Molecular mobility and microscopic structure changes in κ-carrageenan solutions studied by gradient NMR.
Molecular mobility and microscopic structure changes in κ-carrageenan solutions studied by gradient NMR.
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DOI：
10.1016/j.carbpol.2013.02.049
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发表时间：
2013-06
影响因子：
11.2
通讯作者：
Qiuhua Zhao;T. Brenner;S. Matsukawa
中科院分区：
文献类型：
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作者：
Qiuhua Zhao;T. Brenner;S. Matsukawa
采用脉冲场梯度回波（PGSTE）和Carr-Purcell-Meiboom-Gill（CPMG）方法观察了κ-卡拉胶分子迁移率的变化，以阐明溶胶-凝胶转变的分子方面。无梯度的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随温度的降低而增加，表明溶质κ-卡拉胶链的分子量Mw低于参与聚集的链。以普鲁兰多糖（Dpul）为探针分子，测定了其在κ-卡拉胶溶液中的扩散系数，并根据扩散限制程度估算了特征流体力学筛选长度λ.在一定温度下，κ-卡拉胶的分子量Dkap大于Dpul，表明κ-卡拉胶的分子量低于普鲁兰多糖。GPC测量证实了具有比支链淀粉低的Mw的κ-卡拉胶链的存在。提出了κ-卡拉胶溶液中结构变化的简单物理模型，其中κ-卡拉胶具有比普鲁兰探针更高和更低的Mw的双峰分布。较高的分子量链形成限制探针扩散的凝胶网络，而较低的分子量链增加有效粘度。由Ikap（0）和总κ-卡拉胶浓度估算了1%、2%和4% κ-卡拉胶溶液中高分子量κ-卡拉胶链的浓度，并研究了其与普鲁兰多糖扩散的关系。
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 T2and the diffusion coefficient of κ-carrageenan (Dkap) increased with decreasing temperature, indicating that the solute κ-carrageenan chains have a lower molecular weight Mwthan 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, Dkapreached a higher value than that of Dpul, suggesting that the Mwof solute κ-carrageenan became lower than that of pullulan. GPC measurements confirmed the presence of κ-carrageenan chains with a lower Mwthan 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 Mwthan the pullulan probe. The higher Mwchains form the gel network restricting the probe's diffusion, and the lower Mwchains increase the effective viscosity. The concentration of the high Mwsolute 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.