Quantitative Determination of Cysteine and Cystine in Peptides and Proteins Using Hydriodic Acid

Abstract

Determination of the composition of proteins by amino acid analysis after hydrochloric acid hydrolysis is unsatisfactory for cystine which is not all recoverable. When proteins were hydrolysed in hydriodic acid, half-cystine was released quickly and without measurable loss as cystine with freshly distilled acid and as cystine plus cysteine with hydriodic acid containing hypophosphorous acid_ The necessary alterations to the normal operating conditions of an amino acid analyser to enable the resolution of cysteine from proline are discussed. This procedure for cystine analysis provides a simple alternative to determinations involving chemical modification of the amino acid.