Conformational analyses of Xanthan gum polymer: experimental, molecular dynamics and quantum mechanical studies

The effect of temperature on the conformational behaviour of xanthan gum polymer in an aqueous solution (XG (aq)) was investigated. Viscosity values of XG (aq) were measured in the temperature range of 20–60°C. Traditionally, density and concentration have similar nature. However, conformational changes of polymers in aqueous solutions may lead to alteration of density values while such structural changes do not alter concentrations. The plots of density values versus concentrations reveal abnormal behaviours of XG (aq) at 35–60°C, which may be attributed to the conformational changes. The interactions of XG (aq) with human serum albumin (HSA) protein were explored via fluorescence studies. The rate of change of the Stern–Volmer constant as well as bonding parameters demonstrates a turning point at 37°C. Molecular dynamics (MD) simulations of XG (aq) were performed after equilibrium and at the last 60 ns. Three different conformational variables, namely at the backbone of β-1,4-D-glucopyranose glucan linkage, at the side chain of mannose (β-1,4) linkage and at the side chain of glucuronic acid (β-1,2) linkage were defined. Density functional theory (DFT) studies verified the existence of intra- and intermolecular interactions. The combination of xanthan gum polymer with metformin as an anti-diabetic drug has the most stability at normal body temperature.本研究考察了温度对水溶液中黄原胶聚合物（xanthan gum polymer）构象行为的影响。在20~60℃的温度范围内，测定了黄原胶水溶液（XG(aq)）的黏度值。传统认知中，密度与浓度的变化特性相近，但水溶液中聚合物的构象变化可导致密度值发生改变，而此类结构变化并不会改变浓度。密度-浓度曲线显示，黄原胶水溶液在35~60℃区间表现出异常行为，该现象可归因于其构象变化。本研究通过荧光光谱实验探究了黄原胶水溶液与人血清白蛋白（human serum albumin, HSA）蛋白的相互作用。斯特恩-沃尔默常数（Stern–Volmer constant）的变化速率以及结合参数均表明，在37℃处存在一个转折点。本研究对平衡态及最后60纳秒时段的黄原胶水溶液开展了分子动力学（molecular dynamics, MD）模拟，定义了三类不同的构象变量，分别对应β-1,4-D-吡喃葡萄糖葡聚糖主链糖苷键、甘露糖侧链（β-1,4）糖苷键以及葡萄糖醛酸侧链（β-1,2）糖苷键处的构象变化。密度泛函理论（density functional theory, DFT）验证了分子内与分子间相互作用的存在。黄原胶聚合物与作为抗糖尿病药物的二甲双胍的复合体系在正常体温下具有最高稳定性。