128 LECTURE VII. 



hemoglobin contains one hematin molecule and one molecule of globin, 

 an assumption for which adequate proof is lacking. A percentage of iron 

 of 0.4-0.5 per cent indicates a molecule of 14,000-11,200; a sulphur con- 

 tent of 0.43-0.67 per cent gives a molecule of 14,899-9500; and 4-5 per 

 cent of hematin J one of 14,800-11,800. 



Another method for estimating the molecular weight of proteins depends 

 on the formation of metallic compounds. 



Harnack 2 has shown that many proteins can be precipitated from 

 solution by means of copper sulphate. We obtain a precipitate contain- 

 ing copper, called copper albuminate. Harnack obtained the following 

 amounts of copper in the precipitates from egg-albumin: (I) 1 . 34-1 . 37 per 

 cent Cu, and (II) 2 . 48-2 . 73 per cent Cu. Two different copper albumi- 

 nates were formed therefore. We are not acquainted with the conditions 

 governing the formation of one or the other compound. Copper albumin- 

 ate (I) would have a molecular weight of 4700, while the second compound 

 probably has the same value, if the assumption of Harnack is correct, 

 that both albuminates represent the same protein substance, differing 

 only in the fact that the first possesses one atom, while the second has 

 two atoms of copper in the molecule. 



Other metallic albuminates, such as those with silver, calcium, etc., 

 have been prepared. It is difficult to state whether these are salt-like 

 compounds, or not. Recent investigations on the colloids have indicated 

 the necessity of being extremely cautious in passing judgment on such 

 compounds. Zsigmondy 3 has called attention to a peculiar property of 

 a colloidal gold solution, in the presence of albumin. A pure gold solution 

 is coagulated by an addition of electrolytes; for instance, sodium chloride. 

 If, however, albumin is present, the precipitation does not occur. The 

 albumin protects the colloidal gold. Fr. N. Schulz and Zsigmondy 4 have 

 found it possible to express the degree with which each individual albu- 

 min protects the colloidal gold numerically. Thus globulin, under certain 

 conditions, can protect twenty times its weight of gold. If an albumin 

 solution, mixed with a gold solution, is precipitated, the gold is dragged 

 down. A homogeneous red precipitate is obtained. If the globulin be 

 redissolved, the gold will likewise go into solution. It can easily be seen 

 how such a behavior might lead one to believe that there is a compound 

 of albumin and gold, and thus to erroneous conclusions. It is of great 

 interest to know that crystallized egg-albumin also takes up gold, and 

 that the mixture can then be recrystallized. Copper, iron, calcium oxide, 

 etc., can also be held in colloidal solution by means of albumin. 



1 Fr. N. Schulz: Z. physiol. Chem. 24, 449 (1898). 



2 E. Harnack: Z. physiol. Chem. 5, 198 (1881). 



3 R. Zsigmondy: Z. anal. Chem. 40, 597 (1901). 

 * Hofmeister's Beitrage, 3, 137 (1902). 



