Ultrasensitive Detection of Interfacial Water Diffusion on Lipid Vesicle Surfaces at Molecular Length Scales

Dynamical Crossover of Interfacial Water upon Melting of a Lipid Bilayer: Influence of Different Parts of the Headgroups

All-atom molecular dynamics simulations of a 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayer in contact with liquid water were performed at different temperatures ranging from 285 to 320 K. We have investigated the heterogeneity and dynamical transitions in interfacial water as the lipid bilayer undergoes a melting transition. Results are obtained for water at the outer surface of the bilayer and for those buried more deeply in the lipid chains of the bilayer. It is found that lipid bilayer melting influences both the structure and dynamics of interfacial water. The number of interfacial water molecules shows a jump in the melting of the bilayer. The temperature dependence of the diffusivity and orientational relaxation of interfacial water molecules exhibits a dynamical crossover upon melting of the bilayer. The extent of dynamical crossover is found to be rather strong with significant changes in activation barriers for interfacial water around the carbonyl groups, which are deeply buried toward the lipid chains of the bilayer. The dynamical crossover gradually decreases as one moves further away from the outer surface, and it essentially vanishes for water in the region of 5-10 Å from the outer surface. It is found that the lipid melting-induced dynamical crossover of interfacial water is significant only for water that is in close proximity to the bilayer surface or deeply buried into it. The current results reveal that water molecules in different parts of the interface respond differently on melting of the bilayer. The current study also shows that the carbonyl-bound water molecules can play an important role in the phase transition of the bilayer as the temperature is raised through its melting point.

Location of the TEMPO moiety of TEMPO-PC in phosphatidylcholine bilayers is membrane phase-dependent

The (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) moiety tethered to the headgroup of phosphatidylcholine (PC) lipid is employed in spin labeling electron paramagnetic resonance (EPR) spectroscopy to probe the water dynamics near lipid bilayer interfaces. Due to its amphiphilic character, however, TEMPO spin label could partition between aqueous and lipid phases, and may even be stabilized in the lipid phase. Accurate assessment of the TEMPO-PC configuration in bilayer membranes is essential for correctly interpreting the data from measurements. Here, we carry out all-atom molecular dynamics (MD) simulations of TEMPO-PC probe in single-component lipid bilayers at varying temperatures, using two standard MD force fields. We find that for DPPC membrane whose gel-to-fluid lipid phase transition occurs at 314 K, while the TEMPO spin label is stabilized above the bilayer interface in the gel phase, there is a preferential location of TEMPO below the membrane interface in the fluid phase. For bilayers made of unsaturated lipids, DOPC and POPC, which adopt the fluid phase at ambient temperature, TEMPO is unequivocally stabilized inside the bilayers. Our finding of membrane phase-dependent positioning of TEMPO moiety highlights the importance of assessing the packing order and fluidity of lipids under a given measurement condition.

Nanometer-Scale Water Dynamics in Nafion Polymer Electrolyte Membranes: Influence of Molecular Hydrophobicity and Water Content Revisited

The ionic conductivity of polymer electrolyte membranes (PEMs) is an essential parameter for their device applications. In water-swollen PEMs, protons and other ions are transferred through hydrophilic channels of a few nanometers in diameter at most. Thus, optimizing the chemical and physical properties of the channels can enhance the conductivity of PEMs. However, the factors controlling the conductivity have not been completely clarified. Here, we report that measurements taken near the channel walls by a special nuclear magnetic resonance technique with ≤1 nm spatial resolution showed the largest water diffusivity when ∼80% of hydrophilic sulfonic acid groups were blocked, but the proton conductivity was low. The water diffusivity was much less affected by differences in water content. Our results provide a concept for changing the properties of PEMs and a challenge to implement the improved diffusivity in a way that enhances net ion conductivity.

Overhauser Dynamic Nuclear Polarization for the Study of Hydration Dynamics, Explained

We outline the physical properties of hydration water that are captured by Overhauser Dynamic Nuclear Polarization (ODNP) relaxometry and explore the insights that ODNP yields about the water and the surface that this water is coupled to. As ODNP relies on the pairwise cross-relaxation between the electron spin of a spin probe and a proton nuclear spin of water, it captures the dynamics of single-particle diffusion of an ensemble of water molecules moving near the spin probe. ODNP principally utilizes the same physics as other nuclear magnetic resonance (NMR) relaxometry (i.e., relaxation measurement) techniques. However, in ODNP, electron paramagnetic resonance (EPR) excites the electron spins probes and their high net polarization acts as a signal amplifier. Furthermore, it renders ODNP parameters highly sensitive to water moving at rates commensurate with the EPR frequency of the spin probe (typically 10GHz). Also, ODNP selectively enhances the NMR signal contributions of water moving within close proximity to the spin label. As a result, ODNP can capture ps-ns movements of hydration waters with high sensitivity and locality, even in samples with protein concentrations as dilute as 10 µM. To date, the utility of the ODNP technique has been demonstrated for two major applications: the characterization of the spatial variation in the properties of the hydration layer of proteins or other surfaces displaying topological diversity, and the identification of structural properties emerging from highly disordered proteins and protein domains. The former has been shown to correlate well with the properties of hydration water predicted by MD simulations and has been shown capable of evaluating the hydrophilicity or hydrophobicity of a surface. The latter has been demonstrated for studies of an interhelical loop of proteorhodopsin, the partial structure of α-synuclein embedded at the lipid membrane surface, incipient structures adopted by tau proteins en route to fibrils, and the structure and hydration profile of a transmembrane peptide. This chapter focuses on offering a mechanistic understanding of the ODNP measurement and the molecular dynamics encoded in the ODNP parameters. In particular, it clarifies how the electron-nuclear dipolar coupling encodes information about the molecular dynamics in the nuclear spin self-relaxation and, more importantly, the electron-nuclear spin cross-relaxation rates. The clarification of the molecular dynamics underlying ODNP should assist in establishing a connection to theory and computer simulation that will offer far richer interpretations of ODNP results in future studies.

Asymmetric osmotic water permeation through a vesicle membrane

Understanding the water permeation through a cell membrane is of primary importance for biological activities and a key step to capture its shape transformation in salt solution. In this work, we reveal the dynamical behaviors of osmotically driven transport of water molecules across a vesicle membrane by molecular dynamics simulations. Of particular interest is that the water transport in and out of vesicles is highly distinguishable given the osmotic force are the same, suggesting an asymmetric osmotic transportation. This asymmetric phenomenon exists in a broad range of parameter space such as the salt concentration, temperature, and vesicle size and can be ascribed to the similar asymmetric potential energy of lipid-ion, lipid-water, lipid-solution, lipid-lipid, and the lipid-lipid energy fluctuation. Specifically, the water flux has a linear increase with the salt concentration, similar to the prediction by Nernst-Planck equation or Fick's first law. Furthermore, due to the Arrhenius relation between the membrane permeability and temperature, the water flux also exhibits excellent Arrhenius dependence on the temperature. Meanwhile, the water flux shows a linear increase with the vesicle surface area since the flux amount across a unit membrane area should be a constant. Finally, we also present the anonymous diffusion behaviors for the vesicle itself, where transitions from normal diffusion at short times to subdiffusion at long times are identified. Our results provide significant new physical insights for the osmotic water permeation through a vesicle membrane and are helpful for future experimental studies.

Bicontinuous Fluid Structure with Low Cohesive Energy: Molecular Basis for Exceptionally Low Interfacial Tension of Complex Coacervate Fluids

An exceptionally low interfacial tension of a dense fluid of concentrated polyelectrolyte complexes, phase-separated from a biphasic fluid known as complex coacervates, represents a unique and highly sought-after materials property that inspires novel applications from superior coating to wet adhesion. Despite extensive studies and broad interest, the molecular and structural bases for the unique properties of complex coacervates are unclear. Here, a microphase-separated complex coacervate fluid generated by mixing a recombinant mussel foot protein-1 (mfp-1) as the polycation and hyaluronic acid (HA) as the polyanion at stoichiometric ratios was macroscopically phase-separated into a dense complex coacervate and a dilute supernatant phase to enable separate characterization of the two fluid phases. Surprisingly, despite up to 4 orders of magnitude differing density of the polyelectrolytes, the diffusivity of water in these two phases was found to be indistinguishable. The presence of unbound, bulk-like, water in the dense fluid can be reconciled with a water population that is only weakly perturbed by the polyelectrolyte interface and network. This hypothesis was experimentally validated by cryo-TEM of the macroscopically phase-separated dense complex coacervate phase that was found to be a bicontinuous and biphasic nanostructured network, in which one of the phases was confirmed by staining techniques to be water and the other polyelectrolyte complexes. We conclude that a weak cohesive energy between water-water and water-polyelectrolytes manifests itself in a bicontinuous network, and is responsible for the exceptionally low interfacial energy of this complex fluid phase with respect to virtually any surface within an aqueous medium.

DMSO induces dehydration near lipid membrane surfaces

Dimethyl sulfoxide (DMSO) has been broadly used in biology as a cosolvent, a cryoprotectant, and an enhancer of membrane permeability, leading to the general assumption that DMSO-induced structural changes in cell membranes and their hydration water play important functional roles. Although the effects of DMSO on the membrane structure and the headgroup dehydration have been extensively studied, the mechanism by which DMSO invokes its effect on lipid membranes and the direct role of water in this process are unresolved. By directly probing the translational water diffusivity near unconfined lipid vesicle surfaces, the lipid headgroup mobility, and the repeat distances in multilamellar vesicles, we found that DMSO exclusively weakens the surface water network near the lipid membrane at a bulk DMSO mole fraction (XDMSO) of <0.1, regardless of the lipid composition and the lipid phase. Specifically, DMSO was found to effectively destabilize the hydration water structure at the lipid membrane surface at XDMSO <0.1, lower the energetic barrier to dehydrate this surface water, whose displacement otherwise requires a higher activation energy, consequently yielding compressed interbilayer distances in multilamellar vesicles at equilibrium with unaltered bilayer thicknesses. At XDMSO >0.1, DMSO enters the lipid interface and restricts the lipid headgroup motion. We postulate that DMSO acts as an efficient cryoprotectant even at low concentrations by exclusively disrupting the water network near the lipid membrane surface, weakening the cohesion between water and adhesion of water to the lipid headgroups, and so mitigating the stress induced by the volume change of water during freeze-thaw.

Perspectives on DNP-enhanced NMR spectroscopy in solutions

More than 60 years after the seminal work of Albert Overhauser on dynamic nuclear polarization by dynamic cross relaxation of coupled electron-nuclear spin systems, the quest for sensitivity enhancement in NMR spectroscopy is as pressing as ever. In this contribution we will review the status and perspectives for dynamic nuclear polarization in the liquid state. An appealing approach seems to be the use of supercritical solvents that may allow an extension of the Overhauser mechanism towards common high magnetic fields. A complementary approach is the use of solid state DNP on frozen solutions, followed by a rapid dissolution or in-situ melting step and NMR detection with substantially enhanced polarization levels in the liquid state. We will review recent developments in the field and discuss perspectives for the near future.

Anomalously Rapid Hydration Water Diffusion Dynamics Near DNA Surfaces

The emerging Overhauser effect dynamic nuclear polarization (ODNP) technique measures the translational mobility of water within the vicinity (5-15 Å) of preselected sites. The work presented here expands the capabilities of the ODNP technique and illuminates an important, previously unseen, property of the translational diffusion dynamics of water at the surface of DNA duplexes. We attach nitroxide radicals (i.e., spin labels) to multiple phosphate backbone positions of DNA duplexes, allowing ODNP to measure the hydration dynamics at select positions along the DNA surface. With a novel approach to ODNP analysis, we isolate the contributions of water molecules at these sites that undergo free translational diffusion from water molecules that either loosely bind to or exchange protons with the DNA. The results reveal that a significant population of water in a localized volume adjacent to the DNA surface exhibits fast, bulk-like characteristics and moves unusually rapidly compared to water found in similar probe volumes near protein and membrane surfaces. Control studies show that the observation of these characteristics are upheld even when the DNA duplex is tethered to streptavidin or the mobility of the nitroxides is altered. This implies that, as compared to protein or lipid surfaces, it is an intrinsic feature of the DNA duplex surface that it interacts only weakly with a significant fraction of the surface hydration water network. The displacement of this translationally mobile water is energetically less costly than that of more strongly bound water by up to several kBT and thus can lower the activation barrier for interactions involving the DNA surface.

Overhauser Dynamic Nuclear Polarization Studies on Local Water Dynamics

Overhauser dynamic nuclear polarization (ODNP) is an emerging technique for quantifying translational water dynamics in the vicinity (<1 nm) of stable radicals that can be chemically attached to macromolecules of interest. This has led to many in-depth and enlightening studies of hydration water of biomolecules, revolving around the role of solvent dynamics in the structure and function of proteins, nucleic acids, and lipid bilayer membranes. Still to date, a complete and fully automated ODNP instrument is not commercialized. The purpose of this chapter is to share the technical know-how of the hardware, theory, measurement, and data analysis method needed to successfully utilize and disseminate the ODNP technique.

Cholesterol enhances surface water diffusion of phospholipid bilayers

Elucidating the physical effect of cholesterol (Chol) on biological membranes is necessary towards rationalizing their structural and functional role in cell membranes. One of the debated questions is the role of hydration water in Chol-embedding lipid membranes, for which only little direct experimental data are available. Here, we study the hydration dynamics in a series of Chol-rich and depleted bilayer systems using an approach termed (1)H Overhauser dynamic nuclear polarization (ODNP) NMR relaxometry that enables the sensitive and selective determination of water diffusion within 5-10 Å of a nitroxide-based spin label, positioned off the surface of the polar headgroups or within the nonpolar core of lipid membranes. The Chol-rich membrane systems were prepared from mixtures of Chol, dipalmitoyl phosphatidylcholine and/or dioctadecyl phosphatidylcholine lipid that are known to form liquid-ordered, raft-like, domains. Our data reveal that the translational diffusion of local water on the surface and within the hydrocarbon volume of the bilayer is significantly altered, but in opposite directions: accelerated on the membrane surface and dramatically slowed in the bilayer interior with increasing Chol content. Electron paramagnetic resonance (EPR) lineshape analysis shows looser packing of lipid headgroups and concurrently tighter packing in the bilayer core with increasing Chol content, with the effects peaking at lipid compositions reported to form lipid rafts. The complementary capability of ODNP and EPR to site-specifically probe the hydration dynamics and lipid ordering in lipid membrane systems extends the current understanding of how Chol may regulate biological processes. One possible role of Chol is the facilitation of interactions between biological constituents and the lipid membrane through the weakening or disruption of strong hydrogen-bond networks of the surface hydration layers that otherwise exert stronger repulsive forces, as reflected in faster surface water diffusivity. Another is the concurrent tightening of lipid packing that reduces passive, possibly unwanted, diffusion of ions and water across the bilayer.

Coupled diffusion in lipid bilayers upon close approach

Biomembrane interfaces create regions of slowed water dynamics in their vicinity. When two lipid bilayers come together, this effect is further accentuated, and the associated slowdown can affect the dynamics of larger-scale processes such as membrane fusion. We have used molecular dynamics simulations to examine how lipid and water dynamics are affected as two lipid bilayers approach each other. These two interacting fluid systems, lipid and water, both slow and become coupled when the lipid membranes are separated by a thin water layer. We show in particular that the water dynamics become glassy, and diffusion of lipids in the apposed leaflets becomes coupled across the water layer, while the "outer" leaflets remain unaffected. This dynamic coupling between bilayers appears mediated by lipid-water-lipid hydrogen bonding, as it occurs at bilayer separations where water-lipid hydrogen bonds become more common than water-water hydrogen bonds. We further show that such coupling occurs in simulations of vesicle-vesicle fusion prior to the fusion event itself. Such altered dynamics at membrane-membrane interfaces may both stabilize the interfacial contact and slow fusion stalk formation within the interface region.

Probing water density and dynamics in the chaperonin GroEL cavity

ATP-dependent binding of the chaperonin GroEL to its cofactor GroES forms a cavity in which encapsulated substrate proteins can fold in isolation from bulk solution. It has been suggested that folding in the cavity may differ from that in bulk solution owing to steric confinement, interactions with the cavity walls, and differences between the properties of cavity-confined and bulk water. However, experimental data regarding the cavity-confined water are lacking. Here, we report measurements of water density and diffusion dynamics in the vicinity of a spin label attached to a cysteine in the Tyr71 → Cys GroES mutant obtained using two magnetic resonance techniques: electron-spin echo envelope modulation and Overhauser dynamic nuclear polarization. Residue 71 in GroES is fully exposed to bulk water in free GroES and to confined water within the cavity of the GroEL–GroES complex. Our data show that water density and translational dynamics in the vicinity of the label do not change upon complex formation, thus indicating that bulk water-exposed and cavity-confined GroES surface water share similar properties. Interestingly, the diffusion dynamics of water near the GroES surface are found to be unusually fast relative to other protein surfaces studied. The implications of these findings for chaperonin-assisted folding mechanisms are discussed.

Quantitative cw Overhauser effect dynamic nuclear polarization for the analysis of local water dynamics

Liquid state Overhauser effect Dynamic Nuclear Polarization (ODNP) has experienced a recent resurgence of interest. The ODNP technique described here relies on the double resonance of electron spin resonance (ESR) at the most common, i.e. X-band (∼10GHz), frequency and ¹H nuclear magnetic resonance (NMR) at ∼15 MHz. It requires only a standard continuous wave (cw) ESR spectrometer with an NMR probe inserted or built into an X-band cavity. We focus on reviewing a new and powerful manifestation of ODNP as a high frequency NMR relaxometry tool that probes dipolar cross relaxation between the electron spins and the ¹H nuclear spins at X-band frequencies. This technique selectively measures the translational mobility of water within a volume extending 0.5-1.5 nm outward from a nitroxide radical spin probe that is attached to a targeted site of a macromolecule. It allows one to study the dynamics of water that hydrates or permeates the surface or interior of proteins, polymers, and lipid membrane vesicles. We begin by reviewing the recent advances that have helped develop ODNP into a tool for mapping the dynamic landscape of hydration water with sub-nanometer locality. In order to bind this work coherently together and to place it in the context of the extensive body of research in the field of NMR relaxometry, we then rephrase the analytical model and extend the description of the ODNP-derived NMR signal enhancements. This extended model highlights several aspects of ODNP data analysis, including the importance of considering all possible effects of microwave sample heating, the need to consider the error associated with various relaxation rates, and the unique ability of ODNP to probe the electron-¹H cross-relaxation process, which is uniquely sensitive to fast (tens of ps) dynamical processes. By implementing the relevant corrections in a stepwise fashion, this paper draws a consensus result from previous ODNP procedures and then shows how such data can be further corrected to yield clear and reproducible saturation of the NMR hyperpolarization process. Finally, drawing on these results, we broadly survey the previous ODNP dynamics literature. We find that the vast number of published, empirical hydration dynamics data can be reproducibly classified into regimes of surface, interfacial, vs. buried water dynamics.

Overhauser Dynamic Nuclear Polarization-Enhanced NMR Relaxometry

We present a new methodological basis for selectively illuminating a dilute population of fluid within a porous medium. Specifically, transport in porous materials can be analyzed by now-standard nuclear magnetic resonance (NMR) relaxometry and NMR pulsed field gradient (PFG) diffusometry methods in combination with with the prominent NMR signal amplification tool, dynamic nuclear polarization (DNP). The key components of the approach introduced here are (1) to selectively place intrinsic or extrinsic paramagnetic probes at the site or local volume of interest within the sample, (2) to amplify the signal from the local solvent around the paramagnetic probes with Overhauser DNP, which is performed in situ and under ambient conditions, and (3) to observe the ODNP-enhanced solvent signal with 1D or 2D NMR relaxometry methods, thus selectively amplifying only the relaxation dynamics of the fluid that resides in or percolates through the local porous volume that contains the paramagnetic probe. Here, we demonstrate the proof of principle of this approach by selectively amplifying the NMR signal of only one solvent population, which is in contact with a paramagnetic probe and occluded from a second solvent population. An apparent one-component T₂ relaxation decay is shown to actually contain two distinct solvent populations. The approach outlined here should be universally applicable to a wide range of other 1D and 2D relaxometry and PFG diffusometry measurements, including T₁-T₂ or T₁-D correlation maps, where the occluded population containing the paramagnetic probes can be selectively amplified for its enhanced characterization.

Structural insight into proteorhodopsin oligomers

Oligomerization has important functional implications for many membrane proteins. However, obtaining structural insight into oligomeric assemblies is challenging, as they are large and resist crystallization. We focus on proteorhodopsin (PR), a protein with seven transmembrane α-helices that was found to assemble to hexamers in densely packed lipid membrane, or detergent-solubilized environments. Yet, the structural organization and the subunit interface of these PR oligomers were unknown. We used site-directed spin-labeling together with electron spin-resonance lineshape and Overhauser dynamic nuclear polarization analysis to construct a model for the specific orientation of PR subunits within the hexameric complex. We found intersubunit distances to average 16 Å between neighboring 55 residues and that residues 177 are >20 Å apart from each other. These distance constraints show that PR has a defined and radial orientation within a hexamer, with the 55-site of the A-B loop facing the hexamer core and the 177-site of the E-F loop facing the hexamer exterior. Dynamic nuclear polarization measurements of the local solvent dynamics complement the electron spin-resonance-based distance analysis, by resolving whether protein surfaces at positions 55, 58, and 177 are exposed to solvent, or covered by protein-protein or protein-detergent contacts.

Asymmetric collapse in biomimetic complex coacervates revealed by local polymer and water dynamics

Complex coacervation is a phenomenon characterized by the association of oppositely charged polyelectrolytes into micrometer-scale liquid condensates. This process is the purported first step in the formation of underwater adhesives by sessile marine organisms, as well as the process harnessed for the formation of new synthetic and protein-based contemporary materials. Efforts to understand the physical nature of complex coacervates are important for developing robust adhesives, injectable materials, or novel drug delivery vehicles for biomedical applications; however, their internal fluidity necessitates the use of in situ characterization strategies of their local dynamic properties, capabilities not offered by conventional techniques such as X-ray scattering, microscopy, or bulk rheological measurements. Herein, we employ the novel magnetic resonance technique Overhauser dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP), together with electron paramagnetic resonance (EPR) line shape analysis, to concurrently quantify local molecular and hydration dynamics, with species- and site-specificity. We observe striking differences in the structure and dynamics of the protein-based biomimetic complex coacervates from their synthetic analogues, which is an asymmetric collapse of the polyelectrolyte constituents. From this study we suggest charge heterogeneity within a given polyelectrolyte chain to be an important parameter by which the internal structure of complex coacervates may be tuned. Acquiring molecular-level insight to the internal structure and dynamics of dynamic polymer complexes in water through the in situ characterization of site- and species-specific local polymer and hydration dynamics should be a promising general approach that has not been widely employed for materials characterization.

Dynamic Nuclear Polarization Methods in Solids and Solutions to Explore Membrane Proteins and Membrane Systems

Membrane proteins regulate vital cellular processes, including signaling, ion transport, and vesicular trafficking. Obtaining experimental access to their structures, conformational fluctuations, orientations, locations, and hydration in membrane environments, as well as the lipid membrane properties, is critical to understanding their functions. Dynamic nuclear polarization (DNP) of frozen solids can dramatically boost the sensitivity of current solid-state nuclear magnetic resonance tools to enhance access to membrane protein structures in native membrane environments. Overhauser DNP in the solution state can map out the local and site-specific hydration dynamics landscape of membrane proteins and lipid membranes, critically complementing the structural and dynamics information obtained by electron paramagnetic resonance spectroscopy. Here, we provide an overview of how DNP methods in solids and solutions can significantly increase our understanding of membrane protein structures, dynamics, functions, and hydration in complex biological membrane environments.

Nonlinear scaling of surface water diffusion with bulk water viscosity of crowded solutions

The translational hydration dynamics within 0.5-1.5 nm of the surface of a DPPC liposome, a model biomacromolecular surface, is analyzed by the recently developed Overhauser dynamic nuclear polarization (ODNP) technique. We find that dramatic changes to the bulk solvent cause only weak changes in the surface hydration dynamics. 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 <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 solvent, nearly exclusively determine the macromolecule's surface hydration dynamics.

Simulation of influence of bilayer melting on dynamics and thermodynamics of interfacial water

We investigate the effect of bilayer melting transition on thermodynamics and dynamics of interfacial water using molecular dynamics simulation with the two-phase thermodynamic model. We show that the diffusivity of interface water depicts a dynamic crossover at the chain melting transition following an Arrhenius behavior until the transition temperature. The corresponding change in the diffusion coefficient from the bulk to the interface water is comparable with experimental observations found recently for water near 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) vesicles [Phys. Chem. Chem. Phys. 13, 7732 (2011)]. The entropy and potential energy of interfacial water show distinct changes at the bilayer melting transition, indicating a strong correlation in the thermodynamic state of water and the accompanying first-order phase transition of the bilayer membrane.

Quantitative analysis of molecular transport across liposomal bilayer by J-mediated 13C Overhauser dynamic nuclear polarization

We introduce a new NMR technique to dramatically enhance the solution-state (13)C NMR sensitivity and contrast at 0.35 T and at room temperature by actively transferring the spin polarization from Overhauser dynamic nuclear polarization (ODNP)-enhanced (1)H to (13)C nuclei through scalar (J) coupling, a method that we term J-mediated (13)C ODNP. We demonstrate the capability of this technique by quantifying the permeability of glycine across negatively charged liposomal bilayers composed of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG). The permeability coefficient of glycine across this DPPC/DPPG bilayer is measured to be (1.8 ± 0.1) × 10(-11)m/s, in agreement with the literature value. We further observed that the presence of 20 mol % cholesterol within the DPPC/DPPG lipid membrane significantly retards the permeability of glycine by a factor of 4. These findings demonstrate that the high sensitivity and contrast of J-mediated (13)C ODNP affords the measurement of the permeation kinetics of small hydrophilic molecules across lipid bilayers, a quantity that is difficult to accurately measure with existing techniques.

Nature of interactions between PEO-PPO-PEO triblock copolymers and lipid membranes: (II) role of hydration dynamics revealed by dynamic nuclear polarization

Amphiphilic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymers, also known as poloxamers, have broad biomembrane activities. To illustrate the nature of these activities, (1)H Overhauser dynamic nuclear polarization NMR spectroscopy was employed to sensitively detect polymer-lipid membrane interactions through the modulation of local hydration dynamics in lipid membranes. Our study shows P188, the most hydrophilic poloxamer that is a known membrane sealant, weakly adsorbs on the membrane surface, yet effectively retards membrane hydration dynamics. Contrarily, P181, the most hydrophobic poloxamer that is a known membrane permeabilizer, initially embeds at lipid headgroups and enhances intrabilayer water diffusivity. Unprecedented resolution for differentiating weak surface adsorption versus translocation of polymers to membranes is obtained by probing local water diffusivity in lipid bilayer systems. Our results illustrate that the relative hydrophilic/hydrophobic ratio of the polymer dictates its functions. These findings gleaned from local hydration dynamics are well supported by a thermodynamics study presented in the accompanying paper (Wang, J.-Y.; Marks, J. M.; Lee, K. Y. C. Biomacromolecules, 2012, DOI: 10.1021/bm300847x).

An ultrasensitive tool exploiting hydration dynamics to decipher weak lipid membrane-polymer interactions

We introduce a newly developed tool, (1)H Overhauser Dynamic Nuclear Polarization (ODNP), to sensitively explore weak macromolecular interactions by site-specifically probing the modulation of the translational dynamics of hydration water at the interaction interface, in the full presence of bulk water. Here, ODNP is employed on an illustrative example of a membrane-active triblock copolymer, poloxamer 188 (P188), which is known to restore the integrity of structurally compromised cell membranes. We observe a distinct change in the translational dynamics of the hydration layer interacting with the lipid membrane surface and the bilayer-interior as P188 is added to a solution of lipid vesicles, but no measurable changes in the dynamics or structure of the lipid membranes. This study shows that hydration water is an integral constituent of a lipid membrane system, and demonstrates for the first time that the modulation of its translational diffusivity can sensitively report on weak polymer-membrane interactions, as well as mediate essential lipid membrane functions. ODNP holds much promise as a unique tool to unravel molecular interactions at interfaces even in the presence of bulk water under ambient conditions.

Polarizing agents and mechanisms for high-field dynamic nuclear polarization of frozen dielectric solids

This article provides an overview of polarizing mechanisms involved in high-frequency dynamic nuclear polarization (DNP) of frozen biological samples at temperatures maintained using liquid nitrogen, compatible with contemporary magic-angle spinning (MAS) nuclear magnetic resonance (NMR). Typical DNP experiments require unpaired electrons that are usually exogenous in samples via paramagnetic doping with polarizing agents. Thus, the resulting nuclear polarization mechanism depends on the electron and nuclear spin interactions induced by the paramagnetic species. The Overhauser Effect (OE) DNP, which relies on time-dependent spin-spin interactions, is excluded from our discussion due the lack of conducting electrons in frozen aqueous solutions containing biological entities. DNP of particular interest to us relies primarily on time-independent, spin-spin interactions for significant electron-nucleus polarization transfer through mechanisms such as the Solid Effect (SE), the Cross Effect (CE) or Thermal Mixing (TM), involving one, two or multiple electron spins, respectively. Derived from monomeric radicals initially used in high-field DNP experiments, bi- or multiple-radical polarizing agents facilitate CE/TM to generate significant NMR signal enhancements in dielectric solids at low temperatures (<100 K). For example, large DNP enhancements (∼300 times at 5 T) from a biologically compatible biradical, 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-ol (TOTAPOL), have enabled high-resolution MAS NMR in sample systems existing in submicron domains or embedded in larger biomolecular complexes. The scope of this review is focused on recently developed DNP polarizing agents for high-field applications and leads up to future developments per the CE DNP mechanism. Because DNP experiments are feasible with a solid-state microwave source when performed at <20K, nuclear polarization using lower microwave power (<100 mW) is possible by forcing a high proportion of biradicals to fulfill the frequency matching condition of CE (two EPR frequencies separated by the NMR frequency) using the strategies involving hetero-radical moieties and/or molecular alignment. In addition, the combination of an excited triplet and a stable radical might provide alternative DNP mechanisms without the microwave requirement.

Site-specific hydration dynamics in the nonpolar core of a molten globule by dynamic nuclear polarization of water

Water-protein interactions play a direct role in protein folding. The chain collapse that accompanies protein folding involves extrusion of water from the nonpolar core. For many proteins, including apomyoglobin (apoMb), hydrophobic interactions drive an initial collapse to an intermediate state before folding to the final structure. However, the debate continues as to whether the core of the collapsed intermediate state is hydrated and, if so, what the dynamic nature of this water is. A key challenge is that protein hydration dynamics is significantly heterogeneous, yet suitable experimental techniques for measuring hydration dynamics with site-specificity are lacking. Here, we introduce Overhauser dynamic nuclear polarization at 0.35 T via site-specific nitroxide spin labels as a unique tool to probe internal and surface protein hydration dynamics with site-specific resolution in the molten globular, native, and unfolded protein states. The (1)H NMR signal enhancement of water carries information about the local dynamics of the solvent within ∼10 Å of a spin label. EPR is used synergistically to gain insights on local polarity and mobility of the spin-labeled protein. Several buried and solvent-exposed sites of apoMb are examined, each bearing a covalently bound nitroxide spin label. We find that the nonpoloar core of the apoMb molten globule is hydrated with water bearing significant translational dynamics, only 4-6-fold slower than that of bulk water. The hydration dynamics of the native state is heterogeneous, while the acid-unfolded state bears fast-diffusing hydration water. This study provides a high-resolution glimpse at the folding-dependent nature of protein hydration dynamics.

Alzheimer Abeta(1-42) monomer adsorbed on the self-assembled monolayers

Amyloid-beta (Abeta) peptide aggregation on the cell membranes is a key pathological event responsible for neuron cell death in Alzheimer's disease (AD). We present a collection of molecular docking and molecular dynamics simulations to study the conformational dynamics and adsorption behavior of Abeta monomer on the self-assembled monolayer (SAM), in comparison to Abeta structure in bulk solution. Two distinct Abeta conformations (i.e., alpha-helix and beta-hairpin) are selected as initial structures to mimic different adsorption states, whereas four SAM surfaces with different end groups in hydrophobicity and charge distribution are used to examine the effect of surface chemistry on Abeta structure and adsorption. Simulation results show that alpha-helical monomer displays higher structural stability than beta-hairpin monomer on all SAMs, suggesting that the preferential conformation of Abeta monomer could be alpha-helical or random structure when bound to surfaces. Structural stability and adsorption behavior of Abeta monomer on the SAMs originates from competitive interactions between Abeta and SAM and between SAM and interfacial water, which involve the conformation of Abeta, the surface chemistry of SAM, and the structure and dynamics of interfacial waters. The relative net binding affinity of Abeta with the SAMs is in the favorable order of COOH-SAM > NH(2)-SAM > CH(3)-SAM > OH-SAM, highlighting the importance of electrostatic and hydrophobic interactions for driving Abeta adsorption at the SAMs, but both interactions contribute differently to each Abeta-SAM complex. This work provides parallel insights into the understanding of Abeta structure and aggregation on cell membrane.

Comparative molecular dynamics study of Abeta adsorption on the self-assembled monolayers

The adsorption and aggregation of the amyloid-beta (Abeta) peptides on the cell membrane plays a causal role in the pathogenesis of Alzheimer's disease. Here, we report all-atom molecular dynamics (MD) simulations to study the interactions of Abeta oligomer with self-assembled monolayers (SAMs) terminated with hydrophobic CH(3) and hydrophilic OH functional groups, with particular interests in how surface chemistry and Abeta orientation affect the adsorption behavior of Abeta. Simulation results show that the CH(3)-SAM has a stronger binding affinity to Abeta than the OH-SAM does, although both surfaces can induce Abeta adsorption. Regardless of the characteristics of the surface, the hydrophobic C-terminal region is more likely to be adsorbed on the SAMs, indicating a preferential orientation and interface for Abeta adsorption. Structural and energetic comparison among six Abeta-SAM systems further reveals that Abeta orientation, SAM surface hydrophobicity, and interfacial waters all determine Abeta adsorption behavior on the surface, highlighting the importance of hydrophobic interactions at the interface. This work may provide parallel insights into the interactions of Abeta with lipid bilayers.