PURINES 287 



HN C= O HN-C= O CH 3 . N C= O 



0=C C N.CH, H 2 N C C NH O=C C N . CH 9 



n; 



HN C N 



H 



CH 



CH 3 . N C N HN C N CH 3 . N C N 



Theobromine, Guanine, Caffeine, 



or or or 



3, 7-dimethylxanthine. 2-amino, 6-oxypurine. i, 3, y-trimethylxanthine. 



It will be noticed that the complex or ring of atoms is the same in 

 all these compounds, but that according as the compounds contain 

 oxygen atoms attached to a carbon atom the double bond is changed 

 in position so as to keep the nitrogen and carbon atoms trivalent and 

 tetravalent respectively. The double bond between the carbon atoms 

 4 and 5 is the same throughout. 



URIC ACID. 



Scheele discovered uric acid in urinary calculi in 1776 and also 

 isolated it from urine. He made a careful investigation of its properties 

 and reactions, most of which are still used at the present time for its 

 identification. It was called lithic acid or ouric acid by Fourcroy in 

 1793, who showed that it contained urea. It was discovered in guano 

 in 1805 and in bird's excrement in 1815, and later it was shown 

 to be the chief constituent of snakes' excrement. Prout about this 

 time showed that uric acid on oxidation with nitric acid gave alloxan. 

 It was first analysed in 1834 by Liebig and Wohler and found to pos- 

 sess the empirical formula C 5 H 4 N 4 O 3 . These workers by oxidising uric 

 acid with lead peroxide obtained allantoine, which had been previously 

 found in the allantoic fluid of calves. Other ureides were obtained 

 by Strecker and by Baeyer who established their constitution. 



The formation of alloxan and urea, and of allantoine, by the oxida- 

 tion of uric acid, showed that uric acid contained 2 molecules of urea 

 and that it contained the structures of alloxan and allantoine in its 



molecule : 



N C 



C C C N\ 



U LN> 



Two formulae were put forward to represent the constitution of 



uric acid : 



HN CO HN C NH 



L 



NH\ 



I II > 



HN C NH/ 

 Formula 



of 

 Medicus. 



