434 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Properties. 



The various histones vary considerably in properties. They contain about 

 18-19 P er cent, of nitrogen ; some contain sulphur, others do not. 



They are basic and are intermediate between albumins and globulins and 

 the protamines, yielding on hydrolysis a larger proportion of arginine than 

 albumins and globulins, but less than protamines. 



Their properties resemble in part the coagulable proteins, in part the pro- 

 tamines and in part the proteoses. 



A neutral solution, free from salt, is precipitated by ammonia, but accord- 

 ing to Bang the histone from red blood corpuscles is soluble in excess, but 

 comes down on saturating the solution with ammonium sulphate. Histones are 

 also precipitated by caustic alkalies and alkaline earths. Towards nitric acid 

 they behave like the proteoses. They are precipitated by the alkaloida re- 

 agents also in neutral solution. Like the protamines they are precipitated 

 by albumin and primary proteoses. They are not coagulated by heat, but 

 coagulation occurs in the presence of salt. The coagulum dissolves in dilute 

 hydrochloric acid. 



Globin. 



Globin, the protein moiety of the conjugated protein, haemoglobin (p. 

 472), has been considered to be a histone, though in its properties it has many 

 points of difference. 



Preparation. 



A solution of haemoglobin is dialysed to remove salts and treated with 

 very dilute hydrochloric acid 20 c.c. of *iN HC1 to 200 c.c. of haemo- 

 globin solution containing 1*84 gm. (Gamgee and Hill) until a flocculent 

 brown precipitate which forms is just redissolved. One fifth of the volume 

 of 80 per cent, alcohol is added and the solution shaken several times with 

 half its volume of ether. The clear aqueous solution on neutralisation gives 

 a flocculent precipitate. It is rapidly filtered off, washed with water, dis- 

 solved in dilute acetic acid and dialysed. The globin is precipitated on 

 adding alcohol. 



Properties. 



Globin dissolves in water and it differs from histones in that the neutral 

 solution, in the absence of salts, gives a precipitate which is readily soluble in 

 excess of ammonia, and that ammonium chloride only precipitates it when a 

 large excess of ammonia is not present. 



Globin gives most of the colour reactions of proteins ; it is not precipitated 

 by most of the heavy metals. 



It contains about 17 per cent, of nitrogen of which 29 per cent, is in the 

 form of diamino acids. In this respect it resembles the histones, but the 

 chief diamino acid is histidine, not arginine. 



