Hydrogen-bonding behavior of gellan in solution during structural change observed by 1H NMR and circular dichroism methods
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.
Key wordsGellan gum Hydrogen bonding Nuclear magnetic resonance Circular dichroism Helix-coil transition
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