2 THE AMINO ACIDS 



Viewed from the chemical standpoint protein is 

 seen as a huge molecule, complex in structure, labile 

 in character and therefore prone to chemical change. 

 So large and intricate is the make-up of the molecule 

 that chemists for generations have been baffled in 

 their attempts to gain any adequate conception of its 

 nature. At the present stage of our knowledge it is 

 impossible to form any satisfactory definition of a 

 protein based either on its chemical or physiological 

 properties. In general, proteins contain about 15 to 

 19 per cent of nitrogen, 52 per cent of carbon, 7 per 

 cent of hydrogen, 23 per cent of oxygen and 0.5-2.0 

 per cent of sulphur. Some also contain phosphorus 

 or iron. They act like amphoteric electrolytes, that 

 is, they are capable of forming salts with both acids 

 and bases. Proteins belong to that class of substances 

 known as colloids and as such do not possess the 

 power to pass through animal or vegetable mem- 

 branes. In a manner similar to colloids they may be 

 separated from their solutions by suitable treatment 

 with salts, such as sodium chloride, ammonium sul- 

 phate, etc. By a process known as "coagulation," 

 which may be induced by the action of heat or the 

 long continued influence of alcohol the proteins lose 

 their colloidal characteristics which cannot be restored. 



Many proteins are capable of crystallization and 

 indeed may occur in nature in crystalline form. It 

 has been found possible also to cause some to crys- 

 tallize although their presence in nature as crystals 

 is unknown. Some doubt has been cast upon the 



