118 



R. MARKHAM 



thus showing that the material was actually incorporated into the nucleic 

 acid (the nucleoside 2'- and 3'-phosphates were isolated, showing that the 

 nucleotide was originally present in diester linkage). Matthews had been 

 working on the inhibition of virus multiplication which this compound caused. 

 It is very inhibitory to the cucumber mosaic virus and to the alfalfa mosaic 

 virus, and it has a lesser, but still marked, effect on other virus infections, 

 including infection with the tobacco mosaic virus. Virus isolated from plants 

 treated by spraying with the analog (IQ-^M) was hydrolyzed, using KOH, 

 and the nucleotides liberated were separated by electrophoresis and by 

 chromatography (Matthews, 1954). The separation is facilitated by two 

 factors. First, azaguanine and its compounds fluoresce strongly and so may 

 be detected in minute quantities; second, the dissociation constants of the 

 groupings in the analog are changed, so as to make the compounds derived 

 from azaguanine both more acidic and weaker bases than are the natural 

 derivatives. The sensitivity of the detection procedure is such that 0.3 /xg. of 

 azaguanine (Formula VI) may be detected in a mixture, so that the use of 

 radioactive labels is unnecessary. 



,N 



N 



NH, 



\ 



\, 



N 



V 



I 



H 



I 

 H 



8-Azaguanine 



(VI) 



The similarity of structure between azaguanine and guanine is obvious, 

 and, if anything, it is surprising that more of the compound is not incorporated 

 into the virus nucleic acids. In the tobacco mosaic virus some 3-4 % of the 

 guanine residues may be replaced, but this results in only a very minor 

 reduction in the infectivity of the virus produced. In the turnip yellow 

 mosaic virus even less of the unnatural compound is incorporated (Matthews, 

 1955) and the infectivity is reduced to about half that of the control virus. 



One of the main deficiencies of azaguanine as an unnatural agent is its 

 very short half-life. Almost all tissues of plants (and animals) contain an 

 active guanase (guanine deaminase), which converts the compomid to the 

 inactive azaxanthine. This is probably one of the reasons why such high 

 concentrations {10~^M) are needed for any appreciable biological effect 

 to be manifested. However, in systems such as Bacillus cereus in which 



