158 CHEMICAL STATICS 



have been specially studied by Hopkins (45) (46) (47) and Blum 

 and Vaubel (20). The following were the percentages of the 

 different halogens found by Hopkins and by Blum in firm com- 

 bination with egg-albumin.* 



As in the case of the iodo-proteins, two series of bromine com- 

 pounds exist: the one in which the bromine is firmly bound and 

 the percentage of bromine is low, the other in which a part of 

 the bromine is loosely bound and the percentage is high. Hop- 

 kins and Brook found in this latter type of brominated egg- 

 albumin 14.89 per cent of bromine. These compounds, as also 

 the chlorinated egg-albumin, are soluble in hot absolute alcohol, 

 the halogen-protein being precipitated from this solution by the 

 addition of ether (Hopkins and Brook). 



The best-known naturally occurring halogen-protein is the 

 thyreoglobulin of the thyroid gland. It contains only 1.75 per 

 cent of iodine (91). The physiological action of this protein 

 does not appear to be primarily dependent upon its iodine con- 

 tent (108) and it is doubtful whether this is the physiologically 

 active iodine-containing principle of the thyroid (56) . An iodized 

 keratin, gorgonin, is found in corals (26) (40) (79) (81). It 

 contains a high (about 8) percentage of iodine. Sponges also 

 contain iodo-proteins (36) (52) (115). 



Nitro substitution-products of the proteins were first prepared 

 by Loew (73). VonFtirth (29) prepared them by acting upon 

 the protein with nitrous acid, at the same time adding urea to 

 prevent the formation of nitric acid. He obtained a product con- 

 taining 1.78 per cent of N0 2 . The nitro derivatives of the pro- 

 tamins have been very extensively studied by Kossel (65) (64) 

 (67) (68) (119), who believes that the nitration of protein leads 

 to the entry of N0 2 into the guanidine radical of the arginin. 

 The nitrated proteins therefore yield nitroarginin on hydrolysis. 

 On treatment with alkali the nitro-guanidine radical breaks up 

 yielding carbon dioxide, nitrous oxide and ammonia. 



* Cited after Gustav Mann (76). 



