The reaction of mercaptans with carbonyl compounds has received extensive study, but little attention has, until recently, been paid to the case of cysteine. In a study of the effect of formaldehyde on the titration curves of amino acids, Birch and Harris2 observed, on the addition of formaldehyde to cysteine, the disappearance of the buffering action typical of the sulfhydryl group. Shinohara8 in an investigation of the action of cysteine upon phosphotungstic acid, noted that the reducing power is inhibited completely by addition of formaldehyde. During the progress of the present investigation, Schubert4 has described the formation of compounds of cysteine with various aldehydes, among them formaldehyde, and has produced evidence that the latter reacts with cysteine to form a thiazolidinecar boxy lie acid. The present report covers a more extended investigation of this compound, its reactions, and its manner of formation.The reaction between formaldehyde and cysteine can take place over a wide range of hydrogen ion activity. The product, which is remarkably stable toward both acid and alkali, possesses amphoteric properties, as is shown by the titration curve (Fig. 3). The empirical formula, C4H7O2NS, and the conditions of formation indicate that the compound is formed from equimolecular quantities of cysteine and formaldehyde;
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