232 CHEMISTRY OF THE PROTEIDS CHAP. 



In addition to the above, other albumins with a much higher 

 iodine-content also exist, the halogen being partly in the above de- 

 scribed firm union and partly in a very loose combination. To this class 

 belongs the periodo-casein of Liebrecht with 17 '8 per cent of iodine. 



The iodine-compounds are believed to be formed in the following 

 manner : In the aromatic groups of the albumin-molecule, i.e. in 'the 

 tyrosin, tryptophane, and phenylalanin radicals, iodine is substituted 

 for several of the hydrogen atoms. By employing the same method 

 of iodising, as described above, Oswald prepared from tyrosin a tri-iodo- 

 tyrosin. That phenylalanin takes up iodine as well as does tyrosin, 

 Oswald proved by iodising hetero-albumose and gelatine, for these two 

 substances do not contain either tyrosin or tryptophane. He proposes 

 to make use of the' iodine-number, which is readily determined, for 

 estimating the amount of aromatic groups present in an albumin. This 

 is permissible, because, as far as we know, no other groups but the 

 aromatic ones will unite with iodine. The negative results obtained 

 with the reactions of Millon and Adamkiewicz are not to be explained 

 on the ground that the hydroxyl- group in tyrosin has been sub- 

 stituted, for Blum and Vaubel have shown that tyrosin and other 

 substances with similar constitution do not give Millon's reaction, if 

 halogens have been substituted in both ortho- or in both meta- 

 positions. Millon's reaction is obtained again if the halogen is split 

 off under a pressure of 5 to 6 atmospheres. The iodo-albumins 

 behave in exactly the same way. 



Little is known as to the other changes which are produced in 

 proteids by iodisation, but distinct differences are induced according 

 to the temperature used, and according to the duration of iodisation. 

 A certain amount of disintegration of albumin is, however, unavoidable 

 when using the methods of Blum and Hofmeister, even if the greatest 

 care is taken by iodising at 40, and by keeping the alkaline reaction 

 as feeble as possible. The formation of hydriodic acid, HI + H 2 0, 

 speaks in itself for an oxidation of albumins ; according to Vaubel 1 

 and Schmidt the amounts of iodic and bromic acids formed give 

 iodine- and bromine-numbers, which are quite characteristic for the 

 different albumins. The negative result obtained with the lead 

 sulphide reaction shows that the sulphur must have become oxidised 

 or have been changed somehow. Schmidt observed that the splitting 

 off of amino-groups differed with individual albumins ; he also noticed 

 the formation of iodoform, formic, acetic, and carbonic acids. The 

 ratio C : N is unchanged in serum - albumin, while it is greatly 



1 W. Vaubel, Zeitschr, f. analyt. .Chem. 40. 470 (1901) ; according to Chem. 

 Zentralbl. 1901, II. p. 711. ' 



