THE PROTEINS 83 



Some clue to the size of the protein molecule is afforded by deter- 

 minations of the osmotic pressure or molecular concentration of 

 their solutions by physical methods. When we determine the freez- 

 ing-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 determine 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 corre- 

 sponds 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. These results, however, 

 must be received with caution, since we are not justified in applying 

 to these gigantic molecules data derived from a study of smaller mole- 

 cules such as salt or sugar. Even if we accept these, determinations 

 of osmotic pressure as indicating the molecular weights I have just 

 quoted, it is evident that a very slight degree of aggregation of the 

 molecules into larger complexes will bring the osmotic pressure below 

 the point at which it is measurable, and would transform the solution 

 into a suspension of particles in which one could not expect to find any 

 osmotic pressure whatsoever. 



THE STRUCTURE OF THE PROTEIN MOLECULE 



We can arrive at some idea of the manner in which the protein 

 molecule is built up only by breaking it down bit by bit, employing 

 methods which, while resolving the large molecule into its proximate 

 constituents, will not act too forcibly in changing the whole arrange- 

 ments of these constituents. The relation of the starches or poly- 

 saccharides to the sugars was found by studying the hydrolysis of the 

 former, and it is by the hydrolysis of the proteins that we have arrived 

 at most of our present knowledge of their constitution. Contributory 

 evidence may also be gained by the use of oxidising agents or by 

 employing the refined methods of analysis possessed by certain 

 living organisms bacteria, by which means we can effect limited 

 oxidations or reductions or can replace an NH 2 group by H, or a COOH 

 group by H. 



ACID HYDROLYSIS OF PROTEINS. For this purpose rather 

 stronger acids are used than for the hydrolysis of starch. The pro- 

 tein is heated for twenty-four hours in a flask fitted with a reflux 



