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

Abstract

The binding effect of divalent cation Cu(2+) on the gelation process with a coil-helix transition in Cu(2+)/gellan aqueous 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 formation induced by hydrogen bonding between chains without any additional cations at T(ch)(-)(in) ( approximately 29 degrees C) with cooling temperature. An extrinsic coil-helix transition, induced by additional divalent cations in advance of the intrinsic sol-gel transition of gellan gum, is separately detected by CD measurement. The extrinsic coil-helix transition temperatures T(ch)(-)(ex) (>47 degrees C), which increased with the Cu(2+) concentration added, were nearly 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 was elucidated that the helix forming process via divalent cations is composed of two steps ascribed to the different origins, i.e., a chemical binding effect via Cu(2+) ions in the initial stage and hydrogen bonds subsequently. Finally, we propose the coil-helix and the sol-gel transition mechanism initiated by the binding effect with the divalent cation, in which the partial chelate formation can cause local formation of helices and junction zones in the vicinity of the chelates at the initial stage of the process and stabilize the helices and the junction zones. On the other hand, the stabilized helices and junction zones can induce further formation and further stabilization of the Cu(2+)-gellan chelates. The mutual stabilization promotes the formation of three-dimensional network structure at the higher temperature than the intrinsic temperature for network formation.

Al+3 ion cross-linked and acetalated gellan hydrogel network beads for prolonged release of glipizide

“…Chemically, the gelation and aggregation occurs via ionic-linking between divalent Ca +2 ions and two carboxylate groups belonging to glucuronic acid molecules in the gellan chains (Kanesaka, Watanabe, & Matsukawa, 2004). This mechanism is quite similar to that of calcium alginate gel formation (Rees, 1970).…”

Rifabutin-loaded Floating Gellan Gum Beads: Effect of Calcium and Polymer Concentration on Incorporation Efficiency and Drug Release

“…The beads were dispersed in 500 ml of 0.01M HCl (pH 2.0) at 37 ± 1 0 C with continuous agitation at 50 rpm for 24 h. The floating beads were separated from the submerged beads and their proportion (%) was determined as described earlier [3,4].…”

“…In aqueous solution, the gelation of gellan is accompanied by a two-step process, namely, formation of double helices from random coil chains and an aggregation of pairs of double helices [3]. The coil-helix transition is greatly affected by electrostatic interaction with cations present in the solution.…”

“…Gellan forms gels in the presence of monovalent (Na + , K + ) and divalent cations (Ca 2+ , Mg 2+ ) but its affinity for the latter is much stronger than for the former [1]. A number of efforts have been made to understand the mechanism of microencapsulation of gellan gum in the presence of calcium cations but the exact mechanism remains unknown [2,3].…”

Markedly compressible behaviors of gellan hydrogels in a constrained geometry at ultraslow strain rates

“…Aqueous solutions of gellan undergo gelation by cooling when the concentration of gellan is higher than the critical gelling concentration [2]. The sol-gel transition of gellan is thermoreversible, and the driving force for gelation is the random coil-to-helix transition [3]. The junctions are composed of double helix structures and/or their aggregates.…”