THE INDIVIDUAL GROUPS OF PROTEINS 455 



Collagen. 



Collagen forms the greater part of the ground substance surround- 

 ing the connective tissue cells in connective tissue, of tTie corpuscles 

 in bone and of tendon ; it also forms part of the substance of cartilage, 

 cornea and fish scales. 



Bones are treated (i) with dilute hydrochloric acid to remove inorganic 

 calcium salts, (2) with dilute alkali to remove organic matter 



Tendons are digested for several days with trypsin to remove proteins 

 and washed with water 



Collagen is a colourless material which swells up in cold water, in dilute 

 acids and dilute alkalies. It is insoluble in organic solvents ; it dissolves 

 with swelling in strong caust'c alkalies but not in carbonates. It dissolves 

 in pepsin solutions but not in tr)psin solutions unless the collagen has been 

 previously heated with water to 70 or treated with acid. It is changed into 

 derivatives when it is dissolved. It is converted by tannic acid into a form 

 of leather, undergoing shrinkage. It slowly dissolves on boiling with water 

 with evolution of ammonia and conversion into gelatin. 



Gelatin. 



Preparation. 



Gelatin is formed by boiling collagen with water. The collagen of 

 fish is the most easily converted into gelatin ; that from older animals 

 with greater difficulty than from young. The gelatin is obtained from 

 the solution by evaporation and is generally procurable in the form of 

 sheets. 



Purification. 



Commercial gelatin may be purified by soaking it for several days in (i) 

 water containing ether, (2) several weeks in changes of dilute sodium 

 hydroxide, (3) very dilute acetic acid, (4) water. It is hardened with 

 alcohol, dissolved in hot water and precipitated with alcohol. The precipitate 

 is dried with alcohol and ether and placed in a desiccator over sulphuric 

 acid. 



Properties. 



Gelatin swells up in cold water, but does not dissolve. It dissolves 

 in hot water and solutions above about I per cent, set to a jelly on 

 cooling and redissolve on heating. The heating cannot be repeated 

 very often as the gelatin is hydrolysed and the solution no longer sets 

 to a jelly on cooling. It dissolves very slightly in dilute alkali and 

 is insoluble in alcohol. 



Of the general reactions for proteins the xanthoproteic, Millon, 

 sulphur and Adamkiewicz* reactions are very faint. Gelatin, therefore, 

 does not contain those amino acids which are the cause of the reaction. 

 It is not precipitated by concentrated mineral acids. 



Mercuric chloride does not precipitate gelatin in neutral solution, 

 but it precipitates it in presence of hydrochloric acid. 



