ELEMENTARY CHEMICAL PHYSIOLOGY 



193 



Or, alternatively, they may simply be classified as neutral, acid 

 or basic ampholytes, i.e. substances which can either function as 

 acids or bases. In this classification to the neutral group belong the 

 monoamino monocarboxylic acids including tyrosine, phenylalanine, 

 tryptophan and cystine, to the acid group the monoamino di- 

 carboxylic acids and to the basic the diamino acids and histidine. 



The following table gives some idea of the percentage distribu- 

 tion of the various amino acids in different proteins : 



The Physical and Chemical Properties of Proteins 



I. Solubility. Most proteins x are insoluble in alcohol and ether. 

 They vary as to their solubility in water; of the more common 

 proteins, albumins are soluble in water and globulins in weak 

 saline solutions. Some, however, are not soluble, even in concen- 

 trated saline solutions. 



EXPERIMENT. From the undiluted egg-white provided prepare 

 a solution of egg albumin by adding 10 volumes of distilled water 

 and mixing thoroughly in a flask. An opalescent solution is thus 

 obtained, the opalescence being partly due to the colloidal nature 

 of the solution, although in part to some other protein (ovo-mucin), 

 which has not gone into solution. This can be removed by filtering 

 through fine muslin. Note that this solution, like all colloidal 

 solutions, gives a persistent froth on shaking. 



The solution prepared above can be used in the subsequent 

 experiments, unless otherwise stated. 



II. Diffusibility. -As the proteins give only colloidal solutions, 

 these solutions will not dialyse, that is, diffuse through animal 

 membranes or parchment paper. In this they are unlike crystalloids, 

 such as inorganic salts, which readily diffuse through such 

 membranes. Of the various forms of dialyser, a tube of parch- 

 ment is the simplest. 



EXPERIMENT. Place a mixture of diluted blood and of a 10 per 

 1 Some vegetable proteins are soluble in alcohol. 



