Emerging magnetic resonance techniques can offer insight into how water responds to severe confinement. Confinement quenches the ability of water molecules to engage in collective motion, and proves relevant to the structure and function of both synthetic materials and biological macromolecules. However, this study focuses primarily on reverse micelles of various compositions, since they serve as an excellent model system, both for modeling variable confinement effects, and for interrogating how confined water molecules respond to changes in surface chemistry and dynamics. In particular, the development of specialized but robust spectroscopic techniques will highlight the change in translational and rotational dynamics of the water at different levels of confinement. Overhauser Dynamic Nuclear Polarization (ODNP) isolates the translational motion of water at the nanoscale, and reads out the result through variable enhancement of NMR (Nuclear Magnetic Resonance) signal. ODNP does not rely on higher-order spectroscopic effects, requires only a simple, versatile, and non-perturbative labeling strategy, and functions seamlessly in opaque, viscous, and other “dirty” systems. Despite previous applications to biological and polymer systems, ODNP still possesses many fundamental capabilities and a flexibility that remain largely untapped. Similarly, past research employed 2H NMR solution state relaxometry to a limited extent, but we will present how modern equipment can automate this technique and and deploy it across a wide range of samples. Thus, we describe measurements in highly phase-heterogeneous and confined systems and we include comparisons to molecular modeling simulations. We present holistic improvements to the ODNP methodology that enable its application to these systems. Finally, we demonstrate that ODNP can work synergistically with fully automated 2H NMR relaxometry and 1H NMR diffusometry develop a complete picture of translational motion of the water, rotational motion of the water, and translational motion of the reverse micelle assemblies, respectively.

Automated nuclear and electron-nuclear magnetic resonance relaxometry probes the dynamics of confined water molecules

ACS Spring 2022

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