Reaction, Hill

In 1937, Robin Hill succeeded in isolating chloroplasts from leaves which under suitable conditions still retained some form of photosynthetic activity. The material isolated by Hill actually consisted of thylakoid membranes which lacked the necessary enzymes for CO2 fixation as a result of the loss of the outer membrane envelope. When these chloroplasts were illuminated, no oxygen evolution could be observed. However, when an electron acceptor such as ferric oxalate, as was used by Hill initially, was added, oxygen evolution was observed upon illumination, accompanied by the reduction of the ferric oxalate to the ferrous form. Lurthermore, four equivalents of the oxidant were found to be photochemi-cally reduced for each mole of oxygen evolved. This reaction was called the Hill reaction. Later, ferricyanide and benzoquinone were commonly used as the oxidizing agents. 

At this juncture, van Niel s concept of a redox reaction taking place in the reaction center became very useful for understanding the Hill reaction. Although CO2 was not assimilated under illumination, it appeared that the other reactions responsible for the splitting of water were still active. The ferric oxalate used by Hill apparently served as a substitute for the natural oxidant CO2 to intercept electrons produced at the reaction center and thus allow oxygen to be evolved. The Hill reaction shows that CO2 assimilation and oxygen evolution are not obligatorily linked and two physically distinct enzymatic systems may exist. 

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Atrazine enters plants primarily by way of the roots and secondarily by way of the foliage, passively translocated in the xylem with the transpiration stream, and accumulates in the apical meristems and leaves (Hull 1967 Forney 1980 Reed 1982 Wolf and Jackson 1982). The main phytotoxic effect is the inhibition of photosynthesis by blocking the electron transport during Hill reaction of photosystem II. This blockage leads to inhibitory effects on the synthesis of carbohydrate, a reduction in the carbon pool, and a buildup of carbon dioxide within the leaf, which subsequently causes closure of the stomates, thus inhibiting transpiration (Stevenson et al. 1982 Jachetta et al. 1986 Shabana 1987). 

Hill reaction org chem Production of substituted phenylacetic acids by the oxidation of the corresponding alkylbenzene by potassium permanganate in the presence of acetic acid. hil re,ak-shon ... 

Ozone causes both quantitative and qualitative changes in carbon dioxide fixation patterns. Wilkinson and Bames, using carbon dioxide-found a reduction in radioactivity in soluble sugars and increases in free amino acids and sugar phosphates in white pine after a 10-min exposure to ozone at 0.10 ppm. Miller observed a decrease in carbon dioxide-fixation in ponderosa pines that correlated with loss of chlorophyll, after exposure to ozone at 0.30-0.35 ppm. The Hill reaction rates of chloroplasts isolated from healthy and ozone-injured ponderosa pine indicated that both light and dark reactions of the chloroplasts from ozone-injured plants were depressed. Barnes found depressed photosynthesis and stimulated respiration in seedlings of four pine species of the southeastern United States after exposure to ozone at 0.15 ppm. 

In the search for more effective post-emergent herbicides, many laboratories have measured the inhibition of photosystem II in chloroplasts i.e., the Hill reaction. In a continuing investigation of this system, ( ) Corwin Hansch s group at Pomona College, in cooperation with BASF in Germany, analyzed two sets of phenyl substituted ureas 17 1,1-dimethyl-3-phenyl, and 38... 

Table II Inhibition of Hill Reaction in Spinach Chloroplasts...

Figure 1. Optimizing both Transport and Hill Reaction Inhibition...

The eleven equations shown in Table VI express the activity of a group of Hill Reaction inhibitors (4-11). A common structural feature, in all the sets except Number 5, seems to be a nitrogen atom having considerable double bond character. Activity is... 

Another example is the herbicidal activity of substituted nitrophenols, expressed as the inhibition of the Hill reaction. 

In 1937 Robert Hill found that when leaf extracts containing chloroplasts were illuminated, they (1) evolved 02 and (2) reduced a nonbiological electron acceptor added to the medium, according to the Hill reaction ... 

ATP synthesis in chloroplasts. The flow of electrons between PSII and PSI (Fig. 23-18) is of great importance for ATP formation. As previously mentioned, plastocyanin is usually the immediate donor to P700 and serves as a mobile carrier to bring electrons to this reaction center. In this function it is analogous to cytochrome c of mitochondrial membranes. The essentiality of plastocyanin was shown by study of copper-deficient Scenedesmus (Fig. 1-11). The photoreduction of C02 by H2 is impaired in these cells, but the Hill reaction occurs at a normal rate. 

Members of an extensive group of sym-triazine herbicides, usually having one or two secondary amine substituents, block the Hill reaction and inhibit photosynthesis in a manner quite similar to that of the urea herbicides. The most widely used, 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), (10), is one of several hundred herbicidal analogs... 

Distinct differences in cells with regard to the presence or absence of target structures or metabolic processes also offer opportunities for selectivity. Herbicides such as phenylureas, simazine, and so on, block the Hill reaction in chloroplasts, thereby killing plants without harm to animals. This is not always the case because paraquat, which blocks photosynthetic reactions in plants, is a pulmonary toxicant in mammals, due apparently to analogous free-radical reactions (see Figure 18.4) involving enzymes different from those involved in photosynthesis. 

Research had confirmed that no parent simazine residues were found in treated com plants, and additional data on the dissipation pathway of simazine needed to be developed. Research also indicated that triazines interfered with the photosynthetic process on susceptible growing weeds, as evidenced by the appearance of chlorotic leaves. Steps were undertaken to elucidate simazine s dissipation pathway and herbicidal mode of action. In Basel, Dr. Gast (1958) showed that the accumulation of starch by common coleus (Coleus blumei Benth.) plants was inhibited from treatment with 2-chloro-4,6-bis-(alkyl-amino)-triazines due to the inhibition of sugar synthesis. At the same time, Moreland et al. (1958) found weed control activity could be reduced by supplying carbohydrates to the plants through their leaves and that simazine was a strong inhibitor of the Hill reaction in photosynthesis. Exer (1958) found that triazines inhibited the Hill reaction as strongly as urea of the CMU (monuron) type. 

In a study designed to determine the mode of action of atrazine in higher plants, Shimabukuro and Swanson (1969) concluded that atrazine inhibits the Hill reaction and its noncyclic phosphorylation, while being ineffective against cyclic photophosphorylation. Atrazine readily penetrated the chloroplast of resistant as well as susceptible plants. In tolerant plants such as sorghum, the metabolism of atrazine was postulated to occur outside the chloroplasts to form water-soluble and insoluble residues that reduced the concentration of photosynthetic inhibitors in the chloroplasts. 

Shimabukuro et al. (1973) identified 2-chloro-4,6-diamino-.v-triazinc (G-28273), which represented complete dealkylation of the triazine ring, as an organosoluble metabolite in sorghum. This metabolite did not inhibit the Hill reaction and cyclic and noncyclic photophosphorylation in isolated pea chloroplasts. 

Good, N.E. (1961). Inhibitors of the Hill reaction. Plant Physiol., 36 788-803. 

Wessels, J.S.C. and R. van der Veen (1956). Action of some derivatives of phenylurethan and of 3-phenyl-1,1-dimethylurea on the Hill reaction. Biochim. Biophys. Acta, 19 548-549. 

Mi sc. herbicides Hill reaction inhibition with mixture of bean leaves (phaseolus vulgaris) and redox indicator 2,6 dichlorophenol 0.01 mg L [94]... 

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