Wallace Matthew, Iggo Jonathan A, Adams Dave J
Department of Chemistry, University of Liverpool, Crown Street, Liverpool, L69 7ZD, UK.
Soft Matter. 2015 Oct 21;11(39):7739-47. doi: 10.1039/c5sm01760b. Epub 2015 Aug 27.
Supramolecular hydrogels are formed via the self-assembly of gelator molecules upon application of a suitable trigger. The exact nature of this self-assembly process has been widely investigated as a practical understanding is vital for the informed design of these materials. Solution-state NMR spectroscopy is an excellent non-invasive tool to follow the self-assembly of supramolecular hydrogels. However, in most cases the self-assembled aggregates are silent by conventional (1)H NMR spectroscopy due to the low mobility of the constituent molecules, limiting NMR spectroscopy to following only the initial assembly step(s). Here, we present a new solution-state NMR spectroscopic method which allows the entire self-assembly process of a dipeptide gelator to be followed. This gelator forms transparent hydrogels by a multi-stage assembly process when the pH of an initially alkaline solution is lowered via the hydrolysis of glucono-δ-lactone (GdL). Changes in the charge, hydrophobicity and relative arrangement of the supramolecular aggregates can be followed throughout the assembly process by measuring the residual quadrupolar couplings (RQCs) of various molecular probes (here, (14)NH4(+) and isopropanol-d8), along with the NMR relaxation rates of (23)Na(+). The initially-formed aggregates comprise negatively charged fibrils which gradually lose their charge and become increasingly hydrophobic as the pH falls, eventually resulting in a macroscopic contraction of the hydrogel. We also demonstrate that the in situ measurement of pH by NMR spectroscopy is both convenient and accurate, representing a useful tool for the characterisation of self-assembly processes by NMR.
超分子水凝胶是在施加合适的触发因素后通过凝胶剂分子的自组装形成的。由于对这些材料进行明智的设计而言，对这种自组装过程的确切性质有实际的了解至关重要，因此该过程已得到广泛研究。溶液态核磁共振光谱是跟踪超分子水凝胶自组装的一种出色的非侵入性工具。然而，在大多数情况下，由于组成分子的流动性低，传统的（1）H NMR光谱无法检测到自组装聚集体，这限制了NMR光谱仅能跟踪初始组装步骤。在此，我们提出了一种新的溶液态NMR光谱方法，该方法可以跟踪二肽凝胶剂的整个自组装过程。当最初碱性溶液的pH值通过葡萄糖酸 - δ - 内酯（GdL）的水解而降低时，这种凝胶剂通过多阶段组装过程形成透明水凝胶。通过测量各种分子探针（此处为（14）NH4（+）和异丙醇 - d8）的残余四极耦合（RQC）以及（23）Na（+）的NMR弛豫率，可以在整个组装过程中跟踪超分子聚集体的电荷、疏水性和相对排列的变化。最初形成的聚集体由带负电荷的纤维组成，随着pH值下降，这些纤维逐渐失去电荷并变得越来越疏水，最终导致水凝胶发生宏观收缩。我们还证明，通过NMR光谱原位测量pH既方便又准确，是一种用于通过NMR表征自组装过程的有用工具。
