Potentiometric titrations involving the β‐coil transition

Potentiometric titrations of poly( S ‐carboxymethyl‐ L ‐cysteine) and poly( S ‐carboxy‐ethyl‐ L ‐cysteine) were carried out in aqueous sodium chloride solutions and in water. For samples of both polymers of high molecular weight, a new pattern was observed concerning the change of titration curve with time; the β‐coil transition became sharper and the transition free energy increased by about 100 cal mole −1 as the equilibrium was approached. This suggests that equilibrium data were not obtained in most previous studies on the titration involving the β‐coil transition. It also shows that the reversbility is not necessarily sufficient to confirm the equilibrium. Another pattern, which was previously observed, was also confirmed with a low molecular weight sample of poly( S ‐carboxymethytl‐ L ‐cysteine). The titration curves were shown to be insensitive to polymer concentration, even when aggregation or phase separation was present. The validity of the Gouy model to describe the titration curve of the β‐structure was found to depend on molecular weight as well as on the nature of the side chain.