NUCLEIC ACIDS AND RELATED COMPOUNDS 233 



Deoxyribonucleic acid from wheat germ and the leaves of several plants has been reported 

 to contain, in addition to the expected bases, 5-methylcytosine (2, 3). 



Some of these free bases have been found in higher plants although they may be arti- 

 facts arising from decomposition of nucleic acid. Adenine is probably the most widespread, 

 followed by guanine. Free thymine has been found in EqidsetiDii palustre (4). Other pu- 

 rines and pyrimidines which are not components of nucleic acid have been found to occur 

 free in some plants although they are certainly not widespread in the plant kingdom. As 

 nitrogen -containing compounds they are often classed with the alkaloids, but biosyntheti- 

 cally they are presumably closely related to other purines and pyrimidines. Structures 

 of some of these compounds are given in Table 1 with selected natural sources. 



Although uric acid is usually regarded as an end product of purine metabolism in 

 animals, it has been found in several different plants. In many animals uric acid is fur- 

 ther broken down to allantoin and then allantoic acid. These two compounds are also 

 rather widely distributed in plants. They seem to function as major nitrogen storage com- 

 pounds in some trees (5). Although not purines, the structural relationship of these two 

 to the purines is obvious, and tracer experiments have shown the conversion of adenine to 

 allantoin, allantoic acid and urea in Acer sacchariniun leaves (6). 



HO NHj 

 NH2 V I 



A.A 



c=o 



H H 



allantoin allantoic acid 



The free purines and pyrimidines are colorless, crystalline compounds. Many of them 

 are only very slightly soluble in water but all are readily soluble in either dilute acid 

 (e. g. guanine) or alkali (e. g. uric acid). They are generally rather insoluble in organic 

 solvents. Their characteristic ultra-violet absorption spectra are discussed below under 

 "Characterization." Long heating with acid or alkali (as in hydrolysis procedures) may 

 cause some decomposition, but generally the purines and pyrimidines are sufficiently 

 stable so that losses are small. 



Little is known regarding the possible functions of the free purines and pyrimidines 

 in plants. Some may serve as nitrogen storage compounds. Others show pronounced 

 morphological effects when applied to plants artificially and may act in such ways under 

 natural circimustances (7, 8). 



NUCLEOSIDES 



The nucleosides have a sugar molecule bound by a /3-glycosidic bond to position-3 

 of the pyrimidines or position-9 of the purines. Deoxyribonucleosides occur only as break- 

 down products of DNA or of nucleotides, but a few ribonucleosides apparently occur as 



such in plants e.g. adenosine and guanosine (vernine). Some of the nitrogen bases 



found in specific nucleosides are never found as components of any other compound. The 

 vitamin, riboflavin, is usually included with the nucleosides although, strictly speaking, 

 it contains a ribityl moiety related to D-ribitol rather than a ribosyl moiety derived from 

 D-ribose. Structures and occurrence of some of the natural free nucleosides are given 

 in the following pages. A review on the chemistry and natural occurrence of pyriniidine 

 nucleosides has recently appeared (9). 



