Studies on the Spontaneous Oxidation of Cysteine I. the Preparation of Iron-free Cystine and Cysteine Hydrochlorides*

In carrying out studies on the oxidation of cysteine, it is most important that cysteine be used which is known to be completely free from contaminating metals such as iron and copper, both of which are powerful catalysts to this type of oxidation. In a quantitative study of this action, Mathews and Walker (1) showed that this reaction was sensitive to iron in amounts as small as 0.00001 M concentration. This catalytic activity of traces of iron on cysteine oxidation is employed by Warburg (2) in a method devised to measure amounts of iron too small to be detected by the usual methods. Because of the ubiquitous nature of iron, and also of copper, the preparation of a metal-free cysteine has been extremely difficult. The starting point is usually Lcystine, which is quite easily prepared from the keratins such as horn, feathers, hair, or wool by acid hydrolysis, with subsequent separation from the decolorized hydrolysate by careful neutralization. The cystine, which is obtained as typical hexagonal plates on recrystallization, is then reduced, usually by the tin and hydrochloric acid method described by Baumann (3) and by Friedmann (4). But cysteine