THE PROTEINS 85 



It is customary to assign to a compound having an unknown molecular 

 mass, i.e., relative weight in units of the weight of an atom of hydrogen, a 

 formula representing the least mass which the substance could have and 

 preserve its characteristic properties. The simplest formula for oxyhemoglo- 

 bin, the compound protein of the red blood cells, is C 658 H 1181 N 207 S 2 FeO 210 . 

 This formula is based on the relative proportion that the C, H, N, S, 

 and O bear to the Fe as determined by analysis. This protein contains 

 iron, and the least Fe that one molecule can contain is one atom. By addi- 

 tion of the atomic masses of the total number of atoms of each element, the 

 least possible molecular mass for oxyhemoglobin is about 15,000. It might 

 just as well be 30,000 with two atoms of iron in the compound. The follow- 

 ing formulae have been proposed for ovalbumin and seralbumin: 



Ovalbumin, C 239 H 386 N 58 S 2 O 78 

 Seralbumin, C 450 H 720 N 116 S 6 O 140 



the molecular masses being in the neighborhood of 5000 and 6000, re- 

 spectively. 



Besides the amino acids other radicals are present in some proteins. A 

 carbohydrate moiety is evidently present in certain proteins and phosphoric 

 acid in others (as in the milk protein casein). Such proteins are to be dis- 

 tinguished from those which exist as combinations with definite chemical 

 entities, as hematin, nucleic acid, amino sugars, lecithins, etc. 



Properties of Protein. Many proteins have been prepared in crys- 

 talline form, especially the reserve proteins from various seeds. Only three 

 animal proteins have ever as yet been crystallized seralbumin, lactalbumin, 

 and ovalbumin. 



The majority of the proteins are soluble in water or in dilute solutions 

 of neutral salts of strong bases with alkalies. The proteins do not form 

 solutions as do ; for instance, the inorganic salts, but are to be regarded as a 

 suspension of the molecules or molecular aggregates. Such a solution is 

 known as a colloidal solution, and the proteins are frequently spoken of as 

 colloids. Colloidal solutions of the heavy metals can be formed by the 

 interrupted contact of metal electrodes under water. The metallic colloidal 

 solutions and the protein solutions have many properties in common. 



When a true solution of a chemical substance of relatively small molecular 

 weight is placed within a parchment or animal membrane, and the whole 

 immersed in water, the substance in solution will diffuse through the pores 

 of the membrane into the water external to it; similarly, water will pass 

 through the membrane to the interior. After some time the system will 

 come into equilibrium. The force which drives the dissolved substance from 

 the more concentrated to the less concentrated solution is known as osmotic 

 pressure. The large protein molecules and molecular aggregates cannot 

 pass through the pores of the membrane, or, in other words, they are not 



