VIII] 



AROMATIC COMPOUNDS 



123 



is two hydroxy! groups in the ortho position, found in plants, such as 

 catechol, protocatechuic acid, caffeic acid, hydrocaffeic acid, etc., 



COOH CH=CH— COOH 



/\ 



OH 



/\ 



OH 



OH 



OH 



Catechol 



OH 



Protocatechuic acid 



\/' 



OH 



CafFeic acid 



When solutions of such substances are left in air, they slowly aut- 

 oxidize with the production of brownish oxidation products, accompanied, 

 at the same time, by the formation of peroxides, probably hydrogen 

 peroxide (since organic peroxides . tend to decompose in the presence 

 of water with the production of hydrogen peroxide). The oxygen in this 

 form — O — O — can be detected by chemical tests in the solutions. In 

 plants, moreover, which contain catechol compounds, there are present 

 certain enzymes, the oxygenases, which catalyze the autoxidation of the 

 catechol compounds, and these only, with rapid production of a brown 

 colour and of a peroxide \ Since peroxidases are also universally present, 

 these may decompose the peroxide with production of active oxygen: 



catechol substance + oxygenase + molecular oxygen — ►peroxide 

 peroxide + peroxidase — »► active oxygen. 



This system, which constitutes an oxidase, is therefore capable of 

 transforming molecular into active oxygen, and may in this way bring 

 about oxidations in the plant which would not otherwise occur. 



Catechol substances with the accompanying oxygenases are only 

 present, as mentioned above, in about 63 °/o of the higher plants. The}^ 

 are present in about 76% of the Monocotyledons, in about 84^0 of the 

 Sympetalae but only in about 50 "/o of the Archichlamydeae examined. 

 Usually the genera of an order are all of one kind, either oxidase plants 

 or peroxidase plants without the oxygenase and catechol elements. A 

 few examples of oxidase orders are Gramineae, Umbelliferae, Labiatae, 

 Boraginaceae, Solanaceae and Compositae: of peroxidase orders, Liliaceae, 

 Cruciferae and Crassulaceae : of mixed orders, Ranunculaceae, Rosaceae 

 and Leguminosae. 



After death by inj ury, chloroform vapour, etc., the tissues of oxidase 

 plants usually turn brown or reddish-brown in air, e.g. fruit of Apple, 

 petals oi Anemone, Rosa, etc.; peroxidase plants, on the contrary, do not 



1 The term oxygenase was originally applied by Bach and Chodat to ferment like com- 

 pounds which form peroxides. 



