Hydrogen Ion Titration of Poly-S-carboxymethyl-L-cysteine

Abstract Potentiometric titration curves have been determined for aqueous solutions of poly-S-carboxymethyl-l-cysteine. A pH-induced transition of the polymer conformation is manifest from the occurrence of a transition region around pH 5, where the solution has an essentially infinite buffer action. The titration curve at higher ionization corresponds to that of a random coil. At lower ionization the polymer is associated to form a β-structure and its titration behavior is qualitatively accounted for by the Gouy model for a uniformly charged sheet. Deviation from the model would partly be ascribed to the aggregation of sheets in the β-form. The standard free energy of transition from random coil to β-form has been estimated for the unchanged polymer and is found to depend on the molecular weight of polymer. It is suggested that fine structure of the β-form and its mode of aggregation vary with polymer chain length.