Diener Michael, Adamcik Jozef, Bergfreund Jotam, Catalini Sara, Fischer Peter, Mezzenga Raffaele
Department of Health Sciences and Technology, ETH Zürich, 8092 Zürich, Switzerland.
European Laboratory for Non-Linear Spectroscopy (LENS), Università di Firenze, 50121 Florence, Italy.
ACS Macro Lett. 2020 Jan 21;9(1):115-121. doi: 10.1021/acsmacrolett.9b00824. Epub 2020 Jan 6.
Polysaccharides are ubiquitous in nature; they serve fundamental roles in vivo and are used for a multitude of food, pharmaceutical, cosmetic biomaterials, and biomedical applications. Here, the structure-property function for low acetylated Gellan gum hydrogels induced by divalent ions was established by means of optical, rheological, and microscopic techniques. The hydrogels interacted with visible light as revealed by birefringence and multiple scattering, as a consequence of quaternary, supramolecular fibrillar structures. The molecular assembly and structure were elucidated by statistical analysis and polymer physics concepts applied to high-resolution AFM height images and further supported by FTIR. This revealed intramolecular coil-to-single helix transitions, followed by lateral aggregation of single helices into rigid, fibrillar quaternary structures, ultimately responsible for gelation of the system. Calcium and magnesium chloride were shown to lead to fibrils up to heights of 6.0 nm and persistence lengths of several micrometers. The change in molecular structure affected the macroscopic gel stiffness, with the plateau shear modulus reaching ∼10 Pa. These results shed light on the two-step gelation mechanism of linear polysaccharides, their conformational molecular changes at the single polymer level and ultimately the macroscale properties of the ensued gels.
多糖在自然界中无处不在；它们在体内发挥着重要作用，并被用于众多食品、制药、化妆品生物材料及生物医学应用中。在此，通过光学、流变学和显微镜技术建立了二价离子诱导的低乙酰化结冷胶水凝胶的结构-性质-功能关系。水凝胶与可见光相互作用，表现为双折射和多重散射，这是四级超分子纤维结构的结果。通过对高分辨率原子力显微镜高度图像应用统计分析和高分子物理概念阐明了分子组装和结构，并通过傅里叶变换红外光谱进一步证实。这揭示了分子内从卷曲到单螺旋的转变，随后单螺旋横向聚集形成刚性的纤维状四级结构，最终导致体系凝胶化。氯化钙和氯化镁可导致形成高度达6.0纳米、持续长度达几微米的纤维。分子结构的变化影响了宏观凝胶硬度，平台剪切模量达到约10帕斯卡。这些结果揭示了线性多糖的两步凝胶化机制、其在单个聚合物水平上的构象分子变化以及最终所得凝胶的宏观性质。
