Sodium counterion activity of poly(S‐carboxymethyl‐L‐cysteine) and the beta–random‐coil transition

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Abstract
Sodium ion activity was measured using a Na‐glass electrode in a solution of poly(S‐carboxymethyl‐L‐cysteine) with no added salt at various degrees of neutralization and various concentrations for samples of different molecular weights. The conformational change from random coil to the β‐structure was detected from the activity coefficient of counterions, as well as from CD. At a constant degree of neutralization, the activity coefficient is insensitive to a concentration change not only in the random‐coil state, but also in the range of conformational change if the concentration is below about 3 × 10−2 monomolal. At high concentrations of about 5 × 10−2 monomolal, however, the activity coefficient becomes low, probably due to the occurrence of the stacking of the pleated sheets.