S-carboxymethylcysteine sulfone: instability to acid hydrolysis and unreactivity with N-terminal reagents.

An examination of the properties and reactivity of S-carboxymethylcysteine sulfone indicated that, unlike S-carboxymethylcysteine, the sulfone is not stable under acid hydrolysis conditions and decomposes to yield alanine. Unlike S-carboxymethylcysteine, the sulfone is resistant to N-derivatization by the dansyl reagent or by phenylisothiocyanate. Efforts were made to determine if spontaneous cyclization of the sulfone to the corresponding thiazane (lactam) accounts for lack of reactivity. These included i.r. spectroscopy, natural abundance 13C-n.m.r. spectroscopy and differential scanning calorimetry, but yielded equivocal results concerning the existence of the cyclic form in solution. Solubility behavior of the sulfone after lyophilization from strongly acid solutions was consistent with conversion of the open chain form to the cyclic form on addition of water.