



THE PROTEINS 75 



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 1 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,CCO. 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 

 molecules 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 arrangements of these constituents. 

 The relation of the starches or polysaccharides to the sugars was found by 

 studying the hydrolysis of the former, and it is by the hydrolysis of the pro- 

 teins that we have arrived at most of our present knowledge of their con- 

 stitution. 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 protein is heated for 

 twenty-four hours in a flask fitted with a reflux condenser either with con- 

 centrated hydrochloric acid or with a 25 per cent, sulphuric acid. Hydro- 

 chloric acid was first made use of by Hlasiwetz and Habermann, who added a 

 certain amount of stannous chloride to the mixture in order to prevent any 

 oxidation taking place. We obtain in this way an acid fluid containing an 

 extremely complex mixture of various substances, all of which belong to the 



