THE THYROID AND PARATHYROIDS 313 



The change to the open ring form is effected easily in presence 

 of certain products of hydrolyzed protein which contain 

 the indole nucleus. 



(1) The keto form (formula 1) crystallizes in needles and is 

 tasteless and odourless. It is found in the keto form in acid 

 solution. It is insoluble in organic solvents unless they are acid 

 or basic. It is soluble in alcohol in the presence of mineral 

 acid or an alkali. It is not destroyed by heating. The melting 

 point is 250 C. Mol. Wt. = 585, and empirical formula 

 CuH 10 O 3 NI ;l ( c f. tryptophane C n H 12 2 N 2 ). It is amphoteric, 

 It can yield a sulphate, a hydrochloride, a ureide and an 

 acetyl derivative. It gives also mono- and di-basic salts, 

 but these are so easily decomposed that they cannot be 

 obtained in a pure form. 



(2) The enol form (formula 2) exists in alkaline solution. 

 It may be prepared by hydrolysis in the cold of the ammonium 

 salt, solution in pyridine and addition of water, when it 

 separates out in needles (melting point 204 C). It is easily 

 soluble in anhydrous or aqueous pyridine and quinoline and 

 in formic acid. It is easily changed into the keto form. 



(3) The open-ring form (formula 3) is obtained by adding 

 sulphuric acid to an alkaline watery solution of thyroxin. The 

 precipitate which is formed is suspended in water and boiled. 

 Thyroxin is then thrown down in long crystals (melting -point 

 225 C.). It is soluble in alcohol, changing in solution to the 

 keto form. 



A fourth (amino -hydrate) form (formula 4) may be obtained 

 from an alkaline solution of thyroxin by heating and adding 

 10 per cent, ammonium chloride. Fine crystals with a melting- 

 point of 216 C. separate out. If these are suspended in water 

 acidified with formic acid and boiled, the crystals are changed 

 into the open-ring form. 



If nitrous acid be added to an alcoholic solution of thyroxin or 

 to a suspension in water in presence of hydrochloric acid a 

 yellowish colour is developed which, on addition of ammonia, 

 changes to deep red. This reaction serves as a test for thryoxin. 



Thyroxin is more susceptible to reduction than to oxidation. 

 Zinc and other metals in acid or alkaline solution split off iodine 

 and break up the nucleus. Mild oxidizing agents have no 

 effect, stronger ones break down the molecule. Thyroxin is 

 unstable in sunlight ; iodine is split off as hypoiodous acid and 



