THE PHOT EI^S AND THEIR METABOLISM 101 



not seem to unite with the different ions in the same stoichiometrical ra- 

 tios a* they unite with crystalloids, and because of that, the proteins were 

 credited with special "absorption" properties. These were attributed to 

 ail the colloids. 



The recent researches of Jacques Loeb (1919-1921) seem to clarify the 

 whole problem. He proved that the proteins, and perhaps all other am- 

 plK'teric colloids, can exist in three states and that these states depend 

 CUT i rely upon the hydrogen ion concentration of the medium in which 

 rhev are dissolved ; that each protein has a critical point in the hydrogen 

 inn concentration at which it does not dissociate and at which it is incapable 

 (1 f -raving united with either anion or cation. At this point a protein like 

 nchitin is almost completely insoluble, hence all the properties which are 

 dependent upon the solubility of gelatin, like its osmotic pressure, viscosity, 

 .-welling and conductivity, are at a minimum. This point is known a* 

 the "isoclectric" point. For gelatin this isoelectric point lies at a hydro- 

 gen ion concentration of C H = 2.10~ 5 or pH 4.7, for casein 4.7, for 

 OL r ir albumin 4.8, and for oxy hemoglobin at 6.8, and at these points we 

 find the proteins to be almost inert bodies. 



On either side of this isoelectric point the protein molecule dissociates 

 in TWO different states. On the acid side, i. e., if the hydrogen ion con- 

 centration of a gelatin solution is raised and the pH falls below 4.7, the 

 [-.lY'tehi dissociates into a cationic state, carrying a positive electrical charge 

 :md capable of forming salts with anions forming protein chlorids, protein 

 sulphates, etc. In this state the amino radical becomes chemically active, 

 while the carboxyl, the other binding post of the protein molecule, is en- 

 tirely inert. 



On the other hand, if the hydrogen ion concentration of the solution 

 i- lowered and we have a rise in the pH above 4.7, the protein dissociates 

 into an anionic state carrying a negative electrical charge and capable 

 "f forming salts \vith metals or cations, forming metal-proteinates, like 

 sodium gelat.inate, calcium albuminate, potassium casemate, etc. 



lie further found that all proteins at their isoelectric points w r ill aban- 

 don The chemical union they may have had with either anion or cation or 

 "i her protein, and may be obtained in a state of high purity. He also 

 i 'mi. 1 that for each given hydrogen ion concentration the proteins com- 

 1'iit" with the various anions or cations in definite stoichiometrical ratios 

 .-iruilar to those of the crvstalloids. 



The Digestion of the Protein 



During the process of mastication the proteins suffer only physical 

 : oration by being broken up into smaller particles. The saliva contains 



