66 THE GENERAL CHARACTERS OF THE PROTEINS 



iodo derivative some carbon complex which is poor in nitrogen has 

 been eliminated from the molecule ; this Hofmeister assumes to be 

 a carbohydrate group. 



Again, Blum and Vaubel, by their method of forming these 

 halogen derivatives in the presence of alkalis, bring some evidence 

 to show that a separation of some sulphur body takes place, although 

 this probably does not happen when other methods are employed. 



Another significant fact with regard to the properties of the 

 halogen derivatives is their failure to give a positive result when 

 tested by the Millon and Adamkiewicz (Hopkins) reaction. This 

 fact indicates either that the aromatic groups are so substituted by 

 halogen that they fail to give these reactions, or that they are en- 

 tirely destroyed. 



Investigations on the groups that have been eliminated during 

 halogenisation have been undertaken by Schmidt, who has isolated the 

 simpler bye-products of the reaction. He has shown that ammonium 

 iodide and ammonium iodate are always formed when the iodine deriva- 

 tives are prepared from egg-albumin by the Hofmeister reaction. This 

 indicates the scission of some amido group. Investigations with 

 simpler bodies, such as ammonium salts, urea, arginine, aspartic acid 

 and guanidine, showed that iodine is capable of causing the scission 

 of NH 2 groups from those bodies where the group is combined 

 through a carboxyl or imido group to the molecule, such as in urea 

 and in guanidine. In other bodies, such as aspartic acid, where the 

 NH 2 group is directly united to a carbon atom, no iodate could 

 be detected as a bye-product of the reaction. The reaction with 

 ammonium salts may be represented by the following equation : 



6(NH 4 ) 2 SO 4 + 61 + aH 2 O = 5NH 4 I + NH 4 IO 3 + 6NH 4 HSO 4 . 



It appears, therefore, from the somewhat limited number of Schmidt's 

 experiments, that the estimation of the free ammonium salts elimin- 

 ated may form a measure of the number of amido groups in the 

 protein molecule. The reaction is, however, somewhat complicated, 

 and the scission of amido groups cannot be directly measured by 

 the iodate eliminated, for, owing to concurrent oxidation, relatively 

 large quantities of hydriodic acid are formed simultaneously, and 

 this, when it reaches a certain concentration, reduces the iodate 

 according to the following equations: 



HI + NH 4 IO 3 = NH 4 I + HIO 3 . 

 HIO 3 + sHI = 61 + aH 2 O. 



The amount of iodate formed can, therefore, only reach a certain 

 limit. 



Other bye-products isolated by Schmidt are iodoform, carbonic, 

 formic and acetic acids, and possibly also para-iodopyrocatechin. 

 These have been estimated quantitatively, and their formation is 

 ascribed to the destruction of the tyrosine complex. Certain con- 

 clusions of Schmidt's later papers are not in full concordance with 

 those arrived at earlier, especially with reference to the formation 

 of iodates only by certain amino groups. 



Sufficient has been said to show how complex are the possible 

 reactions which can take place when proteins are treated with 

 halogens. Nevertheless, it is conceivable that certain standard con- 

 ditions for halogenisation can be fixed, by means of which halogen- 



