SRB sulfonyl chloride (also called Lissamine rhodamine sulfonyl chloride) is highly fluorescent. It is quite unstable in water, especially at the higher pH required for reaction with aliphatic amines. Protein modification by this reagent is frequently done at low temperature (preferably at 4 C). This reagent reacts with amine compounds such as amino acids, peptides and proteins to give bright red fluorescent conjugates that are extremely stable, and resistant to protease-catalyzed hydrolysis.  Unlike succinimidyl esters, sulfonyl chlorides react with both aliphatic amines and aromatic amines indiscriminatingly.