The instability of S-beta-(4-pyridylethyl)-L-cysteine to performic acid

S-β-(4-pyridylethyl)-l-cysteine (PEC) was transformed to S-β-(4-pyridylethyl)-l-cysteine sulfone (PEC·SO₂) and to cysteine sulfinic acid following exposure to performic acid. Acid hydrolysis (6n HCl, 110°C, 20 h) of the isolated PEC·SO₂ resulted in its partial decomposition to cysteic acid and alanine. Several minor ninhydrin positive derivatives, including glycine and serine, were also observed.The transformations involving PEC and PEC·SO₂ were followed using high-voltage electrophoresis (HVE) at pH 1.6, 2.0, 3.5, and 6.5 and through use of the amino acid analyzer and gas-liquid chromatography.These studies indicate that as a modification form of the thiol groups of proteins, PEC is not the derivative of choice when subsequent treatment of the protein involves the use of performic acid.