Collapse-reexpansion conformational transition of alginate under non-specific ion conditions

As negatively charged polyelectrolytes, alginate conformation is greatly influenced by ionic environment, which is of critical importance for solution properties and practical applications. To advance our understanding of alginate conformation in the presence of non-specific ions (NaCl, KCl, MgCl2), high resolution atomic force microscopy, together with statistical analysis, is employed to provide direct structural information. Alginate chains undergo a collapse-reexpansion transition with increasing salt concentration, which is quantitatively evidenced by a nonmonotonic evolution of persistence length (L-p). At low salt concentrations, a decrease in L-p is caused dominantly by electrostatic screening effect as electrostatic persistence length (L-p(e)) shows a strong dependence on Debye screening length (kappa(-1)) with a scaling relation of L-p(e) similar to kappa(-2), which is in agreement with Odijk-Skolnick-Fixman theory described for semiflexible polyelectrolytes. Above a certain salt concentration, there is a rise in L-p, which can be explained by overcharging and ion-ion correlations. The type of salt also affects the change degree of persistence length, with divalent ions (Mg2+) showing the greatest collapse and reexpansion level. Since alginate chains containing higher alpha-L-guluronate (G) content are more influenced by salts, it can be inferred that non-specific ions have a higher affinity for G units.