The aim of this work is the study of cysteine by its derivative form. Cysteine derivation is done by Edman degradation and we obtain the phenylthiohydantoin of cysteine. Edman degradation is one of the most usual procedures for sequentate proteins. From this reaction, the phenylthiohydantoin amino acid derivative (PTH-amino acid) from the N-terminal one is obtained, leaving the new amino terminus for the next degradation cycle. To identify the produced PTH-derivative, a square-wave cathodic stripping voltammetry method is developed. In this work we show the results obtained for the optimization the instrumental and chemical conditions, and we can conclude that it is an electrochemically irreversible system with adsorptive reduction phemomenon. Under the best conditions we found a new method to analyze proteins with LOD of 2 mg L À1 and LOQ of 5 mg L À1 , a RSD less than 10% and a E r minor than 9%. Results of the application of this methodology to a real protein are included.

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