Multiple quantum filtered nuclear magnetic resonance of Na-23(+) in uniformly stretched and compressed hydrogels

Stretching or compressing hydrogels creates anisotropic environments that lead to motionally averaged alignment of embedded guest quadrupolar nuclear spins such as 23Na+. These distorted hydrogels can elicit a residual quadrupolar coupling that gives an oscillation in the trajectories of single quantum coherences (SQCs) as a function of the evolution time during a spin-echo experiment. We present solutions to equations of motion derived with a Liouvillian superoperator approach, which encompass the coherent quadrupolar interaction in conjunction with relaxation, to give a full analytical description of the evolution trajectories of rank-1 ( <^>T 1 +/- 1), rank-2 ( <^>T 2 +/- 1), and rank-3 ( <^>T 3 +/- 1) SQCs. We performed simultaneous numerical fitting of the experimental 23Na nuclear magnetic resonance (NMR) spectra and rank-2 ( <^>T 2 +/- 1) and rank-3 ( <^>T 3 +/- 1) SQC evolution trajectories measured in double and triple quantum filtered experiments, respectively. We estimated values of the quadrupolar coupling constant CQ, rotational correlation time tC, and 3 x 3 Saupe order matrix. We performed simultaneous fitting of the analytical expressions to the experimental data to estimate values of the quadrupolar coupling frequency.Q/2p, residual quadrupolar coupling..Q./2p, and corresponding spherical order parameter S* 0, which showed a linear dependence on the extent of uniform hydrogel stretching and compression. The analytical expressions were completely concordant with the numerical approach. The insights gained here can be extended to more complicated (biological) systems such as 23Na+ bound to proteins or located inside and outside living cells in high-field NMR experiments and, by extension, to the anisotropic environments found in vivo with 23Na magnetic resonance imaging (c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Automated summary

Stretching or compressing hydrogels leads to alignment of embedded guest quadrupolar nuclear spins. This alignment generates residual quadrupolar coupling and oscillation of single quantum coherences during spin-echo experiments. By using Liouvillian superoperator approach, we provide analytical descriptions of rank-1, rank-2, and rank-3 SQCs' evolution trajectories. The insights gained from this study can be applied to more complex systems and anisotropic environments found in high-field NMR experiments and sodium magnetic resonance imaging.