Overhauser DNP (ODNP) is an emerging and novel magnetic resonance technique that has been used in several of our studies to perform highly localized measurements of translational diffusivity on molecular and lipid membrane surfaces. We find that the local hydration dynamics on the surface and interface of lipid membranes are intimately modulated by the same factors that are known to modulate their key properties, such as Hofmeister cations and anions, various osmolytes, cholesterol, and surface-active polymers, such as poloxamers. As these surface-active species dramatically modulate surface water dynamics, i.e. the compressibility of the surface hydration later, they directly tune the approachability ofbiomacromolecules to lipid membrane surfaces. Interestingly, crowding agents that may dramatically increase the bulk water viscosity, such as sucrose, glycerol or ficoll, do not retard the lipid membrane surface dynamics accordingly. Specifically, both a >10-fold increase in bulk viscosity and the restriction of water diffusion by confinement on a multiple-nm length-scale, do not change the local translational diffusion coefficient of the membrane surface water by much, namely less than 2.5-fold. We believe that the surface topology and chemistry at the ≤ 1 nm scale, rather than the characteristics of the bulk fluid, nearly exclusively determine the surface hydration dynamics of aqueous macromolecular solutes. These studies impact the unraveling of lipid membrane biophysics, as well as the mechanistic understanding of inter-protein and protein-lipid membrane interaction, a long-term goal of our lab.
Our group is interested in elucidating the role of specific ions, osmolytes, cosolvents and lipid composition on the surface properties of membranes under ambient solution conditions. Our experimental tool is the study of local surface hydration dynamics, based on the idea that surface water plays the role of a “gateway” to intermolecular and surface interactions, whose strength is modulated by water hydrogen bonding structure and dynamics. Among others, we study the effects of specific ions (also known as Hofmeister ions) on the local translational motion of water around small chemical defects and near large hydrophilic lipid vesicle surfaces by Overhauser dynamic nuclear polarization (ODNP) measurements. We found specific ions to modulate the diffusion of water near small chemical defects that harbor the unpaired electron spin probe (depicted in image). Experimental findings further suggest that the chemical defects preferentially orient water protons towards the unpaired electron, and thus pull and push nearby polarized water molecules through differential electrostatic forces exerted by the specific ions. Surface water diffusivity reports on the activation barrier of the surface hydration layer, i.e., how tightly water is bound to the hydrophilic surface and surrounding waters. We find this diffusivity to be directly modulated by the presence of specific ions in solution, with its order following the known Hofmeister series. Currently, we are pursuing to find molecular and theoretical descriptions of how ions affect the hydration structure at the hydrophilic surface, and its role in mediating and influencing interactions of other biomolecular constituents to and with membrane surfaces.
We are closely investigating the information content of surface hydration water’s diffusivity that reflect on intrinsic molecular surface properties. Interestingly, we find that whether the surface water dynamics is coupled or decoupled from the bulk solvent viscosity depends on the molecular composition of the “viscogen”. Surface-active molecules may cause a stronger coupling between its effect on bulk v.s. surface water, while many generic viscogens show entirely non-linear coupling between bulk viscosity and surface water dynamics. For example, the translational hydration dynamics within 0.5-1.5 nanometers (nm) of the surface of a DPPC liposome – a model biomacromolecular surface – was analyzed by Overhauser dynamic nuclear polarization (ODNP) measurements. We find that dramatic changes to the bulk solvent cause only weak changes in the surface hydration dynamics, if ficoll or sucrose is used that present generic viscogens. Specifically, both a >10-fold increase in bulk viscosity and the restriction of diffusion by confinement on a multiple-nm length-scale change the local translational diffusion coefficient of the surface water surrounding the lipid bilayer by less than 2.5-fold. By contrast, previous ODNP studies have shown that changes to the biomacromolecular surface induced by folding, binding, or aggregation can cause local hydration dynamics to vary by factors of up to 30. We suggest that the surface topology and chemistry at the 1:5 nm scale, rather than the characteristics of the bulk fluid, nearly exclusively determine the surface hydration dynamics of aqueous macromolecular solutes. The decoupling of the surface and bulk dynamics may prove important to the function of biomolecular systems in cellular environments. Proteins and cell membranes can gather a soft shell of hydrating water molecules.They provide a microenvironment that remains relatively unperturbed by the surrounding crowded environment and is likely crucial for maintaining structure and function. Even in the incredibly crowded cytoplasm, where macromolecules occupy 20-30% of the volume, these results imply that water at the surface of the macromolecule will display relatively unperturbed properties, unless interactions occur that penetrate into and perturb this microenvironment at the nanometer scale.
The emerging Overhauser Dynamic Nuclear Polarization (ODNP) technique allows highly localized (5-10Å) measurements of translational diffusivity of surface water. In collaboration with the group of Prof. Peter Qin at USC, we apply ODNP to measure the water dynamics near the phosphate backbone of generic DNA duplexes (figure shows a short DNA duplex with spin labels positioned at three different positions, with each sample labeled at one position at a time). By implementing a novel approach to ODNP analysis, we are able to separate the contributions of bound/exchanging waters from those undergoing free translational diffusion. We find that a significant population of water near the DNA surface exhibits fast, bulk-like characteristics that, within the localized volume probed by ODNP, moves unusually rapidly compared to near protein and membrane surfaces. This implies that the probed hydration water binds relatively weakly to the DNA surface and its hydration water that we thus refer to as being “soft.” Such water would be more easily moved to rehydrate a new intra- or inter-molecular surface approaching the DNA surface, facilitating interactions. We test for the effects of local motion of the DNA and attached moieties, and find the observation of fast dynamics to be independent of such local motion. Consistently with literature knowledge, we observe contributions from bound/exchanging hydrogen nuclei, but find the property of unusually rapidly diffusing water preserved on DNA surfaces. The high mobility of the solvation water around DNA has implications for how the hydrations shells of protein and DNA molecules have been tailored to their contrasting fundamental roles. We work on the interesting hypotheses that the radically different dynamics of the solvation water around proteins and DNA may be in keeping with the fundamentally divergent roles that the two types of macromolecules play in the central dogma of molecular biology.