82 PHYSIOLOGY 



contain two atoms of sulphur, and we must regard double the molecular 

 weight given in this table as the minimum molecular weights of these 

 various proteins. Some idea of the molecular complexity repre- 

 sented by these weights may be gained by writing out the empirical 

 formulae of the various proteins, e.g., 



Egg albumin . C 204 H 322 N 52 66 S 2 



Protein in haemoglobin (from horse) . . C 680 H 1098 N 210 241 S 2 



Protein in haemoglobin (from dog) . . . C 725 H 1171 N 194 214 S 2 



Crystallised globulin (from pumpkin seeds) C 292 H 481 N 20 83 S 2 



With some proteins we may make use of other elements to arrive 

 at an idea of the approximate molecular weight. Thus, oxyha3nio- 

 globin contains between 0-4 and 0-5 per cent iron. If we assume 

 that each molecule of oxyhaBmoglobin contains one atom of iron, its 

 molecular weight must be from 11,200 to 14,000. 



Attempts have been made to solve the same question by studying 

 the compounds of proteins with inorganic salts or oxides. Thus, 

 the crystals of globulin from pumpkin seeds prepared with magnesia 

 contain 1-4 per cent. MgO. Assuming that one molecule of protein 

 has combined with one molecule MgO, the molecular weight of the 

 protein must be about 2800. 



(If # be the molecular weight 



x 100-1-4 



40 1-4 



.-.3 = 2817) 



Harnack has shown that many proteins are precipitated from 

 their solutions as a copper compound by the addition of copper 

 sulphate. Harnack found that this precipitate of copper contained 

 either 1-34 -1-37 Cu. or 2-48-2-73 per cent, Cu. The smaller 

 percentage would correspond to a molecular weight of 4700, while 

 the second number might be accounted for on the hypothesis that 

 each molecule of protein was combined with two atoms of copper. 

 Similar attempts have been made by determining the amount of acid 

 or alkali necessary to keep certain types of protein in solution. We 

 shall see later on, however, that the amounts vary largely with the 

 physical condition and previous history of the colloidal substance. 

 We are dealing here not with compounds in the strict chemical sense 

 of the term, but with adsorption compounds, where the quantities 

 taken up are determined not only by the chemical nature of the protein 

 itself, but by the state of aggregation of its molecules. It is therefore 

 impossible to lay any great stress on the determinations of the mole- 

 cular weight which have been effected in this way. 



