198 



PLANT BIOCHEMISTRY 



HO- 



OH 



-OH 



cyanidin 



ci- 



HO- 



,^^^o+. / 



V^ 



o 



H 



OH 



OH 

 —OH 

 OH 



delphinidin 



involving positions 5 and 7, are found in different species. Glucose is 

 the most common sugar, although mixed diglycosides occur. Usually 

 the alternate sugar is L-rhamnose. 



The colors of these pigments are sensitive to pH changes. For ex- 

 ample, pelargonidin and cyanidin are red in acid solution and blue 

 in an alkaline solution, whereas delphinidin follows a reverse pat- 

 tern. In some species, such as the hydrangea, it is possible to control 

 the color of the flower by adjusting the soil pH. Additional color 

 variations are obtained by substituting methoxyl (CHg — O — ) for hy- 

 droxy! groups in positions V and/or 5', and sometimes in the 7 posi- 

 tion. Hirsutidin, the coloring agent in blue grapes, is the 4-hydroxy- 

 3'-5'-7-trimethoxy-2-phenylbenzopyrylium salt. Flavones, the water- 

 soluble yellow and orange pigments, are derivatives of 2-phenylben- 

 zopyrone. 



2-phenylbenzopyrone 



Again, as in the anthocyanins, the members differ in the degree of 

 hydroxylation. The flavonols have the basic structure of the flavones 

 with the addition of a hydroxyl group in the 3 position. Generally 

 flavones and flavonols carry hydroxyl groups in the 5 and 7 positions. 



Citronetin, the coloring agent of the citrus peel, is shown as an 



