PROTEINS 259 



ii. Preparation of Cystin from Wool. The student will be 

 furnished with a mixture resulting from the hydrolysis of wool 

 with concentrated hydrochloric acid. To about 50 c.c. of the 

 liquid add solid sodium acetate until congo red paper no longer 

 indicates an acid reaction. This indicator is blue in acid solu- 

 tion, red in alkaline. A precipitate consisting mainly of cystin 

 is thrown down. If the precipitate is not heavy, allow the mix- 

 ture to stand. Filter off the precipitate, wash with a little cold 

 water, and dissolve in a small volume of 5% hydrochloric acid. 

 If the solution is dark colored, boil with animal charcoal until a 

 small filtered portion is colorless or pale yellow. Filter, and to 

 the hot solution add hot sodium acetate solution. The cystin 

 will separate out in crystal form as large pentagons, hexagons 

 or plates. 



(1) Examine the cystin crystals under the microscope. 



(2) Dissolve a small amount of cystin in caustic soda, add lead 

 acetate and boil. Observe the positive "neutral sulphur" test 

 and recall that as given by proteins, this test depends upon their 

 content of cystin or cystein. 



