Coil-to-helix transition of anionic polysaccharides: Comparison of voltammetric and conductometric results for kappa-carrageenan

Two electroanalytical techniques, steady-state voltammetry and conductance measurements were utilized to study the conformational coil to double helix transition of an ionic polysaccharide kappa-carrageenan, kappa-car, in sol utions of low ionic strengths as effected by temperature and concentration of the polyion. The interactions between counterions, Na+, or electroactive probe ions, Tli, and polyanions during such transition were quantified by the ratio of the diffusion coefficient of Na+ or Ti+ ions in the polyelectr olyte solution (kappa-car sodium salt) to that in the solution without poly eleclrolyte (NaCl or NaClO4). The results for native counterions, Na+, from conductance measurements were very close to those obtained by voltammetry of TI+ probe ions. The conformational transition was a reversible coil-to-d ouble helix process resulting from temperature decrease. This process did n ot depend on the concentration of kappa-car for the concentration range fro m 10 to 40 mmol l(-1). The charge spacing in kappa-car conformers was deter mined by employing Manning's theory for polyelectrolytes. The average charg e spacing calculated from the conductivity and voltammetric results was 0.3 9 nm and 0.90 nm for the double helix and the coil form of kappa-car, respe ctively.