Abstract
•Size, heterogeneity, stability, conformation, crucial for lambda-carrageenan.•Combination of modern analytical ultracentrifugation with other methods.•Extract sedimentation coefficient, MHKS parameter, Wales–van Holde ratio.•Flexible molecule in mol. wt. range Mw=340,000–870,000g/mol.•No clear evidence for association but could be hidden by non-ideality.
The conformation and heterogeneity of lambda-carrageenan, a sulphonated galactan from red seaweed, solubilised in aqueous solvent with the assistance of microwave irradiation, has been assessed by a combination of analytical ultracentrifugation, size-exclusion chromatography, light scattering and capillary viscometry. Preparations appeared generally unimodal on the basis of sedimentation coefficient distributions from sedimentation velocity although at the highest concentrations a shoulder appears with a sedimentation coefficient approximately 1.1 times greater than that of the main component. Even under conditions commensurate with charge suppression simple linear regression was insufficient to represent non-ideal concentration dependence and the extraction of the Grálen concentration dependence parameter ks. A more general fitting algorithm was therefore employed. Mark–Houwink–Kuhn–Sakurada analysis of the change in intrinsic viscosity [η] with molecular weight, together with the Wales–van Holde ratio (combination of ks with [η]) point to an extended flexible conformation for lambda-carrageenan in the (weight average) molecular weight range Mw=340,000–870,000g/mol. The origin of the larger sedimentation coefficient component appearing at the higher concentrations is considered.