Spin diffusion transfer difference (SDTD) NMR: An advanced method for the characterisation of water structuration within particle networks

Abstract

Saturation transfer difference (STD) NMR spectroscopy is a well-known ligand-based solution NMR technique used extensively for ligand epitope mapping, the identification of the nature of ligand binding sites, and the determination of ligand binding affinity. Recently, we have shown that STD NMR can be also applied to monitor changes in bound water during gelation of particulate dispersions. However, this technique is strongly dependent on gelator and solvent concentrations and does not report on the degree of organisation of the solvent within the particle network. This obscures the detailed understanding of the role of the solvent on gelation and precludes the comparison of solvation properties between dispersions prepared under different experimental conditions. In this work we report a novel STD NMR method to characterise the degree of solvent structuration in carbohydrate-based particulate dispersions by demonstrating for the first time that, for solvents interacting with large particles, the spin diffusion transfer build-up curves can be modelled by the general one-dimensional diffusion equation. Our novel approach, called Spin Diffusion Transfer Difference (SDTD) NMR, is independent of the gelator and solvent concentrations, allowing 2 to monitor and compare the degree of solvent structuration in different gel networks. In addition, the simulation of SDTD build-up curves report on minimum distances (r) and spin diffusion rates (D) at the particle-solvent interface. As a case study, we have characterised the degree of structuration of water and low molecular weight alcohols during the alcohol-induced gelation of TEMPO-oxidised cellulose hydrogels by SDTD NMR, demonstrating the key role of water structuration on gel properties. SDTD NMR is a fast, robust and easy-to-implement solution NMR protocol that overcomes some of the limitations of the classical STD NMR approach when applied to the study of solvation. This technique can be readily extended to characterise the solvent(s) organisation in any type of particulate gels. Hence, the SDTD NMR method provides key insights on the role of water in the mechanism of gelation and the macroscopic properties of particulate gels, of fundamental importance for the design of soft matter materials with tuneable properties.

Insights into the Gelation Mechanism of Metal-Coordinated Hydrogels by Paramagnetic NMR Spectroscopy and Molecular Dynamics

“…The faster accumulation of magnetization in the systems in the presence of Fe 3+ ions might be correlated to a structuration of water molecules in the metal-coordinated system. To further confirm our observation, we applied the SDTD NMR methodology . This methodology relies on the normalization of the STD factors, followed by fitting of the curves to the SDTD equation (eq , see Materials and Methods).…”

“…STD NMR is a well-established technique for investigating the dynamic binding of small molecules with a low-to-medium binding affinity to proteins . In soft matter, STD NMR has been used to study both gelator and solvent molecules, , with recent application for elucidation of supramolecular protein–polymer interactions …”

“…The obtained SDTD build-up curves were then fitted to the SDTD equation, derived from Fick’s second law of diffusion where SDTD is the normalized STD intensity values, C is the proportionality constant of the fit, erfc is the complementary error function, r is the minimum distance between the gelator particles and the HDO molecule (expressed in nm), D SD is the spin diffusion coefficient at the particle–solvent interface (expressed in nm 2 /ms), t sat is saturation time (expressed in ms), and b is a correction factor to center the Gaussian distribution. Fitting of the curves was performed by keeping the r and b parameters constant.…”

“…9 STD NMR is a well-established technique for investigating the dynamic binding of small molecules with a low-to-medium binding affinity to proteins. 56 In soft matter, STD NMR has been used to study both gelator 8 and solvent molecules, 10,57 with recent application for elucidation of supramolecular protein−polymer interactions. 58 To monitor the interaction of CMC 1% w/v dispersions with residually protonated water (HDO, present in solution following the exchange of one of the deuterium atoms of D 2 O by a proton), we recorded a series of STD build-up curves with saturation time ranging from 1 to 20 s and from 0.1 to 8 s for CMC dispersions in the absence and in the presence of Fe 3+ ions, respectively (Figures 2a and S2).…”

“…In this context, nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) calculations have proven to be excellent tools to achieve a fundamental understanding and atomistic details of the mechanism of gelation, unraveling the dynamics and structure of the interacting constituents of the hydrogel, including the crucial role that water plays during gelation. − In addition, combination with other analytical techniques, such as vibrational and X-ray spectroscopies, is an effective strategy to achieve a multiscale view of metal-coordinated hydrogels, establishing a link between the hydrogel’ chemical structure, the nature of metal-coordinated and cross-linked molecular structures, and the larger-scale organization.…”