Probing the surface chemistry of self-assembled peptide hydrogels using solution-state NMR spectroscopy

Abstract

The surface chemistry of self-assembled hydrogel fibres - their charge, hydrophobicity and ion-binding dynamics - is recognised to play an important role in determining how the gels develop as well as their suitability for different applications. However, to date there are no established methodologies for the study of this surface chemistry. Here, we demonstrate how solution-state NMR spectroscopy can be employed to measure the surface chemical properties of the fibres in a range of hydrogels formed from N-functionalised dipeptides, an effective and versatile class of gelator that has attracted much attention. By studying the interactions with the gel fibres of a diverse range of probe molecules and ions, we can simultaneously study a number of surface chemical properties of the NMR invisible fibres in an essentially non-invasive manner. Our results yield fresh insights into the materials. Most notably, gel fibres assembled using different tiggering methods bear differing amounts of negative charge as a result of a partial deprotonation of the carboxylic acid groups of the gelators. We also demonstrate how chemical shift imaging (CSI) techniques can be applied to follow the formation of hydrogels along chemical gradients. We apply CSI to study the binding of Ca and subsequent gelation of peptide assemblies at alkaline pH. Using metal ion-binding molecules as probes, we are able to detect the presence of bound Ca ions on the surface of the gel fibres. We briefly explore how knowledge of the surface chemical properties of hydrogels could be used to inform their practical application in fields such as drug delivery and environmental remediation.

Probing the self-assembled structures and pKa of hydrogels using electrochemical methods

“…Methods to characterise the fibre interactions during self-assembly are also limited. 29 The charge, hydrophobicity and ion-binding dynamics of the gelators and fibres play an important role in self-assembly which ultimately determines the suitability of the gels for specific applications. 29 The pK a is formally defined as the pH when 50% of the molecules in aqueous solution exist in a protonated form and the other half are deprotonated.…”

“…29 The charge, hydrophobicity and ion-binding dynamics of the gelators and fibres play an important role in self-assembly which ultimately determines the suitability of the gels for specific applications. 29 The pK a is formally defined as the pH when 50% of the molecules in aqueous solution exist in a protonated form and the other half are deprotonated. 30 If gelators are aggregated, in a multicomponent system, or have two or more apparent pK a values, then the definition is not as simple.…”

Controlling the properties of the micellar and gel phase by varying the counterion in functionalised-dipeptide systems

“…D 2 O and dioxane-d 8 were used as complementary 2 H probes. 23 The apparent pK a of the terminal carboxylic acid is different in all cases, varying from 9.6 to 4.9 (titration data shown in Fig. S5, ESI †).…”

“…3). The 7 Li, 23 Na and 133 Cs resonances exhibit quadrupolar splitting while the 87 Rb resonance is significantly broadened relative to the native hydroxide (Fig. S4, ESI †).…”

The electrochemical fabrication of hydrogels: a short review

“…Advances in in situ analysis techniques for gelation such as NMR [71][72][73] have limited use in electrochemical gels due to the inability to physically apply the methods. Wallace et al [71][72][73] have shown how NMR can be used to determine the gelator pK a and pore size in bulk gelation. Development of these techniques for an electrochemical system would allow for greater analysis of gelation kinetics.…”