [76]. There are many studies which follow adsorption/desorption characteristics of pesticides in soils (with and without organic matter), on clay minerals (bentonite, montmorillonite and kaolinite) and organic matter (humic acid) [77]. The compound adsorption capacity in soil increases with the organic matter content, and can be insignificant in soils with