Specific sulfonation of tyrosine, tryptophan and hydroxy-amino acids in peptides

1. ClSO3H in trifluoroacetic acid rapidly converts serine and threonine into O-sulfate ester derivatives while tyrosine and tryptophan are converted into arylsulfonic acids. 2. H2SO4 in trifluoroacetic acid reacts more slowly with serine, threonine and tyrosine while is not able to modify tryptophan. 3. All other amino acids are perfectly stable under the above reaction conditions. 4. Peptides containing susceptible amino acid residues are specifically converted into the corresponding sulfonated derivatives in high or quantitative yield.