184 GUANIN. 



which case it is also found in many of their tissues 1 ; additionally in 

 the retinal tapetum of fishes and in their scales 2 , as also in the 

 integument of amphibia and reptiles 3 and in vegetable tissues 4 . 



It is a white amorphous powder, insoluble in water, alcohol, ether 

 and ammonia. Its insolubility in the latter distinguishes it from 

 xanthin and hypoxanthin. It unites with acids, alkalis and salts to 

 form crystallisable compounds. Of its compounds with acids the most 

 characteristic are those witli hydrochloric and nitric acids. 



The compound with nitrate of silver is extremely insoluble in strong 

 boiling nitric acid. 



Reactions. By treatment with nitric acid and caustic soda 

 (Strecker's test) it yields a colouration closely resembling that given 

 by xanthin, but does not respond to Weidel's test (see above, p. 177). 



Capranica's reactions 5 , (i) A yellow crystalline precipitate on the 

 addition of a saturated aqueous solution of picric acid to a solution of 

 guanin-hydrochloride ; insoluble in cold water, (ii) An orange- 

 coloured crystalline precipitate, very insoluble in water, on the addition 

 of a concentrated solution of potassium chromate. (iii) Prismatic 

 yellowish-brown crystals on the addition of a concentrated solution of 

 ferricyanide of potassium. Xanthin and hypoxanthin when similarly 

 treated do not yield the last two precipitates. 



By treatment with nitrous acid guanin may be readily converted 

 into xanthin. (Cf. adenin into hypoxanthin by similar treatment.) 

 By oxidation it yields guanidin NH :C(NH 2 ) 2 , parabanic acid (see 

 above, p. 171) and carbonic anhydride, a decomposition which obviously 

 corresponds to the formula given above for guanin. 



C 5 H 5 N 5 + 3 . O + H 2 = NH : C (NH 2 ) 2 + C 3 H 2 N 2 3 + C0 2 



'2' 



8. Guanidin. CN 3 H 5 . NH 2 



Although this substance does not occur in the free state in any 

 tissue or fluid of the animal body, it is of considerable interest, for it 

 has been obtained by the direct oxidation of proteids (p. 161) and may 



1 Pecile, Ann. d. Chem. u. Pharm. Bd. CLXXXIII. (1876), S. 141. Cf. Salomon, 

 Arch. f. Physiol. 1884, S. 175. Arch. f. Path. Anat. Bd. xcv. (1884), S. 527. 

 Virchow, Arch. f. path. Anat. Bde. xxxv. (1866), S. 358, xxxvi. S. 147. 



2 Kiihne u. Sewall, Unters. a. d. physiol. Inst. Heidelb. Bd. ra. (1880), S. 221. 



3 Ewald u. Krukenberg, Ibid. Bd. iv. Hft. 3. (1882), S. 253, Zt. f. Biol Bd. xix. 

 (1883), S. 154. 



4 Schulze u. Bosshard, Zt. f. physiol. Chem. Bd. ix. (1885), S. 420. 



5 Zt. f. physiol. Chem. Bd. iv. (1880), S. 233. 



