On the Molar Mass of κ-Carrageenan in the Course of Conformational Transition from the Disordered to the Fundamental Ordered Form

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.