HALOGEN AND NITRO COMPOUNDS 157 



9. The Halogen and Nitro Substitution-Compounds of the 

 Proteins. — Besides the combinations of the proteins with 

 inorganic substances which are formed through the ionization 

 of — N.HOC— groups, other types of combination between 

 proteins and inorganic substances are unquestionably possible. 

 Among these the best known are the substitution-compounds of 

 the proteins with the halogens and with NO2. 



According to Blum (18) (20) and to Hofmeister (43) (112) (41) 

 (85), the halogen compounds are formed by the replacement 

 of hydrogen atoms in the aromatic groups of the proteins by 

 halogen atoms. The best known examples, both artificially pre- 

 pared and naturally occurring, are the iodoalbumins. They can 

 be prepared (Cf. papers just cited) by allowing a mixture of 

 potassium iodate and potassium iodide to act upon proteins at 

 a moderately high temperature (40 degrees). Hopkins and 

 Brook (46), however, prepared iodoproteins by allowing powdered 

 iodine to act directly on the protein, in solution. When the 

 former method of preparation is employed, part of the iodine 

 enters into combination with the protein and part is liberated 

 as HI. Hence excess of a carbonate, of an alkali, or of an alka- 

 line earth must be added to the mixture to neutralize the hydro- 

 gen iodide set free, and prevent it from hydrolysing the protein, 

 through the catalytic action of hydrogen ions. The iodo-proteins 

 are, in the dry condition, similar in appearance to ordinary pro- 

 teins; they are insoluble in water or alcohol, and they appear 

 to be more predominantly acid than the non-iodized proteins 

 from which they are derived, since they are readily soluble in 

 dilute solutions of alkahes or of alkaline carbonates, but are 

 soluble in acid only in the presence of a considerable excess. 



The percentage of iodine contained in the iodo-proteins varies, 

 as might be expected, with the nature of the protein, — it also 

 appears to vary somewhat with the mode of preparation or 

 subsequent treatment, since two types of compounds are formed, 

 the one containing a very high percentage of iodine, partly in 

 firm and partly in loose combination (the per-iodo-casein of 

 Liebrecht (70)), and the other a lower percentage of iodine en- 

 tirely in firm combination. 



Improved methods of preparing iodized proteins have recently 

 been described by Oswald (92) . 



The bromine and chlorine substitution-products of the proteins 



