Determination of Cystine and Cysteine in Altered Human Hair Fibers

The study developed a method to determine cystine and cysteine in altered human hair fibers by hydrolyzing the hair with hydrochloric acid and using the Sullivan method for measurement. To differentiate between cystine and cysteine, iodoacetic acid was used to block sulfhydryl groups. Optimal hydrolysis was achieved at 118°-120°C for 6 hours. The study found that cold waving solutions reduce cystine to cysteine within 4 minutes, with minimal further effect. Additionally, hair reduced with a cold waving solution and potassium bromate at room temperature for 15 minutes showed residual cystine values of 3% or less, with most reoxidation occurring within the first 3 minutes. This method is reliable for understanding hair treatment chemistry.