A method for the preparation of cystin

Abstract

A number of years ago Folin 2 described an improved method for the preparation of cystin which has come into general use. It is based on the fact that the solubility of cystin is a minimum in solutions possessing an acidity between P_H 4-5. To obtain the optimum acidity for the precipitation of cystin, the HCl used to hydrolyze the protein is neutralized by the addition of sodium acetate. Although good yields of the amino acid are obtained by this method it nevertheless is not economical for the production of cystin in quantity since large amounts of relatively expensive materials are required. Neutralization of HCl with sodium acetate results in the simultaneous precipitation of humin which later necessitates the repeated use of large quantities of charcoal to effect its removal.

In the method described below the HCl is in large part recovered by vacuum distillation. Use is then made of commercial finishing lime to neutralize the remaining HCl, to precipitate the humin 1 and to hold the cystin in solution. The advantage in the use of lime lies in the fact that it is comparatively insoluble, gives a solution of low alkalinity, thus minimizing the destruction of cystin, and is cheap.

Human hair or wool which has been freed from oil by extraction with gasoline is hydrolyzed by heating at 100°C. with twice its weight of concentrated HC1. It requires about 12 hours to effect complete hydrolysis. The mixture must not be heated for any length of time beyond the point at which the biuret test is either negative or feebly positive since, as Van Slyke 2 has shown, cystin is destroyed during the process of hydrolyzing the protein. The greater part of the protein is removed by distilling in vacuo at a temperature between 60–70°C. and the original volume of the solution is restored by the addition of water.