Phenol-type herbicides

This approach has been used primarily to study photosynthesis, for example to introduce tolerance to a variety of stress conditions. Many studies have been successful in verifying the effea of single amino acid modifications on herbicide binding affinity. High affinity binding to the D1 protein is a useful property for the detection of herbicides. A fluorescence biosensor based on mutants resistant to various herbicide subclasses was developed, it makes possible to distinguish between subclasses of herbicides (e.g., triazines from urea and phenolic type herbicides). ... 

The distinctly different behavior of the phenol-type herbicides following trypsin treatment suggests that different determinants within the PS II protein complex establish the "domains" that regulate the binding properties of these inhibitors. In spite of the fact that phenol-type herbicides will displace bound radiolabeled herbicides such as diuron, these inhibitors show noncompetitive inhibition (29, 30). At present, there are three lines of evidence which favor TH e involvement of two domains within the PS II complex that participate in creating the binding sites for these herbicides (a) isolated PS II particles can be selectively depleted of a polypeptide with parallel loss of atrazine sensitivity, but not dinoseb inhibition activity (33) (b) in resistant weed biotypes, chloroplast membranes that exhibit extreme triazine resistance have increased sensitivity to the phenol-type herbicides (13) and (c) experiments with azido (photoaffinity) derivatives of phenol and triazine herbicides result in the covalent labeling of different PS II polypeptides (, 31). 

Thus far, no mention has been made of the inhibitory binding characteristics of certain phenol-type herbicides such as the hydroxybenzonitriles, DNOC, and dinoseb. There are clear chemical differences between these... 

Alkylated phenol derivatives are used as raw materials for the production of resins, novolaks (alcohol-soluble resins of the phenol—formaldehyde type), herbicides, insecticides, antioxidants, and other chemicals. The synthesis of 2,6-xylenol [576-26-1] h.a.s become commercially important since PPO resin, poly(2,6-dimethyl phenylene oxide), an engineering thermoplastic, was developed (114,115). The demand for (9-cresol and 2,6-xylenol (2,6-dimethylphenol) increased further in the 1980s along with the growing use of epoxy cresol novolak (ECN) in the electronics industries and poly(phenylene ether) resin in the automobile industries. The ECN is derived from o-cresol, and poly(phenylene ether) resin is derived from 2,6-xylenol. 

Mild trypsin treatment has been shown also to alter the affinity for a number of other chemical families of PS II directed herbicides in a manner similar to that of the -triazines. Trypsin-mediated decreases in inhibitory activity are found for uracil (19), urea, pyridazinone ( 9, 28) and triazinone (28) herbicides. In contrast, phenol-type"Terbicides increase in inhibitory activity following brief trypsin treatment (j, 28), although the trend was reversed over longer treatment perio 3 . 

Many herbicides, lilce ureas and triazines of the serine family share a common substructure a sp carbon attached to a nitrogen with a free electron pair and a positive n-charge (2,18,28). Their QSARs show usually a dependence on electronic and lipophilicity parameters. Individual compounds, chemically different, displace each other from the membrane (14.29). This family looses inhibitory potency in tris-treated cbloroplast membranes (7,18). Cross resistance studies of chloroplasts in triazine/triazinone or DCHU tolerant plants and algae have indicated subfamilies (reviewed in 13,18). None of these mutants are tolerant to phenol-type inhibitors. 

Photoaffinity labels are an efficient tool for identification of inhibitor binding proteins in the photosynthetic electron transport chain. [ H]Azido-dinoseb, an azido-deri-vative of the phenolic herbicide dinoseb, was synthesized almost a decade ago and was shown to bind primarily to a 41 kDa protein (1,2). Contrary, labeling with azido-deri-vates of diuron-type herbicides revealed that these herbicides bind to a 32 kDa protein, which has now been recognized as the D-1 protein of the photosystem II reaction center core complex (see references in (3)). Tyrosine residues in positions 237 and 254 of the D-1 sequence were demonstrated to be the primary target of [ CJazido-monuron (3). The phenolic herbicide [ I]azido-ioxynil also labels predominantly the D-1 protein in position of Val249 and only in trace amounts a 41 kDa protein (4). 

The types of compounds that are of concern as contaminants are chlorinated insecticides, organophosphates, herbicides, fungicides, fas-ciolicides (phenolic compounds administered to cattle to control liver flukes), antibiotics and sulfonamides, detergents and disinfectants, and polychlorinated biphenyls (PCBs). Contaminants in milk have been reviewed by Kroger (1974) and Snelson (1979). In several cases, allowable levels for specific contaminants in milk have been set by the World Health Organization. Surveys have seldom revealed levels in excess of such standards. 

Although hydrolysis of the triazine herbicides is temperature and pH dependent, these herbicides are considered to be hydrolytically stable under the pH and temperature conditions encountered in natural waters. However, the relatively slow hydrolysis rates in natural waters may be enhanced somewhat by the presence of dissolved organic carbon (DOC) (in the form of fulvic acids and a variety of low-molecular-weight carboxylic acids and phenols) that has been shown to catalyze the hydrolysis of several triazine herbicides. Although microbial degradation is probably the most important mechanism of dissipation of the triazine herbicides in soils, abiotic hydrolysis of these herbicides also occurs. Hydrolysis in soils is affected by the pH, organic matter (humic acid) content, and the type and content of clay in the soil. 

Gas chromatography has been applied to the determination of a wide range of organic compounds in trade effluents including the following types of compounds which are reviewed in Table 15.15 aromatic hydrocarbons, carboxylic acids aldehydes, non ionic surfactants (alkyl ethoxylated type) phenols monosaccharides chlorinated aliphatics and haloforms polychlorobiphenyls chlorlignosulphonates aliphatic and aromatic amines benzidine chloroanilines chloronitroanilines nitrocompounds nitrosamines dimethylformamide diethanolamine nitriloacetic acid pyridine pyridazinones substituted pyrrolidones alkyl hydantoins alkyl sulphides dialkyl suphides dithiocaibamate insecticides triazine herbicides and miscellaneous organic compounds. 

In this section, examples of laboratory studies concerning 03/UV/H202 advanced oxidations of some water pollutants are discussed. They have been chosen because of the high interest that their oxidation treatment has attracted among researchers in the field. Three different types of pollutants have been chosen phenols of different nature, s-triazine herbicides, and some volatile compounds, mainly chlorinated organics of low molecular weight (VOCs). Information is also given on the treatment of 1,4-dioxane, another important priority pollutant. These studies represent the scope and objectives to be reached in this type of laboratory research. 

The phenolic photoaffinity label azidodinoseb (Figure 4) binds less specifically than either azidoatrazine or azidotriazinone (14). In addition to other proteins, it labels predominantly the photosystem II reaction center proteins (spinach 43 and 47 kDa Chlamydomo-nas 47 and 51 kDa) (17). Because of the unspecific binding of azidodinoseb, this can best be seen in photosystem II preparations (17). Thus, the phenolic herbicides bind predominantly to the photosystem II reaction center, which might explain many of the differences observed between "DCMU-type" and phenolic herbicides (9). The photosystem II reaction center proteins and the 34 kDa herbicide binding protein must be located closely to and interact with each other in order to explain the mutual displacement of both types of herbicides (8,12,21). Furthermore, it should be noted that for phenolic herbicides, some effects at the donor side of photosystem II (22) and on carotenoid oxidation in the photosystem II reaction center have been found (23). 

Another example of the coordinated use of herbicides and allelopathic residues is the situation where a herbicide is used to desiccate a cover crop, Lehle and Putnam (131) recognized that the allelochemical content of a residue, such as sorghum, was dependent on the stage of growth at the time of desiccation. However, no studies have been undertaken on other factors that may control the allelochemical content of the residue. It is possible that the quantity and type of allelochemicals in a cover crop can be manipulated by adjusting the formulation and application rate of the herbicides used for desiccation. Evidence in the literature demonstrates that certain herbicide treatments and other stress factors can result in elevations of several coumarins and phenolic compounds (14-19,21). Thus, it may be reasonable to assume that a herbicide used to kill a cover crop can also be used as a stimulus for the synthesis of allelochemicals prior to senescence. 

Due to their difference in chemistry, all PSII-inhibiting herbicides demonstrate different binding properties. For example, urea/triazine type inhibitors were proposed to be oriented towards Set 264, triazinones towards Ala 251 and phenolic herbicides were oriented towards His 215 (Table 1). ... 

Nonlinear isotherms have been reported particularly at low Cg values (Fig. 3.10a). The sorption of both dibromoethane (EDB) and the herbicide diuron (DUN) on a peat soil (49.3%OC) both show this response, which is more pronounced with the latter. A similar type of response is also observed in soils of lower organic carbon content. A competitive effect is also demonstrated with trichloroethylene and phenol suppressing the sorption of EDB and EDB (at a sufficient concentration), monuron and dichlorophenol affecting the sorption of diuron. The... 

The photochemistry of chlorophenols has been studied by many other authors, and depending on the conditions chosen, numerous product types have been identified. Crosby and co-workers examined the photoreactions of 2,4-dichlorophenol (Crosby and Tutass, 1966) and 2,4,5-trichlorophenol (Crosby and Wong, 1973), model compounds for the herbicides 2,4-D and 2,4,5-T. The products isolated appeared to have been formed by photonucleophilic aromatic substitution of OH groups for Cl atoms (Figure 6.16). Photolysis of 3-chlorophenol as well as other 3-halogenated phenols afforded resorcinols, presumably by similar mechanisms... 

The use of photosynthetic enzymes isolated from plants has been implemented in a toxicity monitor (LuminoTox, Lab Bell Inc., Shawinigan, Canada). This system can detect a range of compounds such as hydrocarbons, herbicides, phenols, polycyclic aromatic hydrocarbons (PAHs), and aromatic hydrocarbons. These enzymes have been coupled to screen-printed electrode and have been demonstrated to be able to detect triazine and phenylurea herbicides [79]. Other enzyme inhibitions have been used to detect biotoxins from plant, animals, bacterial, algae, and fungal species (e.g., ricin, botulinum toxins, mycotoxins, cyanobacterial toxins). However, since the identity and specificity of the above toxic compound can be very important during the analysis, other sensor systems such as immunosensors may be preferred to give a better indication to toxin type and identity than the use of enzyme inhibition tests. 

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