206 SOAPS AND PROTEINS 



proteins and the materials which may be produced from them. 

 It is of interest to bear in mind that the proteins are not only 

 polymerized amino-acids, but that frequently their constituent 

 amino-acids are amino-fatty-acids, as witness, amino-acetic, 

 amino-valeric, amino-caproic and amino-succinic acids. The 

 alleged " neutral" " native " or " genuine " proteins are no more 

 neutral than are any of the higher fatty acids. As the fatty adds may 

 combine with base to form " soaps," even so may the polymerized 

 amino-fatty-acids combine with base to form analogous " soap- 

 like " compounds. It is important for our future discussion that 

 this comparison of the neutral proteins with the free fatty acids 

 be clearly kept in mind. 1 



What, now, are the solubility characteristics of the pure fatty 

 acids and the pure neutral proteins in water and for water? 

 Just as certain fatty acids (like the lowermost members of the 

 acetic series) are readily soluble in water, so also do certain native 

 proteins prove " soluble " in water (as witness the various salt-, 

 acid- and alkali-free, " pure " albumins). On the other hand, 

 as other fatty acids (like the higher members of the acetic series) 

 prove insoluble in water, so also do various native proteins (as 

 witness casein, fibrin, alkali-, acid- and salt-free globulins, etc.). 



The solubility of water in the fatty acids or in the pure pro- 

 teins is hardly to be found discussed as such. The simpler fatty 

 acids " dissolve " so readily in water and are so universally 

 thought of as " aqueous solutions " that the mere raising of the 

 obverse question in their case will seem, to many, absurd. Water 

 is, however, sufficiently souble in the higher fatty acids to make 

 necessary its consideration, when, commercially, a given weight 

 or volume of material is to be bought and paid for as fatty acid. 

 In the case of the pure proteins these things are variable. In 

 those commonly designated as " insoluble " (casein, for example) 

 the solubility for water is so low as to be generally neglected. 



1 Of great interest in connection with this similarity between the colloid- 

 chemistry of the fatty acids and that of the amino-fatty-acids (the proteins) 

 are some observations of KRAFFT and WIGLOW [quoted by LEWKOWITSCH: 

 Oils, Fats and Waxes, 1, 133 (1913)] whose work unfortunately we have not 

 been able to find in the original. These authors note that the amins of the 

 fatty adds behave like the corresponding fatty adds. While the alkali salts of 

 the lower amino-fatty-acids on solution in water behave like crystalloids, 

 those of the higher amino-fatty-acids fail to raise the boiling point of water 

 the calculated amount and in other physico-chemical respects betray them- 

 selves as colloids. 



