AROMATIC COMPOUNDS 65 



Solvents containing a good proportion of water seem to be most useful. Spots are revealed 

 by exposure to ammonia and examination in ultraviolet light, or by spraying with ferric 

 chloride solution. 



METABOLIC PATHWAYS 



Biosynthetic pathways of aromatic compounds in plants have been reviewed by Neish 

 (49). The present discussion is restricted to those aromatic compounds derived from 

 5-dehydroquinic acid. Other types of aromatic compounds will be found in Chapters 6 

 and 9. The pathways showin Figures 1 and 2 are at least probable for higher plants, al- 

 though certain details have been strictly established only for microorganisms. The steps 

 leading from carbohydrates to 5-dehydroquinic acid have been showin Figure 8 of Chapter 

 2. Only a few points will be made here in clarification of the figures: 



1. The conversion of 5-dehydroshikimic acid to protocatechuic acid and gallic 

 acid appears reasonable, and there is some experimental evidence to support 

 it, at least to indicate that gallic acid is formed between glucose and phenylala- 

 nine (50). 



2. It is now generally believed that the main pathway for forming cinnamic and 

 p-coumaric acids from phenylalanine and tyrosine respectively does not go 

 through the corresponding keto and hydroxy acids but occurs by a one-step 

 elimination of ammonia. Deaminases catalyzing these reactions have been 

 studied and the reactions found to be irreversible (51). Monocots have both 

 enzymes, but dicots have only phenylalanine deaminase and are therefore un- 

 able to make p-coumaric acid (or compounds derived from it) from tyrosine. 



3. Hydroxylation of the aromatic ring evidently must occur at several points in 

 the scheme, but the exact location of these points is not clear — i. e. o-coumaric 

 acid may first be hydroxylated and then go on to form hydroxycoumarins, or the 

 parent coumarin may be made first and then hydroxylated. Tracer experiments 

 have established that cinnamic acid fed to plants is readily hydroxylated in sev- 

 eral positions (52). Model experiments have shown that aromatic hydroxylation 

 may be non-enzymatic or could involve a peroxidase system (53). 



4. Several types of evidence (26) indicate that polygalloyl glucose is the parent 

 compound of many, if not all, the hydrolyzable tannins. Thus, it is believed 

 that the ellagitannins are derived by oxidative coupling of two molecules of 

 gallic acid which are already esterified to glucose, rather than by esterifica- 

 tion of the sugar with preformed hexaoxydiphenic acid. On the other hand, 

 Wenkert (54) has suggested that diphenyl and diphenyl ether systems may be 

 formed by carbohydrate -type condensation of hydroaromatic precursors rather 

 than by oxidative coupling of aromatic rings. 



5. Since the complete structure of lignin remains unknown, the exact mechanism 

 of its formation cannot be shown. Freudenberg (55) has suggested that the first 

 step is enzymatic removal of a phenolic hydrogen atom from coniferyl alcohol 

 to produce a free radical which can undergo non-enzymatic rearrangements 

 and reactions with other molecules leading first to dimers (of which lignans 

 are one type) and finally to lignin. The removal of hydrogen is a reaction which 

 may be catalyzed by phenol oxidase or peroxidase. Stafford (56) incubated leaf 

 sections of timothy grass with hydrogen peroxide. and various cinnamic acid 

 derivatives. Ferulic acid gave rise to a product resembling natural lignin. A 

 recent review on lignin biosynthesis is by Brown (57). 



6. Freudenberg and Grion (58) suggest that the well-known binding of lignin to car- 

 bohydrate in cell walls may come about by coupling of one of the free radical 

 intermediates mentioned above with the hydroxyl group of a carbohydrate to 

 form an ether bond. 



