THE PROTEINS ; - 



MgO. Assuming that one molecule of protein has combined with , 

 molecule MgO, the molecular weight of the protein must be about 2800. 

 (If x be the molecular weight 

 x 100-14 

 40~ 14 



.'.a? = 2817) 



Harnack has shown that many proteins are precipitated from their 

 solutions as copper compounds by the addition of copper sulphate. Harnack 

 found that this precipitate of copper contained either 1-34 1-37 Cu. or 

 248 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 aggrega- 

 tion of its molecules. It is therefore impossible to lay any great stress on the 

 determinations of the molecular weight which have been effected in this way. 



Some clue to the size of the protein molecule is afforded by determinations 

 of the osmotic pressure or molecular concentration of their solutions by 

 physical methods. When we determine the freezing-point or boiling-point 

 of protein solutions, the depression of freezing-point, or elevation of boiling- 

 point is so small that it falls within the limit of experimental error or is 

 no greater than can be accounted for by the inorganic salts present in the 

 solution. Since, however, colloidal membranes, such as films of gelatin 

 or vegetable parchment, are impervious to proteins, we can directly deter- 

 mine the osmotic pressure of their solutions. In many cases no osmotic 

 pressure whatever is found. In other cases, e.g. egg albumin, or serum, the 

 colloidal constituents of these solutions are found to give an osmotic pressure 

 of such a height that 1 per cent, protein corresponds to about 4 mm. Hg. 

 pressure. Such an osmotic pressure would indicate a molecular weight for 

 the serum proteins of about 30,000. A determination of the osmotic pressure 

 of haemoglobin by Hiifner gave a molecular weight about 16,000. 

 results, however, must be received with caution, since we are not justi 

 in applying to these gigantic molecules data derived from a study of a 

 molecules such as salt or sugar. Even if we accept these deterrmr 

 osmotic pressure as indicating the molecular weights I have just quo 

 evident that a very slight degree of aggregation of the molecules 

 complexes will bring the osmotic pressure below the point , 

 measurable, and would transform the solution into a suspend 

 in which one could not expect to find any osmotic pressure what 



