Binding Effect of Cu2+ as a Trigger on the Sol-to-Gel and the Coil-to-Helix Transition Processes of Polysaccharide, Gellan Gum

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• 作者：Sho Kanesaka ; Tokuko Watanabe ; and Shingo Matsukawa
• 刊名：Biomacromolecules
• 出版年：2004
• 出版时间：May 2004
• 年：2004
• 卷：5
• 期：3
• 页码：863 - 868
• 全文大小：119K
• 年卷期：v.5,no.3(May 2004)
• ISSN：1526-4602

文摘

The binding effect of divalent cation Cu²⁺ on the gelation process with a coil-helix transition in Cu²⁺/gellanaqueous solutions has been successfully elucidated by EPR, CD, and viscoelasticity measurements. Generally,Na-type gellan gum in aqueous solution can make gel when accompanied by an intrinsic coil-helix formationinduced by hydrogen bonding between chains without any additional cations at T_ch-in (29 C) with coolingtemperature. An extrinsic coil-helix transition, induced by additional divalent cations in advance of theintrinsic sol-gel transition of gellan gum, is separately detected by CD measurement. The extrinsic coil-helix transition temperatures T_ch-ex (>47 C), which increased with the Cu²⁺ concentration added, werenearly identical to the sol-gel transition temperature, T_sg, determined by the viscoelasticity measurement.Judging from the molar ellipticity by CD measurement and quantitative analysis of EPR spectra, it waselucidated that the helix forming process via divalent cations is composed of two steps ascribed to thedifferent origins, i.e., a chemical binding effect via Cu²⁺ ions in the initial stage and hydrogen bondssubsequently. Finally, we propose the coil-helix and the sol-gel transition mechanism initiated by thebinding effect with the divalent cation, in which the partial chelate formation can cause local formation ofhelices and junction zones in the vicinity of the chelates at the initial stage of the process and stabilize thehelices and the junction zones. On the other hand, the stabilized helices and junction zones can inducefurther formation and further stabilization of the Cu²⁺-gellan chelates. The mutual stabilization promotesthe formation of three-dimensional network structure at the higher temperature than the intrinsic temperaturefor network formation.