PROTEINS 75 



Molecular Weight. The molecular weight of the proteins is 

 very high. Estimation of the molecular weight has been at- 

 tended by great difficulties, because of the practical impos- 

 sibility of obtaining pure proteins, and because of the unstable 

 nature of the proteins themselves. The molecular weight of 

 these compounds is so great that they cause only a very slight 

 lowering of the freezing point, so that this method of determin- 

 ing molecular weight has given results of questionable reliabil- 

 ity. The boiling point method cannot be used, as proteins coag- 

 ulate on heating. Various other methods have been devised, 

 however, and as fairly uniform results are obtainable by differ- 

 ent methods of analysis, we may draw fairly accurate conclu- 

 sions as to the molecular weights of members of the group. One 

 of the methods employed consists in estimating the sulphur con- 

 tent of the protein. If the substance contains 0.5% sulphur 

 then sulphur makes up 1/200 of the protein, and the molecular 

 weight will be 200 times that of the sulphur present. If there 

 are two sulphur atoms in the protein, they have a molecular 

 weight of 64 (2X32). The molecular weight of the protein will 

 thus be 200X64=12,800. Calculated on this basis most of the 

 proteins will have a molecular weight of from 14,000-16,000. 

 Calculating the molecular weight in other ways, such as from 

 the amount of oxygen some of the proteins will take up, gives 

 practically identical figures. More direct methods (measure- 

 ments of osmotic pressure, etc.) confirm these figures, so that 

 we are justified in assuming the molecular weight of most pro- 

 teins to be roughly in the neighborhood of 15,000, although 

 some recent evidence makes it seem that perhaps these figures, 

 .after all, are far too high. Comparing this figure with the mole- 

 cular weights of some familiar compounds such as hydrochloric 

 acid (36+), sodium hydrate (40) and sodium chloride (58+), 

 we get an idea of the relative hugeness of the protein molecule. 



Hydrolysis. Any consideration of the molecular structure 

 of compounds having such enormous molecules would seem to 

 present almost insurmountable obstacles. As the result of the 

 work chifly of Kossel, Emil Fischer, Emil Abderhalden, and 



