122 AROMATIC COMPOUNDS [ch. 



(see Expt. 110). Acidify with a little strong hydrochloric acid and heat on a water- 

 bath in an evaporating basin. Add a little zinc dust from time to time. A brilliant 

 pink or magenta colour due to allocyanidin is produced. To a little of this solution 

 add some alkali : a green colour is produced. If the alcohol and hydrochloric acid 

 are evaporated off, and a little water and sulphuric acid added, on shaking up with 

 amyl alcohol, all the allocyanidin passes into the amyl alcohol. (The distribution of 

 the allocyanidin in the amyl alcohol is greater with aqueous sulphuric acid than with 

 aqueous hydrochloric acid.) 



Expt. 116. Formation of allocyanin from quercetin. Make a suspension of the 

 glucoside of quercetin from Cheiranthus or Narcissus (see Expt. 110) in about 2N 

 sulphuric acid, and then add zinc dust (or a drop of mercury about the size of a pea 

 and a little magnesium powder) in the cold. The pink or magenta colour is gradually 

 developed. Divide the coloured solution into two parts in two test tubes. Boil one 

 for 5-10 minutes. Then add amyl alcohol to each. In the unboiled test-tube the 

 amyl alcohol extracts no colour, since allocyanin is present. In the boiled test-tube 

 allocyanidin is taken up by the amyl alcohol as in Expt. 115. 



Expt. 117. Formation of allocyanin and allocyanidin from plant extracts. For 

 this purpose the yellow varieties "Primrose" or "Cloth of Gold" of the Wallflower 

 {Cheiranthus Cheiri) can be used. The flowers are pounded in a mortar, extracted 

 with cold water, the water extract acidified with sulphuric acid, and zinc dust (or 

 mercury and magnesium powder as above) added. A red coloration is slowly 

 developed. To some of the red solution add amyl alcohol. The colour is not 

 abstracted (allocyanin). Boil another portion. The allocyanin is thus converted into 

 allocyanidin which is then taken up on addition of amyl alcohol. 



Oxidizing Enzymes. 



There are certain enzymes in the plant which are concerned with 

 processes of oxidation and reduction (Chodat, 1). They are considered at 

 this point since we have most information of them in their connexion 

 with aromatic substances. 



Peroxidases. A peroxidase is practically always present in the tissues 

 of the Higher Plants. These enzymes are able to decompose hydrogen 

 peroxide with the formation of "active" or atomic oxygen: 

 H2O2 -f peroxidase = H2O -f O. 



The tests for peroxidases will be considered later. 



Oxidases (synonymous with laccases or phenolases) are only present 

 in about 63 "/o of the Higher Plants. A plant oxidase, moreover, is made 

 up of three components, i.e. (1) an enzyme, termed an oxygenase, (2) an 

 aromatic substance containing an ortho-dihydroxy grouping such as that 

 in catechol and (3) a peroxidase as above described (Wheldale Onslow, 9). 



There are a number of substances with the catechol grouping, that 



