Phenolic herbicides displacement

Herbicides that inhibit photosynthetic electron flow prevent reduction of plastoquinone by the photosystem II acceptor complex. The properties of the photosystem II herbicide receptor proteins have been investigated by binding and displacement studies with radiolabeled herbicides. The herbicide receptor proteins have been identified with herbicide-derived photoaffinity labels. Herbicides, similar in their mode of action to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) bind to a 34 kDa protein, whereas phenolic herbicides bind to the 43-51 kDa photosystem II reaction center proteins. At these receptor proteins, plastoquinone/herbicide interactions and plastoquinone binding sites have been studied, the latter by means of a plastoquinone-deriv-ed photoaffinity label. For the 34 kDa herbicide binding protein, whose amino acid sequence is known, herbicide and plastoquinone binding are discussed at the molecular level. 

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). 

DicblorobenzotnfIuoride. This compound is produced by chlorination of 4-chloroben2otrifluoride and exhibits sufficient activation to undergo nucleophilic displacement with phenols to form diaryl ether herbicides, eg, acifluorofen sodium [62476-59-9]. 

Some nonionic organic compounds exhibit much stronger mineral surface affinities than we see for apolar and weakly monopolar compounds like chlorobenzenes and PAHs. In these cases, the organic sorbates are able to displace water from the mineral surface and participate in fairly strong sorbate sorbent intermolecular interactions. Example compounds include mtroaromatic compounds (NACs) such as the explosive, trinitrotoluene (TNT), or the herbicide, 2,4-dinitro-6-methyl-phenol, also called dinitro-o-cresol (DNOC). 

Many commercial herbicides inhibit photosynthesis by displacing Qg from its binding site in D1 and thus block electron transport from to They belong to various chemical classes like triazines, ureas or phenols (For a review see ref. 29) and pollute soil and water due to their massive use in agriculture. This in turn can be harmful for human and animal health and necessitates the development of fast and sensitive detection methods. Coincidentally, the herbicidal target itself is part of the PSII complex, which represents a reporter system directly coupled to an analyte binding site. Thus the most obvious application of the Dl protein in association with other central PSII proteins is its use as a biosensor for herbicides. 

A large number of commercial herbicides such as arylureas, triazines, triazinones and phenolic compounds act as competitors to plastoquinones (Fig. 1). They occupy the Qp-binding site of the D1 protein, thereby displacing from its binding niche and prevent the oxidation of reduced Q/v. The displacement of electron mediator Qp from the D1 protein leads to interruption of the electron flow and, consequently, results in plant s death. 

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). 

Two families of inhibitors interfere with the plastoquinone or herbicide binding site on the D-1 polypeptide, i.e. on one of the reaction center subunits of PS II. The phenol and urea/triazinone family of PS II inhibitors are different in their functional inhibitory pattern (reviewed in [1]), although they both bind to the D-1 polypeptide and displace each other from the binding site (1). Both QSAR studies (2) and - more refined - quantum mechanical calculations... 

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. 

The chief outlets are for polyurethane (di-isocyanates) 40%, rubber chemicals, herbicides minor users include dye makers (approx. 5%) and pharmaceutical manufacturers. Benzene is the feedstock and the traditional route is to nitrate this and then to reduce the nitrobenzene to aniline. Catalytic hydrogenation has displaced iron/ferrous chloride reduction in this and analogous reductions e.g. in the manufacture of toluidines. Amination of phenol manufactured from cumene (Vol. I, p. 366) has been patented (Figure 2.8). The yield claimed is 99% though the economic viability is uncertain. 

The initial synthesis of compound 1, the lead compound in this series, had its origin in the chemistry of a series of phenoxypyridazine herbicides. 3,6-Dichloro-4-(l,l-dimethylethyl) pyridazine (15) was available to us because of the interest of one of our chemists in radical alkylations (2J. The 6-chlorine of this molecule may be displaced selectively by phenols to give herbicidally-active molecules. It had been demonstrated in a series of... 

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