Weight-average (M̄ w) molar mass data were obtained by light scattering on aqueous solutions of a sample of κ-carrageenan with a relative molar mass of about 157 000. They were compared with literature data of several samples, having M̄ w values from 35 000 to 720 000, using a variety of methods. Temperature and concentration of supporting 1:1 electrolyte were chosen so as to span across the disorder-to-order conformational transition, monitored by the change of optical activity. The present comparative study indicates that the single helix is the fundamental ordered (secondary) conformation of κ-carrageenan in aqueous solution. This conclusion appears to be universally valid on the basis of a very large number of cases, including polymer samples of different average molar mass, in the presence of different counterions and co-ions, and at different temperatures. The single-helical structure is the conformational “building-block” of a higher (“tertiary”) ordered structure that stems from the association of (at least) two of such secondary structures according to a topology that cannot be determined by light-scattering data only.

A large number of light-scattering data indicate that the sulfated polysaccharide κ-carrageenan undergoes an intramolecular conformational transition under suitable conditions from a semiflexible, disordered conformation to a rather rigid, helical one. By varying the parameters controlling the conformational phase diagram (i.e., by increasing the concentration of polymer or that of the supporting electrolyte, or both, or by decreasing temperature), a tertiary structure is manifested, through a progressive increase of the weight-average molar mass (M̄ w) as determined by light scattering. This can be very well explained in terms of a reversible association of stretches of such intramolecular helical structures and quantitatively described by a model of “open association” of two or more chains. The association of κ-carrageenan was demonstrated to take place at around room temperature also in the presence of iodide ions, as long as their concentration is ≥0.20 M. In this way the apparent inconsistency of a number of published M̄ w results can be reconciled. Additional light-scattering results indicated that the demonstrated thermodynamic tendency to reversible interchain association may lead to an irreversible aggregation of polymer chains if an improper solution preparation procedure is used.

The conformational transition of ι-carrageenan in different salt solutions under nongelling conditions has been studied by light scattering (WALLS). The focus is on the thermodynamic interactions and the concentration dependence of the reduced scattering intensity at zero angle. An upward curvature of (KC p /R 0) in conditions of conformational ordering has been interpreted as due to a concentration-dependent reversible chain association. The open association model of Elias has been applied to analyze the data. In conditions of conformational ordering a decrease of polymer concentration, C p , is accompanied by a decrease of (M̄ w)assoc. These results have been confirmed by using low-angle light scattering (LALLS). In the lowest range of C p , the decreasing values of (M̄ w)assoc are already very close to that of the single-stranded disordered conformation. In the same range, the values of the specific optical activity remain constant. This is a confirmatory evidence that the fundamental ordered conformation of ι-carrageenan is a single helix.