Paraquat, reduction potentials

The redox system i (R = alkyl Iqx Viologenes was the first to be interpreted correctly (E. Weitz ). It is especially well suited for studying the effects of N-substituents because steric effects are virtually absent. In spite of the great importance of some of these quaternary salts as universal herbicides (R = CH3, paraquat only potentials Ej were known for a long period of time, since the reductions SEM/RED in aqueous medium are mostly irreversible In aprotic media, however, as in acetonitrile and DMF, E2 and Ei are ideally reversibleThis holds true for most of the investi ted substituents as can be seen from Table 1. 

Aromatic hydrocarbons are called 7r bases because of their rather low ionization potentials. On the other hand, aromatic compounds containing a nitrogen atom possess lower MO energy than the parent hydrocarbons, and they show less electron-donating character or electron-accepting character. The mode of action of the herbicide paraquat 300 (1,1 -dimethyl-4,4 -bipyridinium salt) is thought to be connected with its reversible reduction-oxidation reactivity. The compounds with a lower level of activity possess higher reduction potentials, and their one-electron transfer is not quite quantitatively reversed. 

Publications of Mees, Homer and Tomlinson in the 1960s on general herbicidal properties indicated that the phytotoxic action is connected with chlorophyll and light. The authors presumed, on the basis of the relationships between the reducibility of the single compounds and the phytotoxic action, that a reduction to a stable free radical occurs in the plant and that this free radical is responsible for phytotoxic action. The redox potentials of -0.446 and 0.346 mV of paraquat and diquat, respectively, are ensured by the reduction potential of light reaction I of photosynthesis (Calderbank, 1968). 

The bipyridylium herbicides, used as a substitute for ploughing in erosion-prone country, are contact herbicides. Because they are not appreciably translocated, weeding with them produces a razor-sharp boundary. The most potent of these is paraquat 4.65), l,r-dimethyl-4,-4 -bipyridylium cation (used as the dichloride). This substance has long been in use in another connection, namely as an indicator for low reduction potentials. This colourless substance, known as methyl viologen, functions by forming a violet-coloured stable free radical 4.66) when the potential falls to —446 mV (Michaelis and Hill, 1933). 

The redox potential diagram in eq. 1 illustrates that the effect of optical excitation is to create an excited state which has enhanced properties both as an oxidant and reductant, compared to the ground state. The results of a number of experiments have illustrated that it is possible for the excited state to undergo either oxidative or reductive electron transfer quenching (2). An example of oxidative electron transfer quenching is shown in eq. 2 where the oxidant is the alkyl pyridinium ion, paraquat (3). 

Some bipyridinium salts are remarkable herbicides. They rapidly desiccate all green plant tissue with which they come into contact, and they are inactivated by adsorption on to clay minerals in the soil. This potent herbicidal activity is found only in quaternary salts, e.g. diquat (254) and paraquat (255), with redox potentials for the first reduction step between -300 and -500 mV (equations 158 and 159) (B-80MI20504). The first reduction step, which is involved in herbicidal activity, involves a completely reversible, pH independent, one-electron transfer to yield the resonance stabilized radicals (256) and (257). The second reduction step, (256 -> 258) and (257 -> 259), is pH dependent and the p-quinoid species formed are good reducing agents that may readily be oxidized to diquatemary salts. 

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. 

Fig. 2 Observed shifts in the formal potential for the monoelectronic reduction of the surface confined paraquat groups of Au/S -hCioHjiSH electrodes immersed in phosphate buffer (pH = 7) solutions also containing variable concentrations of indole...

Until recently, it was accepted that paraquat is reduced by the primary electron acceptor of PSI, which is also responsible for the reduction of NADP. Recent advances in this area, however, suggest that electron flow after PSI is branched (see Ref. 91). One branch is involved in NADP reduction whereas another branch is involved in paraquat- or ferredoxin-mediated O2 reduction. The branch for reduction of O2 offers a potential site for regulating paraquat toxicity since electron flow to paraquat could be inhibited without a block of NADP reduction. An unidentified inhibitor from hemolyzed rabbit sera was found to inhibit the paraquat- or ferredoxin-mediated O2 evolution without inhibiting NADP -dependent O2 evolution. Similar results were obtained with hemoglobin, which offered partial protection of pea chloroplasts against paraquat by inhibiting only the paraquat-mediated O2 reduction. ... 

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