Fungicides Oxine

S-hydroxyquinoline, oxine, C9H7ON. Light brown needles, m.p. 15-16 C. Forms insoluble complexes with metals. The solubilities of the derivatives vary with pH, etc. and hence oxine is widely used in analysis. Used for estimating Mg, Al, Zn and many other metals. Many oxinates are extracted and the metal is estimated spectrophotometrically. Derivatives, e.g. 2-meIhyl tend to be specific, for, e.g.. Copper derivatives are used as fungicides. 

The range of action of carboxamides can be extended also to fungus species less sensitive to them by combination with protective fungicides, which also produce a partial synergistic effect in the combined seed-dressings. Copper oxine, chloroneb, mancozeb and other fungicides are used in combination with carboxamides (Richard and Vallier, 1969 Ranney, 1972 Baicu and Nagler, 1974). 

Powell (1946) and Mason (1948) found that the 2 1 complex of 8-hydroxyquinoline with copper, oxine-copper, has a stronger fungicidal action than 8-hydroxyquinoline. Albert et al. (1953) attributed this to the fact that the organic part of the compound makes copper lipoid-soluble and thus accelerates its penetration into the cell. Inside the cell the 2 1 complex dissociates into a 1 1 complex and free 8-hydroxyquinoline. The active toxic agent is the ionised 1 1 complex of nonlipoid properties, which reacts with the enzymes in the fungus and blocks their function. This theory is also supported by the research work of McNew and Gershon (1969). 

The antimicrobial activity of chelate formers bases partly on their ability to compete for the complexion of metal cations necessary for a functional cell metabolism. However, membrane-activity of the compounds also plays a role. Antimicrobial agents which form chelates include besides the already mentioned azole fungicides 2-mercaptopyridine-N-oxide (pyrithione) [II, 13.1.3.] (Cooney and Felix, 1972 Chandler and Segel, 1978 Khattar et al., 1988), 8-hydroxyquinoline (oxine) [II, 13.3.](Albert et al, 1947 Albert, 1968), and dithiocarbamates [II, 11.11.], e.g. zineb, thiram (Ludwig and Thorn, 1960). 

Other chelating antimicrobials have been found that, while having a totally different structure, mimic the action of oxine by being active only in the presence of a variable-valence metal, and hence show concentration quenching. Such a substance is i-hydroxypyridine-2-thione (pyrithione), (2.2j) (Albert, Rees, and Tomlinson, 1956), which is much used in the dermatology of the scalp. Another example is dimethyldithiocarbamic acid (2.2 ), whose salts are widely used as selective fungicides in agriculture. 

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