The Optical Activity of Cysteine.*

The exact optical activity of cysteine has been a matter of doubt. Baumann (1) reported that [aID = -12.6” for the hydrochloride and -8.8” for the free ampholyte. Morner (2), on the other hand, stated that while some preparations of cysteine showed values similar to those of Baumann, other preparations were inactive or weakly dextrorotatory. Apparently no other investigator has reported dextrorotatory cysteine; the figures usually reported for [a], range from 10” to 20”. Cysteine has usually been prepared by reduction of cystine with metallic tin in hydrochloric acid solution (l-3). Other reducing agents such as sulfides (4, 5), sulfites (6-8), hydriodic acid (l), cyanides (9), and other metals have been used: However, they have generally resulted in incomplete reduction. According to Mathews and Walker (lo), cysteine reoxidizes to cystine most rapidly at about pH 8. This harmonizes with the usual practice of effecting the opposite reaction at pH levels above or below this point. While tin and hydrochloric acid have been the usual agents, satisfactory reductions are obtained by the Folin and Looney (7) method in sodium carbonate solution. At pH values below the isoelectric point of cystine and cysteine (it is to be presumed that they are not far apart) the reaction would undoubtedly be :