The structure and dynamics of water has captivated the interest of researchers for decades, but the devlopment techniques that can routinely deal with delicate, dynamic, and heterogenous natural systems, such as proteins embedded near or in lipid membranes, remains an area of active development. Magnetic Resonance (MR) experiments rely on resonance frequencies many orders of magnitude slower than those of, e.g., ultrafast laser experiments that have shown earlier promise for interrogating interfacial systems. Nonetheless, MR relaxometry techniques can still probe fast molecular dynamics in the frequency domain. These techniques prove capable of dealing with very heterogeneous systems. Furthermore, they offer the opportunity to investigate the spatial distributions of chemical species at the nanoscale. Currently, by utilizing a NMR-ESR dual resonance technique known as ODNP (Overhauser Dynamic Nuclear Polarization) one can both improve the sensitivity of measurements, allowing amplification of a surface layer against a bulk background, and also move to much faster timescales, thus unlocking a capability to interrogate the dynamics of water molecules in the hydration layer. Unique advantages inherent in NMR, such as directly measuring – rather than inferring – nuclear motion, yield a concomitant ability to compare directly to molecular structures and molecular dynamics simulations. Furthermore, 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 experimentally difficult systems. Notably, ODNP can interrogate differences in hydration water residing in different patches on the surface of the same macromolecule. In the context of recent technological advances our group has introduced, we explore various case studies mapping out the hydration dynamics of macromolecular assemblies. Water molecules subjected to severe confinement inside reverse micelles exhibit clear signatures of very slow motion, in contrast to water molecules permeating the lipid bilayer adjacent to functional transmembrane proteins, which prove unexpectedly mobile and prevalent. We discuss how these techniques and observations underscore the narrative that proteins and other macromolecules engage in a complicated interplay with the surrounding solvent, and that several circumstances can blur the boundaries between lipid and aqueous phases.

Water and lipid molecules at protein surfaces: Nanoscale distributions and dynamics

ACS Spring 2022

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