Eric Daniel Clarke

Characterization of the safener-induced glutathione S-transferase isoform II from maize

The safener-induced maize (Zea mays L.) glutathione S-transferase, GST II (EC 2.5.1.18) and another predominant isoform, GST I, were purified from extracts of maize roots treated with the safeners R-25788 (N,N-diallyl-2-dichloroacetamide) or R-29148 (3-dichloroace-tyl-2,2,5-trimethyl-1,3-oxazolidone). The isoforms GST I and GST II are respectively a homodimer of 29-kDa (GST-29) subunits and a heterodimer of 29 and 27-kDa (GST-27) subunits, while GST I is twice as active with 1-chloro-2,4-dinitrobenzene as GST II, GST II is about seven times more active against the herbicide, alachlor. Western blotting using antisera raised against GST-29 and GST-27 showed that GST-29 is present throughout the maize plant prior to safener treatment. In contrast, GST-27 is only present in roots of untreated plants but is induced in all the major aerial organs of maize after root-drenching with safener. The amino-acid sequences of proteolytic fragments of GST-27 show that it is related to GST-29 and identical to the 27-kDa subunit of GST IV.

Metabolism‐related assays and their application to agrochemical research: reactivity of pesticides with glutathione and glutathione transferases†

An HPLC-based assay system has been developed to measure the reactivity of agrochemicals with glutathione (GSH) with and without catalysis by glutathione transferases (GSTs). Metabolism-related parameters based on second-order related rate constants from non-enzymatic GSH and enzymatic GSH+GST assays have been derived for use in structure–activity and structure–reactivity relationship studies of exploratory agrochemicals. The versatility and sensitivity of the assay system has been established using a diverse range of agrochemicals and model compounds, e.g. 4-nitrobenzyl chloride, 1-chloro-2,4-dinitrobenzene, atrazine, acetochlor, fluorodifen, fluazifop-butyl, tridiphane, fluazinam, chlorothalonil and diazinon. For the enzymatic GSH+GST assay, second-order related rate constants, ratioed to the assay standard, 4-nitrobenzyl chloride to provide a parameter independent of assay conditions, spanned five orders of magnitude, fluazinam being the most reactive and atrazine the least. Within chemical classes significant variations in reactivity were observed, alachlor being c.15-fold more reactive than pretilachlor. Applications of this assay system based on comparative measures of reactivity across and within chemical classes are discussed.

Beyond physical properties—Application of Abraham descriptors and LFER analysis in agrochemical research

This paper provides an overview of the Abraham approach to the determination of molecular descriptors for agrochemicals, and their potential in defining bioavailability related property-profiles and processes. The prediction of Abraham descriptors from structure has been demonstrated using agrochemical products and a simple guide to bioavailability defined in terms of the McGowan volume descriptor V (3), the hydrogen bond acidity descriptor A (1), and the hydrogen bond basicity descriptor, B (3). The use of measured physical property data to yield experimental descriptors is illustrated for 28 representative agrochemicals. Experimental descriptors were used to derive linear free energy relationships for soil binding and root uptake, to provide insights to these processes.

Henry’s Law constants or air to water partition coefficients for 1,3,5-triazines by an LFER method

Solvation descriptors have been obtained for 19 substituted 1,3,5-triazines, using literature data on water to solvent partitions, and our own experimental determinations of water to solvent partition coefficients and micellar electrokinetic chromatography (MEKC) retention factors. The solvation descriptors can then be used to predict environmentally important air to water partition coefficients, K(w), which are the reciprocal of Henrys Law constants, with due regard to units. For this class of chemistry it is shown that the values of log K(w) so obtained are generally in good agreement with the experimental values compiled in this work and also compare well with the calculated values from the established bond contribution method of Meylan and Howard as implemented in the EPI Suite, but not with calculated values from the semi-empirical quantum chemical solvation models, SM2 and SM3, recently reported by Delgado and Alderete (J. Chem. Inf. Comput. Sci., 2003, 41, 1226-1230).

1,3,4(2H)-Isoquinolinetriones: evaluation of amino-substituted derivatives as redox mediator herbicides

Amino-substituted derivatives of 2-ethyl-1,3,4(2H)-isoquinolinetrione have been prepared with the aim of enhancing their hydrolytic stabilities relative to other isoquinolinetriones, and hence potentially improving their herbicidal effects. 5-Amino-2-ethyl-1,3,4(2H)-isoquinolinetrione has been shown to be 12 and 8 times more stable towards hydrolysis than the isomeric 7- and 8-amino derivatives, respectively, and 120 times more stable than 2-ethyl-1,3,4(2H)-isoquinolinetrione itself. Pulse radiolysis studies have shown 5-amino-2-ethyl-1,3,4(2H)-isoquinolinetrione to have free-radical properties which could enhance the generation of superoxide radicals in plants. The compound is a potent stimulator of the light-dependent consumption of oxygen at photosystem I in isolated chloroplasts and also shows high activity in isolated leaf discs. Evidence is presented which indicates that the rate of foliar uptake is a major factor affecting herbicidal activity for this compound.

Synthesis and characterisation of some 4-keto derivatives of 1,3,4(2H)-isoquinolinetrione redox mediator herbicides.

Derivatives of 2-ethyl-1,3,4(2H)-isoquinolinetrione in which the 4-keto group has been modified to (Z)-oxime, (E)- and (Z)-O-methyl oxime, (Z)-N,N-dimethyl hydrazone, cyano-imine and dicyanomethylene moieties have been prepared and evaluated as redox mediator herbicides. All of the compounds have the free-radical properties required to function as redox mediators, as determined by cyclic voltammetry, though only the O-methyl oximes, the N,N-dimethyl hydrazone and the cyano-imine have reduction potentials in the range required to stimulate the light-dependent consumption of oxygen at photosystem I in isolated chloroplasts. The O-methyl oximes and the cyano-imine are fast-acting post-emergence herbicides, producing symptoms of rapid desiccation; the (E)-O-methyl oxime is the most active herbicide, being somewhat more potent than the parent isoquinolinetrione. Hydrolysis studies indicate that it is unlikely that any compound generates the parent isoquinolinetrione in vivo. Attempts to explain differences between in vitro and in vivo activities using hydrolytic stabilities and physical properties were unsuccessful, and it was concluded that these factors probably play a less significant role in moderating the herbicidal activity of isoquinolinetrione derivatives than originally thought.

An Improved Process for the Preparation of an α,α-Difluorosulfonylisoxazoline Herbicide.

An efficient synthesis of a difluorosulfone-containing herbicide has been achieved by selective reductive silylation of a symmetrical bis(trifluoromethyl)-1,2,3-triazole. Subsequently, a fluoride-induced reaction led to a difluoromethyl anion equivalent, which was reacted with a sulfur electrophile leading ultimately to the key difluorosulfide moiety.

The Development and Evaluation of a Microfluidic Device for the Determination of Organic-Aqueous Partition Coefficients to Support Agrochemical and Drug Discovery

A prototype multi-channel microfluidic device has been developed and tested for the determination of organic-aqueous partition coefficients.