Oxygen herbicide formation

Senesi and Testini [147,156] and Senesi et al. [150,153] showed by ESR the interaction of HA from different sources with a number of substituted urea herbicides by electron donor-acceptor processes involves organic free radicals which lead to the formation of charge-transfer complexes. The chemical structures and properties of the substituted urea herbicides influence the extent of formation of electron donor-acceptor systems with HA. Substituted ureas are, in fact, expected to act as electron donors from the nitrogen (or oxygen) atoms to electron acceptor sites on quinone or similar units in HA molecules. 

Triazine (e.g., atrazine, simazine) and substituted urea (e.g., diuron, monuron) herbicides bind to the plastoquinone (PQ)-binding site on the D1 protein in the PS II reaction center of the photosynthetic electron transport chain. This blocks the transfer of electrons from the electron donor, QA, to the mobile electron carrier, QB. The resultant inhibition of electron transport has two major consequences (i) a shortage of reduced nicotinamide adenine dinucleotide phosphate (NADP+), which is required for C02 fixation and (ii) the formation of oxygen radicals (H202, OH, etc.), which cause photooxidation of important molecules in the chloroplast (e.g., chlorophylls, unsaturated lipids, etc.). The latter is the major herbicidal consequence of the inhibition of photosynthetic electron transport. 

The herbicidal activity of 1,1-dimethyl-4,4 -dimethylbipyridylium dication ( Paraquat ) seems to depend on the formation of the cation radical through reaction with a component of photosystem 1 in the chloroplast, followed by reaction with oxygen to form superoxide ion (Eq, 36) (for a summary of references, see >). The rate constant for the latter reaction has been reported... 

The herbicidal effect of paraquat is attributable to the formation of superoxide anion (02 ). Superoxide anion is very toxic compound and is formed by the reaction of oxygen with paraquat radical (paraquat ). Plants, algae, and cyanobacteria have ferredoxin-NADP reductase to form NADPH for the reduction of carbon dioxide (see below). The chemolithoautotrophs also have NAD(P) (NAD and NADP) reductase to form NAD(P)H for the reduction of carbon dioxide. Paraquat [mid-point redox potential at pH 7.0 (Emj 0) = -0.43 V] radical is produced when paraquat is reduced by the catalysis of ferredoxin-NAD(P) reductase or NAD(P) reductase, which catalyzes the reduction of many compounds with of around -0.4 V. Although the aerobic organisms (and even many anaerobic organisms) have superoxide dismutase (SOD) which detoxifies superoxide anion in cooperation with catalase [ascorbate peroxidase in the case of plants (Asada, 1999)], the anion accumulates in the organisms when it is over-produced beyond the capacity of SOD. 

The other point of action by herbicides is light reaction II, in which the formation of NADPHj is blocked. This is the mode of action, for example, of paraquat. In this case, the formation of oxygen bubbles in illuminated algal cultures is unchanged (proof of the normal light reaction I). 

Blockage of electron transport on the reducing side of PS II is just the first in a series of steps that ultimately leads to plant death. Much of the energy from absorbed photons that is normally directed into electron transport is redirected into fluorescence and triplet formation when the herbicides are bound. The triplet states are of special interest because of the destructive interaction between excited chlorophyll and molecular oxygen through the following four step mechanism, where... 

Figure 1. Epr spectrvun of the non-haem iron oxidised in the dark with 5mM potassium ferricyanide. (a) untreated spinach PS2 (b) trypsin treated PS2. Chi concentration lOmg/ml. Epr conditions power lOmW, temperature 5K. Figure 2. The effect of trypsin on the formate induced Qa—Fe g=1.8 signal. (a) untreated spinach PS2 (b) trypsin treated PS2 (c) with 20mM then trypsin treated. All samples contain 50mM HCOa . Chi concentration 5mg/ml. Epr conditions as Fig. 1. Figure 3. The rate of oxygen evolution during trypsin digestion. (a) PS2 without herbicide (b) with 6.7 uM DCMU added after the digestion.

Interest in acylcyclohexanediones has been increasing because of their recently discovered occurrence as insect-derived natural products (1- and because they and their derivatives constitute a rapidly emerging class of herbicides (. Unlike many exercises in organic synthesis, which focus on carbon-carbon bond formation, much of our synthetic work on these systems has involved the manipulation both location and oxidation state or oxygen substituents. This p >er will review some of our efforts in that area. 

The formation of inclusion complexes can stabilize labile compounds and provide protection for light- or oxygen-sensitive compounds. Inclusion complexes can also be used to control the release, stability, solubility, and utilization of biologically active compounds such as drugs, vitamins, flavors, odors, insecticides, herbicides, and so forth. The cyclomaltodextrins can also be used to mask, alter, and/or eliminate undesirable flavors or odors, and they can be used to increase the water solubility of compounds that otherwise have low solubility [16]. 

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