50 UNITY AND DIVERSITY IN BIOCHEMISTRY 



Substances of this nature have a function in the transfer of metabolic 

 hydrogen in plants. In the process, hydrogen peroxide formed by the 

 combination of hydrogen and oxygen oxidizes the flavone derivative in the 

 presence of peroxidase, and in turn the oxidized flavone oxidizes a molecule 

 of ascorbic acid. 



This function is only possessed by derivatives having two hydroxyls in 

 positions 3 and 4 in ring 3, as is the case for quercetin and its glycoside 

 quercitrine, in which the glucose molecule is combined with the hydroxyl 

 group of ring 1. Hesperidin is inactive. 



However, in the lemon when it is ripe, a demethylation of hesperidin 

 takes place and eriodictyol is formed and it is the glucoside of this latter 

 substance which is chiefly found in the ripe fruit. 



OH O 

 /V^CH, OH 



"'^VAo-""^^^" 



Eriodictyol 



The anthocyanins, the blue, red or violet pigments found in many 

 flowers, are glucosides in which there is a six-membered oxygen-containing 

 ring. Their aglucones (non-sugar portion) or anthocyanidins are hydroxy- 

 lated derivatives of a benzopyrilium nucleus : 



H H 



C C , 



HC C CH 



I II I 



HC C CH 



% y ^ <^ 



C 0_j- 



H 

 in which oxygen is the central atom of a complex monovalent ion whose 

 coordination number is three. Certain anthocyanidins have a structure 

 similar to that of flavone, for example, this is the case with oenin, the 

 anthocyanidin present in the skin of the black grape. 



H 



H 

 HO — C. C^ 



I II I ^^ ^ — O— CH3 



HO— C C C C^ ^C — OH 



% ^ ^ ^ C=C— OH 



C 0+ ^ '^ UH 



H CI- 



Oenin chloride 



