Hydrogen-bonding behavior of gellan in solution during structural change observed by ¹H NMR and circular dichroism methods

Abstract

¹H NMR relaxation times for water and the circular dichroism for gellan solutions were measured in order to elucidate the hydrogen-bonding behavior and the structural change of the gellan molecule. From the results of the temperature dependence, it was found that ¹H T₂ for water significantly changes with the conformational change of gellan, such as the random coil-double helix transition or the formation of aggregates of gellan molecules, which alters the proton-exchange rate between a water molecule and the hydroxyl groups of gellan. From results for gellan solutions with various concentrations of K⁺, it is suggested that the hydrogen bonding between gellan molecules is accelerated by the shielding effect of K⁺ ions and reinforces the double-helix structure and the aggregates of double helices, and that an aggregate even among the random coils is induced at high concentrations of K⁺. For a solution with Ca²⁺, it is suggested that ionic bonds are formed between carboxylic acids of gellan and Ca²⁺, and these reinforce the aggregates of double helices.

From the result of the dependence of the ¹H NMR relaxation times on ion concentration, it was found that a certain amount of the cation is required for the structural change of gellan in the case of monovalent cations; on the other hand, divalent cations lead to a continuous structural change in proportion to the concentration of the divalent cations.