Probing the hydration water diffusion of macromolecular surfaces and interfaces

We probe the translational dynamics of the hydration water surrounding the macromolecular surfaces of selected polyelectrolytes, lipid vesicles and intrinsically disordered proteins with site specificity in aqueous solutions. These measurements are made possible by the recent development of a new instrumental and methodological approach based on Overhauser dynamic nuclear polarization (DNP)-enhanced nuclear magnetic resonance (NMR) spectroscopy. This technique selectively amplifies ¹H NMR signals of hydration water around a spin label that is attached to a molecular site of interest. The selective ¹H NMR amplification within molecular length scales of a spin label is achieved by utilizing short-distance range (∼ r^-3) magnetic dipolar interactions between the ¹H spin of water and the electron spin of a nitroxide radical-based label. Key features include the fact that only minute quantities (<10μl) and dilute (≥ 100μM) sample concentrations are needed. There is no size limit on the macromolecule or molecular assembly to be analyzed. Hydration water with translational correlation times between 10 and 800 ps is measured within ∼10 Å distance of the spin label, encompassing the typical thickness of a hydration layer with three water molecules across. The hydration water moving within this time scale has significant implications, as this is what is modulated whenever macromolecules or molecular assemblies undergo interactions, binding or conformational changes. We demonstrate, with the examples of polymer complexation, protein aggregation and lipid-polymer interaction, that the measurements of interfacial hydration dynamics can sensitively and site specifically probe macromolecular interactions.