Probing the Surface Chemistry of Peptide Hydrogels using NMR Spectroscopy

Self-assembled hydrogels are formed upon the assembly in aqueous solution of low molecular weight gelators (LMWGs) into fibrous networks. These networks immobilise the water, conveying solid-like properties to the material. Self-assembled hydrogels have a wide range of applications, from cell culturing to environmental sensing and remediation. In the majority of cases, the utility of the gels arises from their self-assembled nature and high water content. These two properties, however, mean that the gels are notoriously difficult to characterise. Over the past two decades, techniques to characterise the hydrogels on a molecular level in a non-invasive manner have become well established. For example, the existence of hydrogen-bonding interactions between the assembled molecules can be probed using infra-red spectroscopy while the interactions between aromatic groups can be probed using UV-Vis spectroscopy. A neglected aspect of self-assembled hydrogels has been the interfacial, or surface, chemistry of the self-assembled fibres. Properties such as the charge, hydrophobicity and ion binding dynamics of the fibres determine the fundamental structure and behaviour of the materials. For example, these properties determine the interactions between adjacent fibres forming the gel networks which, in turn, determine the macroscopic mechanical properties of the materials. Nevertheless, established analytical techniques cannot directly provide information on the surface chemistry of the gel fibres. In this Thesis, novel techniques based on nuclear magnetic resonance spectroscopy (NMR) were developed to study the surface chemistry. The techniques can be implemented on most modern NMR equipment and are thus available to the majority of researchers in the field. Focussing on N-functionalised dipeptide LMWGs, it is demonstrated how the surface chemical properties of the gels can be measured by studying the interaction with the fibres of a range of probe molecules and ions. The probes include cations, cation-binders and hydrophobic organic solvents. A range of advanced NMR techniques has been developed to detect and quantify the interaction of these probe species with the NMR-invisible gel fibres. It is thus possible to study, for example, the loss of negative charge and increase in hydrophobicity of the gel fibres as the pH of a sample is decreased. The surface chemical properties of the gels can also be used to predict the interaction with the gel fibres of a number of model drug compounds as well as the stability of the gels when exposed to external solutions. Methods for the direct measurement of the solution pH and Ca2+ concentration by NMR spectroscopy are also presented in this Thesis. These methods greatly aid the analysis of samples by NMR as they avoid the need for separate electrochemical measurements. It is also demonstrated how controlled pH and Ca2+ concentration gradients may be established in standard 5 mm NMR tubes and analysed using standard NMR equipment