Binding effect of Cu(2+) 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.