Reversible and irreversible conversion between the intermolecular beta-structure and the disordered state of...

Conversion between the intermolecular beta-structure and the disordered state of a fractionated low molecular weight sample of poly(S-carboxymethyl-L-cysteine) was examined mainly by the measurements of circular dichroism in the absence of salt as well as in the presence of 20 mM NaClO4 or NaCl. In 20 mM NaClO4 or NaCl solutions, the conversion was reversible. Under this condition, it was confirmed by direct and unambiguous evidence provided from the viscosity and the reduced scattering intensity that the beta-structure was formed by intermolecular association. At low degrees of neutralization, the pH increased on dilution while it remained constant over a wide range of concentration at a high degree of neutralization. In the absence of salt, the conversion was often irreversible with respect to a concentration change at a constant degree of neutralization or to a change in the degree of neutralization at a constant concentration. The extent of the irreversible conversion decreased with the amount of beta-structure in the solution. The dissociation of aggregates was very slow at low ionic strengths. It was inferred that the irreversible nature of the conversion arose from this slow dissociation of aggregates.