Department of Food Science and Technology, Tokyo University of Marine Science and Technology, 4-5-7 Konan, Tokyo 108-8477, Japan.
Carbohydr Polym. 2013 Jun 5;95(1):458-64. doi: 10.1016/j.carbpol.2013.02.049. Epub 2013 Mar 5.
Changes in the molecular mobility of κ-carrageenan were observed by the pulsed field gradient stimulated echo (PGSTE) and Carr-Purcell-Meiboom-Gill (CPMG) methods for elucidating the molecular aspect of the sol-to-gel transition. The echo signal intensity of κ-carrageenan without a gradient, Ikap(0), decreased steeply near the sol-to-gel temperature (Tsg), suggesting that κ-carrageenan chains formed aggregates and a network structure. Below Tsg, the spin-spin relaxation time T2 and the diffusion coefficient of κ-carrageenan (Dkap) increased with decreasing temperature, indicating that the solute κ-carrageenan chains have a lower molecular weight Mw than chains involved in the aggregation. The diffusion coefficient of pullulan (Dpul) added as a probe molecule in κ-carrageenan solutions was measured, and the characteristic hydrodynamic screening length, ξ, was then estimated from the degree of diffusion restriction. Below a certain temperature, Dkap reached a higher value than that of Dpul, suggesting that the Mw of solute κ-carrageenan became lower than that of pullulan. GPC measurements confirmed the presence of κ-carrageenan chains with a lower Mw than that of pullulan. A simple physical model of the structural change in κ-carrageenan solution was proposed with a bimodal distribution of κ-carrageenan with higher and lower Mw than the pullulan probe. The higher Mw chains form the gel network restricting the probe's diffusion, and the lower Mw chains increase the effective viscosity. The concentration of the high Mw solute κ-carrageenan chains in 1%, 2% and 4% κ-carrageenan solutions was estimated from Ikap(0) and the total κ-carrageenan concentration, and the relation with pullulan diffusion was studied.
通过脉冲梯度场回波（PGSTE）和 Carr-Purcell-Meiboom-Gill（CPMG）方法观察κ-卡拉胶的分子迁移率的变化，以阐明溶胶-凝胶转变的分子方面。没有梯度的κ-卡拉胶的回波信号强度，Ikap（0），在溶胶-凝胶温度（Tsg）附近急剧下降，表明κ-卡拉胶链形成了聚集体和网络结构。在 Tsg 以下，κ-卡拉胶的自旋-自旋弛豫时间 T2 和扩散系数（Dkap）随温度降低而增加，表明溶质κ-卡拉胶链的分子量 Mw 低于参与聚集的链。作为探针分子添加到κ-卡拉胶溶液中的普鲁兰（Dpul）的扩散系数进行了测量，然后根据扩散限制的程度估计了特征流体力学屏蔽长度ξ。在一定温度以下，Dkap 达到了高于 Dpul 的值，表明溶质κ-卡拉胶的 Mw 低于普鲁兰。GPC 测量证实了存在分子量低于普鲁兰的κ-卡拉胶链。提出了κ-卡拉胶溶液结构变化的简单物理模型，具有高于和低于普鲁兰探针的κ-卡拉胶的双峰分布。较高 Mw 的链形成凝胶网络，限制探针的扩散，而较低 Mw 的链增加有效粘度。从 Ikap（0）和总κ-卡拉胶浓度估计了 1％、2％和 4％κ-卡拉胶溶液中高 Mw 溶质κ-卡拉胶链的浓度，并研究了与普鲁兰扩散的关系。
