多检测器凝胶色谱测定海藻酸钠的摩尔质量及其分布和稀溶液构象

Determination of molar mass,distribution and conformational properties of alginate by TDA-GPC

本文用多检测器凝胶色谱技术测定了2种化学结构类似但动力黏度不同的海藻酸钠的摩尔质量及其分布和特性黏数([η])、Mark-Houwink指数(α)、流体力学半径(R_(h))及均方回转半径(R_(g))等参数.通过Mark-Houwink曲线,R_(g),R_(h)νs.M_(w)曲线,形状因子ρ,及蠕虫链模型对样品的稀溶液构象进行了分析.实验证明2个样品均为单一分布的海藻酸钠,其动力黏度的差异由其摩尔质量及其分布的不同所导致.构象分析显示在该溶解体系下,样品构象受摩尔质量的影响,以5×10^(5) g·mol^(-1)为界,摩尔质量较小的分子链呈刚性略强的伸展链构象,而摩尔质量较大的分子链则呈柔性的无规线团构象.

As a biomacromolecule extracted from the marine plant-brown algae,the Weight-average molar mass(M_(w)),number-average molar mass(M_(n)),weight distribution(M_(w)/M_(n))and conformational properties of sodium alginate are seriously affected by the raw material(the species of brown algae,produced see area,manufacturing season,etc.)and the extraction procedure.These properties have great effect on the processing and the mechanical properties of algi-nate materials.However,commercial sodium alginate powders on sale are usually distinguished by viscosity instead of M_(w) and M_(w)/M_(n),and sometimes the products are even compounded to achieve a particular viscosity,which certainly brings much difficulty for the further processing.Here,gel permeation chromatography(GPC)was used to detect the M_(w),M_(w)/M_(n) and other conformational properties of two types of sodium alginate,which had similar chemical structures but quite different dynamic viscosities.As a polyelectrolyte,the detection of alginate cannot be performed by traditional calibration method with GPC equipped with a single concentration detector.Herein,tridetector array GPC(TDA-GPC)equipped with three detectors-a differential refraction detector(RI),two laser light scattering detectors(placed at 7°and 90°and named LALS and RALS,respectively)and a viscosity detector(DP),were used to obtain the secondary structures of two alginate samples.Detailed test method and parameter such as M_(w),M_(w)/M_(n),intrinsic viscosity ([η]),Mark-Houwink index(α),hydrodynamic radius(R_(h)),mean square radius of gyration(R_(g))and refractive index increment(dn/dc)were calculated and detailed calculation formulas were listed for reference.In the actual course of testing,the repeatability of measurement results was firstly confirmed to determine whether the results were reliable.Then all the parameters were automatic calculated by thesoftware based on the method established with standard polyethylene oxide.The results showed that both alginate samples were single distributed,and the different dynamic viscosities derived from the differences of molecular weight and distribution.Besides,conformational properties were investigated through Mark-Houwink curve,R_(g) νs.M_(w),R_(h) νs.M_(w),the average structure parameterρ(R_(g)/R_(h))value and wormlike cylinder model.All results indicated a stretched coil conformation for molecules with relatively small Mwand flexible random coil conformation for large M_(w).It was interesting to note that the division points of the conformational rigidity and flexibility for both samples were both around 5.0×10^(5).Whether this is a universal property of alginate macromolecules worth need to be further studied.Besides,the result of dn/dc offered an empirical value for the TDA-GPC measurement of alginate with similar resource and extraction procedure.