Supramolecular Amino Acid Based Hydrogels: Probing the Contribution of Additive Molecules using NMR Spectroscopy.

Susana M Ramalhete,Jesús Angulo,Gareth O Lloyd,Jamie S Foster,Andrew N Round,Karol P Nartowski,Nichola Sarathchandra,Yaroslav Z Khimyak

doi:10.1002/chem.201700793

Abstract

Supramolecular hydrogels are composed of self‐assembled solid networks that restrict the flow of water. l‐Phenylalanine is the smallest molecule reported to date to form gel networks in water, and it is of particular interest due to its crystalline gel state. Single and multi‐component hydrogels of l‐phenylalanine are used herein as model materials to develop an NMR‐based analytical approach to gain insight into the mechanisms of supramolecular gelation. Structure and composition of the gel fibres were probed using PXRD, solid‐state NMR experiments and microscopic techniques. Solution‐state NMR studies probed the properties of free gelator molecules in an equilibrium with bound molecules. The dynamics of exchange at the gel/solution interfaces was investigated further using high‐resolution magic angle spinning (HR‐MAS) and saturation transfer difference (STD) NMR experiments. This approach allowed the identification of which additive molecules contributed in modifying the material properties.

Highlights

Hydrogels are semi-solid colloidal materials which contain a three-dimensional (3D) network with a fine structure that can often be defined by non-covalent interactions.[1, 2] They contain large amounts of water, entrapped by capillary and surface forces[1, 2], which provide liquid-like properties to these solid-like rheological systems.[3, 4] This feature explains their great resemblance to human tissues.[3]
The development of multi-component systems proved to be an elegant strategy to modify the rheological properties of the “gel-crystal” supramolecular system
It was possible to correlate the strength of the gel with the composition of the gel fibres and solid-liquid interface in the presence of additive gelator molecules

Summary

Introduction

Hydrogels are semi-solid colloidal materials which contain a three-dimensional (3D) network with a fine structure that can often be defined by non-covalent interactions.[1, 2] They contain large amounts of water, entrapped by capillary and surface forces[1, 2], which provide liquid-like properties to these solid-like rheological systems.[3, 4] This feature explains their great resemblance to human tissues.[3]. The introduction of co-gelators or non-gelating additive molecules has been reported previously in other gel systems to create an additional level of control and allow tailoring of the physical properties of gels through modifications to their supramolecular structure (Figure 1).[14]

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