Protected derivative for the introduction of this highly polar amino acid. Cya-containing peptides are obtained by oxidation of cysteine or cystine, as a post-translational modification, as a by-product during peptide synthesis, or, intentionally (by using oxidants as H₂O₂/formic acid), for quantification of these sensitive amino acids. Use of B-4030 allows to obtain Cya-peptides containing sensitive amino acids, as an additional oxidation step is not required.