Development of an acid hydrolysis method with high recoveries of tryptophan and cysteine for microquantities of protein.

High recoveries of tryptophan and cysteine were achieved by 12.5 min of hydrolysis with mercaptoethanesulfonic acid vapor. Proteins (1-100 micrograms) were modified by vapor-phase S-pyridylethylation before hydrolysis. The modified proteins were hydrolyzed with the vapor of 2.5 M mercaptoethanesulfonic acid at 176 degrees C. This method promoted efficient hydrolysis of the peptide bonds in proteins and resulted in high recoveries of both tryptophan and cysteine, of 90% or greater, in addition to the other amino acids.