Probing Macromolecular Interactions through the Modulation of Hydration Dynamics at the Lipid Membrane Interfaces by Overhauser Dynamic Nuclear Polarization

Abstract

Hydration water at molecular interfaces is a sensitively responsive media that is critically coupled to molecular interactions and their associated functions. We study the interactions between lipid membranes and macromolecules (e.g. proteins and polymers) by an ultra-sensitive technique, 1H Overhauser Dynamic Nuclear Polarization (ODNP), through the modulation of translational hydration dynamics at molecular interfaces. It relies on selectively amplified 1H-NMR signals within 10∼20A distance of localized spin labels by r−3-distance dependence of dipolar interaction between electrons and water protons. This powerful approach provides the capability to probe hydration dynamics in deeply buried as well as solvent-exposed molecular interfaces, and enables to explore a wide range of molecular interactions at lipid membrane interfaces with sensitivity and site-specificity under ambient conditions.Here we present two examples to illustrate that the underlying functions of membrane-active polymers and membrane proteins are strongly mediated through interfacial hydration dynamics. Despite poloxamers, amphiphilic triblock copolymers, are employed as a membrane sealant or permeabilizer, the molecular basis behind their functions is unclear. (collaboration: Jia-Yu Wang, Ka Yee Lee; University of Chicago) We found poloxamers present vastly different functions to hydration diffusivity in the lipid membranes, depending on their hydrophobicities and architectures. Using this approach, we study the interaction interface of membrane-bound α-synuclein, the protein that is critically related to the Parkinson's disease. (collaboration: Jobin Varkey, Ralf Langen; University of Southern California) Our results confirm α-synuclein forms α-helix upon membrane binding, whereas its C-terminus remains unstructured. Remarkably, we found α-synuclein can form a large twist as the extended α-helix proceeds to the C-terminus. These findings showcase the strength of ODNP to unravel the biophysical functions of macromolecules upon their interactions with lipid membranes through the sensitive detection of modulated hydration dynamics at interaction interfaces.