Ultrafast Vibrational Dynamics of Water Confined in Phospholipid Reverse Micelles

Anomalous proton transfer of a photoacid HPTS in nonaqueous reverse micelles

The proton transfer reaction is one of the fundamental chemical reactions where the reaction dynamics strongly depend on solvent properties such as acidity or basicity. A photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) shows a sharp decrease of pKa (7.7 → 0.5) upon photoexcitation, and the excited-state proton transfer (ESPT) occurs with ultrafast time constants of 2.5 and 89 ps in bulk aqueous solution. However, the two-step proton transfers via the contact ion pair formation and the proton diffusion are strongly limited inside the nanopools of reverse micelles (RMs). The confinement in small RMs strongly impeded the proton transfer reactions. In this work, we report the ESPT of HPTS confined in methanol-in-oil RMs by steady-state and time-resolved electronic spectroscopy. Interestingly, HPTS shows substantial deprotonation in the excited state only in small RMs, while the ESPT of HPTS does not occur in bulk methanol solution due to the low basicity of aliphatic alcohols. The kinetic analysis of time-resolved fluorescence and transient absorption measurements will compare the proton transfer dynamics of HPTS in the water-in-oil and methanol-in-oil RMs. The ESPT of photoacids, especially in the nonaqueous RMs, can be crucial in understanding many important chemical reactions involving proton transfer in the confined environments of cells and membranes.

Determination of the Thickness of Interfacial Water by Time-Resolved Sum-Frequency Generation Vibrational Spectroscopy

The physics and chemistry of a charged interface are governed by the structure of the electrical double layer (EDL). Determination of the interfacial water thickness (diw) of the charged interface is crucial to quantitatively describe the EDL structure, but it can be utilized with very scarce experimental methods. Here, we propose and verify that the vibrational relaxation time (T1) of the OH stretching mode at 3200 cm-1, obtained by time-resolved sum frequency generation vibrational spectroscopy with ssp polarizations, provides an effective tool to determine diw. By investigating the T1 values at the SiO2/NaCl solution interface, we established a time-space (T1-diw) relationship. We find that water has a T1 lifetime of ≥0.5 ps for diw ≤ 3 Å, while it displays bulk-like dynamics with T1 ≤ 0.2 ps for diw ≥ 9 Å. T1 decreases as diw increases from ∼3 Å to 9 Å. The hydration water at the DPPG lipid bilayer and LK15β protein interfaces has a thickness of ≥9 Å and shows a bulk-like feature. The time-space relationship will provide a novel tool to pattern the interfacial topography and heterogeneity in Ångstrom-depth resolution by imaging the T1 values.

Ultraslow Biological Water-Like Dynamics in Waterless Liquid Protein

Fluorescence correlation spectroscopy and time-dependent fluorescence Stokes shift have been employed to elucidate dynamics in different time scales, ranging from picoseconds to nanoseconds, for human serum albumin, in its native and cationized forms as well as in the self-assembled complex of the cationized protein with the polymer surfactant (PS) glycolic acid ethoxylate lauryl ether. The effect of crowding in this complex, especially in the waterless condition, is of prime importance in this context. Excellent correlation of the dynamics with the structures, obtained by circular dichroism and Fourier transform infrared spectroscopy, has been observed. Slow solvation, associated classically with biological water, has been observed in these systems, even in the waterless condition. This apparently intriguing observation has been rationalized by the relaxation of segments of the protein and the PS in the microenvironment of the fluorescent probe.

Membrane-Mimicking Reverse Micelles for High-Resolution Interfacial Study of Proteins and Membranes

Despite substantial advances, the study of proteins interacting with membranes remains a significant challenge. While integral membrane proteins have been a major focus of recent efforts, peripheral membrane proteins (PMPs) and their interactions with membranes and lipids have far less high-resolution information available. Their small size and the dynamic nature of their interactions have stalled detailed interfacial study using structural methods like cryo-EM and X-ray crystallography. A major roadblock for the structural analysis of PMP interactions is limitations in membrane models to study the membrane recruited state. Commonly used membrane mimics such as liposomes, bicelles, nanodiscs, and micelles are either very large or composed of non-biological detergents, limiting their utility for the NMR study of PMPs. While there have been previous successes with integral and peripheral membrane proteins, currently employed reverse micelle (RM) compositions are optimized for their inertness with proteins rather than their ability to mimic membranes. Applying more native, membrane-like lipids and surfactants promises to be a valuable advancement for the study of interfacial interactions between proteins and membranes. Here, we describe the development of phosphocholine-based RM systems that mimic biological membranes and are compatible with high-resolution protein NMR. We demonstrate new formulations that are able to encapsulate the model soluble protein, ubiquitin, with minimal perturbations of the protein structure. Furthermore, one formula, DLPC:DPC, allowed the encapsulation of the PMPs glutathione peroxidase 4 (GPx4) and phosphatidylethanolamine-binding protein 1 (PEBP1) and enabled the embedment of these proteins, matching the expected interactions with biological membranes. Dynamic light scattering and small-angle X-ray scattering characterization of the RMs reveals small, approximately spherical, and non-aggregated particles, a prerequisite for protein NMR and other avenues of study. The formulations presented here represent a new tool for the study of elusive PMP interactions and other membrane interfacial investigations.

Proton Traffic Jam: Effect of Nanoconfinement and Acid Concentration on Proton Hopping Mechanism

The properties of the water network in concentrated HCl acid pools in nanometer-sized reverse non-ionic micelles were probed with TeraHertz absorption, dielectric relaxation spectroscopy, and reactive force field simulations capable of describing proton hopping mechanisms. We identify that only at a critical micelle size of W0=9 do solvated proton complexes form in the water pool, accompanied by a change in mechanism from Grotthuss forward shuttling to one that favors local oscillatory hopping. This is due to a preference for H+ and Cl- ions to adsorb to the micelle interface, together with an acid concentration effect that causes a "traffic jam" in which the short-circuiting of the hydrogen-bonding motif of the hydronium ion decreases the forward hopping rate throughout the water interior even as the micelle size increases. These findings have implications for atmospheric chemistry, biochemical and biophysical environments, and energy materials, as transport of protons vital to these processes can be suppressed due to confinement, aggregation, and/or concentration.

Flexible to rigid: IR spectroscopic investigation of a rhenium-tricarbonyl-complex at a buried interface

This work explores the solid-liquid interface of a rhenium-tricarbonyl complex embedded in a layer of zirconium oxide deposited by atomic layer deposition (ALD). Time-resolved and steady state infrared spectroscopy were applied to reveal the correlations between the thickness of the ALD layer and the spectroscopic response of the system. We observed a transition of the molecular environment from flexible to rigid, as well as limitations to ligand exchange and excited state quenching on the embedded complexes, when the ALD layer is roughly of the same height as the molecules.

Ultrafast Dynamics at Lipid-Water Interfaces

Lipid membranes are more than just barriers between cell compartments; they provide molecular environments with a finely tuned balance between hydrophilic and hydrophobic interactions that enable proteins to dynamically fold and self-assemble to regulate biological function. Characterizing dynamics at the lipid-water interface is essential to understanding molecular complexities from the thermodynamics of liquid-liquid phase separation down to picosecond-scale reorganization of interfacial hydrogen-bond networks.Ultrafast vibrational spectroscopy, including two-dimensional infrared (2D IR) and vibrational sum-frequency generation (VSFG) spectroscopies, is a powerful tool to examine picosecond interfacial dynamics. Two-dimensional IR spectroscopy provides a bond-centered view of dynamics with subpicosecond time resolutions, as vibrational frequencies are highly sensitive to the local environment. Recently, 2D IR spectroscopy has been applied to carbonyl and phosphate vibrations intrinsically located at the lipid-water interface. Interface-specific VSFG spectroscopy probes the water vibrational modes directly, accessing H-bond strength and water organization at lipid headgroup positions. Signals in VSFG arise from the interfacial dipole contributions, directly probing headgroup ordering and water orientation to provide a structural view of the interface.In this Account we discuss novel applications of ultrafast spectroscopy to lipid membranes, a field that has experienced significant growth over the past decade. In particular, ultrafast experiments now offer a molecular perspective on increasingly complex membranes. The powerful combination of ultrafast, interface-selective spectroscopy and simulations opens up new routes to understanding multicomponent membranes and their function. This Account highlights key prevailing views that have emerged from recent experiments: (1) Water dynamics at the lipid-water interface are slow compared to those of bulk water as a result of disrupted H-bond networks near the headgroups. (2) Peptides, ions, osmolytes, and cosolvents perturb interfacial dynamics, indicating that dynamics at the interface are affected by bulk solvent dynamics and vice versa. (3) The interfacial environment is generally dictated by the headgroup structure and orientation, but hydrophobic interactions within the acyl chains also modulate interfacial dynamics. Ultrafast spectroscopy has been essential to characterizing the biophysical chemistry of the lipid-water interface; however, challenges remain in interpreting congested spectra as well as designing appropriate model systems to capture the complexity of a membrane environment.

Water Structure and Dynamics in the Stern Layer of Micelles: Femtosecond Mid-Infrared Pump-Probe Spectroscopy Study

Molecular-level understanding of the water structure and dynamics in the Stern layer of micelles is important to elucidate the active role of water in biological processes on membrane surfaces. Micelles and reverse micelles are considered to be excellent membrane model systems. Here, to address the question of whether or not the spatial confinement effect on water in reverse micelles and nanometric water pool systems plays a role in modulating water dynamics, we consider four different aqueous micelle solutions and study the water dynamics in the Stern layer of micelles using a femtosecond mid-infrared pump-probe spectroscopy technique. Vibrational energy relaxation and rotational dynamics of the O?D stretch mode of HDO and the azido stretch mode of hydrazoic acid are critically dependent on the charge, polarity, and chemical structure of the surfactant head group. In particular, water molecules in the Stern layer of micelles, which are not in spatially confined environments, are notably different from those in bulk water. This finding clearly indicates that changes in the vibrational and rotational dynamics of water molecules, even in spatially confined systems, are mainly induced by surface effects instead of spatial confinement effects. We believe that the present experimental results are of importance for understanding water-involved biochemical processes on biological membranes.

Water at surfaces with tunable surface chemistries

Aqueous interfaces are ubiquitous in natural environments, spanning atmospheric, geological, oceanographic, and biological systems, as well as in technical applications, such as fuel cells and membrane filtration. Where liquid water terminates at a surface, an interfacial region is formed, which exhibits distinct properties from the bulk aqueous phase. The unique properties of water are governed by the hydrogen-bonded network. The chemical and physical properties of the surface dictate the boundary conditions of the bulk hydrogen-bonded network and thus the interfacial properties of the water and any molecules in that region. Understanding the properties of interfacial water requires systematically characterizing the structure and dynamics of interfacial water as a function of the surface chemistry. In this review, we focus on the use of experimental surface-specific spectroscopic methods to understand the properties of interfacial water as a function of surface chemistry. Investigations of the air-water interface, as well as efforts in tuning the properties of the air-water interface by adding solutes or surfactants, are briefly discussed. Buried aqueous interfaces can be accessed with careful selection of spectroscopic technique and sample configuration, further expanding the range of chemical environments that can be probed, including solid inorganic materials, polymers, and water immiscible liquids. Solid substrates can be finely tuned by functionalization with self-assembled monolayers, polymers, or biomolecules. These variables provide a platform for systematically tuning the chemical nature of the interface and examining the resulting water structure. Finally, time-resolved methods to probe the dynamics of interfacial water are briefly summarized before discussing the current status and future directions in studying the structure and dynamics of interfacial water.

On the Structural and Dynamical Properties of DOPC Reverse Micelles

The structure and dynamics of phospholipid reverse micelles are studied by molecular dynamics. We report all-atom unconstrained simulations of 1,2-dioleoyl-sn-phosphatidylcholine (DOPC) reverse micelles in benzene of increasing sizes, with water-to-surfactant number ratios ranging from W₀ = 1 to 16. The aggregation number, i.e., the number of DOPC molecules per reverse micelle, is determined to fit experimental light-scattering measurements of the reverse micelle diameter. The simulated reverse micelles are found to be approximately spherical. Larger reverse micelles (W₀ > 4) exhibit a layered structure with a water core and the hydration structure of DOPC phosphate head groups is similar to that found in phospholipid membranes. In contrast, the structure of smaller reverse micelles (W₀ ≤ 4) cannot be described as a series of concentric layers successively containing water, surfactant head groups, and surfactant tails, and the head groups are only partly hydrated and frequently present in the core. The dynamics of water molecules within the phospholipid reverse micelles slow down as the reverse micelle size decreases, in agreement with prior studies on AOT and Igepal reverse micelles. However, the average water reorientation dynamics in DOPC reverse micelles is found to be much slower than in AOT and Igepal reverse micelles with the same W₀ ratio. This is explained by the smaller water pool and by the stronger interactions between water and the charged head groups, as confirmed by the red-shift of the computed infrared line shape with decreasing W₀.

Femtosecond Hydrogen Bond Dynamics of Bulk-like and Bound Water at Positively and Negatively Charged Lipid Interfaces Revealed by 2D HD-VSFG Spectroscopy

Interfacial water in the vicinity of lipids plays an important role in many biological processes, such as drug delivery, ion transportation, and lipid fusion. Hence, molecular-level elucidation of the properties of water at lipid interfaces is of the utmost importance. We report the two-dimensional heterodyne-detected vibrational sum frequency generation (2D HD-VSFG) study of the OH stretch of HOD at charged lipid interfaces, which shows that the hydrogen bond dynamics of interfacial water differ drastically, depending on the lipids. The data indicate that the spectral diffusion of the OH stretch at a positively charged lipid interface is dominated by the ultrafast (<∼100 fs) component, followed by the minor sub-picosecond slow dynamics, while the dynamics at a negatively charged lipid interface exhibit sub-picosecond dynamics almost exclusively, implying that fast hydrogen bond fluctuation is prohibited. These results reveal that the ultrafast hydrogen bond dynamics at the positively charged lipid-water interface are attributable to the bulk-like property of interfacial water, whereas the slow dynamics at the negatively charged lipid interface are due to bound water, which is hydrogen-bonded to the hydrophilic head group.

Water Structure at the Lipid Multibilayer Surface: Anionic Versus Cationic Head Group Effects

Membrane water interface is a potential reaction site for many biochemical reactions. Therefore, a molecular level understanding of water structure and dynamics that strongly depend on the chemical structure of lipid is prerequisite for elucidating the role of water in biological reactions on membrane surface. Recently, we carried out femtosecond infrared pump-probe studies of water structure and dynamics at multibilayer surfaces of zwitterionic phosphatidylcholine-analogue lipid ( J. Phys. Chem. Lett. 2016 , 7 , 741 ). Here, to further elucidate the anionic and cationic headgroup effects on water, we study vibrational dynamics of water on lipid multibilayers formed by anionic phospho-glycerol lipid molecules as well as by cationic choline-derivatized lipid molecules. We observed two significantly different vibrational lifetime components (very fast 0.5 ps and slow 1.9 ps) of the OD stretch mode of HOD molecules at the negatively charged phospho-lipid multibilayer whereas only one vibrational lifetime component (1.6 ps) was observed at the positively charged choline-derivatized lipid multibilayer. From the detailed analyses about the vibrational energy and rotational relaxations of HOD molecules in lipid multibilayers composed of anionic lipid with phosphate and cationic lipid without phosphate, the role of phosphate group in structuring water molecules at phospholipid membrane interface is revealed.

Water Hydrogen-Bonding Network Structure and Dynamics at Phospholipid Multibilayer Surface: Femtosecond Mid-IR Pump-Probe Spectroscopy

The water hydrogen-bonding network at a lipid bilayer surface is crucial to understanding membrane structures and its functional activities. With a phospholipid multibilayer mimicking a biological membrane, we study the temperature dependence of water hydrogen-bonding structure, distribution, and dynamics at a lipid multibilayer surface using femtosecond mid-IR pump-probe spectroscopy. We observe two distinguished vibrational lifetime components. The fast component (0.6 ps) is associated with water interacting with a phosphate part, whereas the slow component (1.9 ps) is with bulk-like choline-associated water. With increasing temperature, the vibrational lifetime of phosphate-associated water remains constant though its relative fraction dramatically increases. The OD stretch vibrational lifetime of choline-bound water slows down in a sigmoidal fashion with respect to temperature, indicating a noticeable change of the water environment upon the phase transition. The water structure and dynamics are thus shown to be in quantitative correlation with the structural change of liquid multibilayer upon the gel-to-liquid crystal phase transition.

Exploration of the presence of bulk-like water in AOT reverse micelles and water-in-oil nanodroplets: the role of charged interfaces, confinement size and properties of water

We show that the distance from the interface at which bulk-like properties are recovered strongly depends on the choice of order parameter being probed: translational < tetrahedral ≪ dipolar orientation.

Computational Study of Phosphate Vibrations as Reporters of DNA Hydration

The sensitivity of the phosphate asymmetric stretch vibrational frequency to DNA hydration was investigated with molecular dynamics (MD) simulations and a spectroscopic map relating the vibrational frequency to the electrostatics of its environment. 95% of the phosphate vibrational frequency shift in fully hydrated DNA was due to water within two hydration layers. The phosphate vibration was relatively insensitive to water in the major and minor grooves and to the sodium counterions but was enormously sensitive to water interacting with the DNA backbone. Comparisons to experimental measurements on DNA as a function of relative humidity suggest that one water molecule per phosphate group likely persists at the lowest values of the relative humidity. Finally, the calculated spectral diffusion dynamics show that water in the vicinity of the DNA backbone is slowed by a factor of ∼5, in agreement with NMR and solvation dynamics experiments, as well as previous MD simulations.

Vibrational probing of the hydrogen-bond structure and dynamics of water in aqueous NaPF6 solutions

The H-bond structures and dynamics of water in bulk and ionic hydration shells in aqueous NaPF₆ solutions were measured at different NaPF₆ concentrations.

Ultrafast vibrational dynamics of BH4(-) ions in liquid and crystalline environments

Ultrafast vibrational dynamics of BH4(-) ions, the key units in boron hydride materials for hydrogen storage, are studied in diluted polar liquid solution and in NaBH4 crystallites by femtosecond infrared spectroscopy. Two-color pump-probe experiments reveal v = 1 lifetimes of 3 ps for the asymmetric BH4(-) stretching mode ν3 and of 3.6 ps for the asymmetric bending mode ν4 in the solvent isopropylamine. We provide direct evidence for the BH4(-) stretching relaxation pathway via the asymmetric bending mode ν4 by probing the latter after femtosecond excitation of ν3. Pump-probe traces measured in the crystalline phase show signatures of radiative coupling between the densely packed BH4(-) oscillators, most clearly manifested in an accelerated subpicosecond depopulation of the v = 1 state of the ν4 mode. The radiative decay is followed by incoherent vibrational relaxation similar to the liquid phase. The excess energy released in the relaxation processes of the BH4(-) intramolecular modes is transferred into the environment with thermal pump-probe signals being much more pronounced in the dense solid than in the diluted solution.

Phospholipid-based nonlamellar mesophases for delivery systems: bridging the gap between empirical and rational design

Phospholipids are ubiquitous cell membrane components and relatively well-accepted ingredients due to their natural origin. Phosphatidylcholine (PC) in particular offers a promising alternative to monoglycerides for lyotropic liquid crystalline (LLC) delivery system applications in the food, cosmetics and pharmaceutical industries, provided its strong tendency to form zero-mean curvature lamellar mesophases in water can be overcome. Higher negative curvatures are usually reached through the addition of a third lipid component, forming a ternary diagram phospholipid/water/oil. The initial part of this work summarizes the potential advantages and the challenges of phospholipid-based delivery system applications. In the next part, various ternary PC/water/oil systems are discussed, with a special emphasis on the PC/water/cyclohexane and PC/water/α-tocopherol systems. We report that R-(+)-limonene has a quantitatively similar effect as cyclohexane. The last part is devoted to the theoretical interpretation of the observed phase behaviors. A fruitful parallel is drawn with PC polymer-like reverse micelles, leading to a thermodynamic description in terms of interfacial bending energy. Investigations at the molecular level are reviewed to help in bridging the empirical and theoretical approaches. Predictive rules are finally derived from this wide-ranging overview, thereby opening the way to a future rational design of PC-based LLC delivery systems.

Vibrational spectroscopy of water in hydrated lipid multi-bilayers. III. Water clustering and vibrational energy transfer

Water clustering and connectivity around lipid bilayers strongly influences the properties of membranes and is important for functions such as proton and ion transport. Vibrational anisotropic pump-probe spectroscopy is a powerful tool for understanding such clustering, as the measured anisotropy depends upon the time-scale and degree of intra- and intermolecular vibrational energy transfer. In this article, we use molecular dynamics simulations and theoretical vibrational spectroscopy to help interpret recent experimental measurements of the anisotropy of water in lipid multi-bilayers as a function of both lipid hydration level and isotopic substitution. Our calculations are in satisfactory agreement with the experiments of Piatkowski, Heij, and Bakker, and from our simulations we can directly probe water clustering and connectivity. We find that at low hydration levels, many water molecules are in fact isolated, although up to 70% of hydration water forms small water clusters or chains. At intermediate hydration levels, water forms a wide range of cluster sizes, while at higher hydration levels, the majority of water molecules are part of a large, percolating water cluster. Therefore, the size, number, and nature of water clusters are strongly dependent on lipid hydration level, and the measured anisotropy reflects this through its dependence on intermolecular energy transfer.

Dynamic morphologies of microscale droplet interface bilayers

Droplet interface bilayers (DIBs) are a powerful platform for studying the dynamics of synthetic cellular membranes; however, very little has been done to exploit the unique dynamical features of DIBs. Here, we generate microscale droplet interface bilayers (μDIBs) by bringing together femtoliter-volume water droplets in a microfluidic oil channel, and characterize morphological changes of the μDIBs as the droplets shrink due to evaporation. By varying the initial conditions of the system, we identify three distinct classes of dynamic morphology. (1) Buckling and fission: when forming μDIBs using the lipid-out method (lipids in oil phase), lipids in the shrinking monolayers continually pair together and slide into the bilayer to conserve their mass. As the bilayer continues to grow, it becomes confined, buckles, and eventually fissions one or more vesicles. (2) Uniform shrinking: when using the lipid-in method (lipids in water phase) to form μDIBs, lipids uniformly transfer from the monolayers and bilayer into vesicles contained inside the water droplets. (3) Stretching and unzipping: finally, when the droplets are pinned to the wall(s) of the microfluidic channel, the droplets become stretched during evaporation, culminating in the unzipping of the bilayer and droplet separation. These findings offer a better understanding of the dynamics of coupled lipid interfaces.

Structural Dynamics of Hydrated Phospholipid Surfaces Probed by Ultrafast 2D Spectroscopy of Phosphate Vibrations

The properties of biomembranes depend in a decisive way on interactions of phospholipids with hydrating water molecules. To map structural dynamics of a phospholipid-water interface on the length and time scale of molecular motions, we introduce the phospholipid symmetric and asymmetric phosphate stretch vibrations as probes of interfacial hydrogen bonds and electrostatic interactions. The first two-dimensional infrared spectra of such modes and a line shape analysis by density matrix theory reveal two distinct structural dynamics components; the first 300 fs contribution is related to spatial fluctuations of charged phospholipid head groups with additional water contributions at high hydration levels; the second accounts for water-phosphate hydrogen bonds persisting longer than 10 ps. Our results reveal a relatively rigid hydration shell around phosphate groups, a behavior relevant for numerous biomolecular systems.

Signatures of nanoconfinement on the linear and nonlinear vibrational spectroscopy of a model hydrogen-bonded complex dissolved in a polar solvent

The one-dimensional IR (1D-IR) absorption and IR pump-probe spectra of a hydrogen stretch in a model hydrogen-bonded complex dissolved in a polar solvent confined in spherical hydrophobic cavities of different sizes were simulated using ground-state mixed quantum-classical dynamics. Due to a thorough analysis of key properties of the complex and solvent from equilibrium trajectory data, we were able to gain insight into the microscopic details underlying the spectra. Both the 1D-IR and IR pump-probe spectra manifested the effects of confinement on the relative stabilities of the covalent and ionic forms of the complex through pronounced changes in their peak intensities and numbers. However, in contrast to the 1D-IR spectra, the time-resolved pump-probe spectra were found to be uniquely sensitive to the changes in the molecular dynamics as the cavity size is varied. In particular, it was found that the variations in the time evolutions of the peak intensities in the pump-probe spectra reflect the differences in the solvation dynamics associated with the various forms of the complex in different locations within the cavities. The ability to detect these differences underscores the advantage of using pump-probe spectroscopy for studying nanoconfined systems.

Dynamics of water confined in reversed micelles: multidimensional vibrational spectroscopy study

Here we perform a comprehensive study of ultrafast molecular and vibrational dynamics of water confined in small reversed micelles (RMs). The molecular picture is elucidated with two-dimensional infrared (2D IR) spectroscopy of water OH stretch vibrations and molecular dynamics simulations, bridged by theoretical calculations of linear and 2D IR vibrational spectra. To investigate the effects of intermolecular coupling, experiments and modeling are performed for isotopically diluted (HDO in D2O) and undiluted (H2O) water. We put a separation of water inside RMs into two subensembles (water-bound and surfactant-bound molecules), observed by many before, on a solid theoretical basis. Water molecules fully attached to the lipid interface ("shell" water) are decoupled from one another and from the central water nanopool ("core" water). The environmental fluctuations are largely "frozen" for the shell water, while the core waters demonstrate much faster dynamics but still not as fast as in the bulk case. A substantial nanoconfinement effect on the dynamics of the core water is observed after disentanglement of the shell water contribution, which is fully confirmed by the simulations of 2D IR spectra. Current results provide new insights into interaction between biological objects like membranes or proteins with the surrounding aqueous bath, and highlight peculiarities in vibrational energy redistribution near the lipid surface.

Site-specific measurements of lipid membrane interfacial water dynamics with multidimensional infrared spectroscopy

One route to accessing site-specific dynamical information available with ultrafast multidimensional infrared spectroscopy is the development of robust and versatile vibrational probes. Here we synthesize and characterize a vibrationally labeled cholesterol derivative, (cholesteryl benzoate) chromium tricarbonyl, to probe model lipid membranes, focusing specifically on the membrane-water interface. Utilizing FTIR and polarized-ATR spectroscopies, we determine the location of the chromium tricarbonyl motif to be situated at the water-membrane interface with an orientation of 46 ± 2° relative to the vector normal to the membrane surface. We test the dynamical sensitivity of the (cholesteryl benzoate) chromium tricarbonyl label with two different nonlinear infrared spectroscopy methods, both of which show that the probe is well-suited to the study of membrane dynamics as well as the dynamics of water at the membrane interface. The metal carbonyl vibrational probe located at the surface of a bicelle exhibits spectral diffusion dynamics induced by membrane hydration water that is roughly three times slower than observed using a nearly identical vibrational probe in bulk water.

Ultrafast Energy Redistribution in Local Hydration Shells of Phospholipids: A Two-Dimensional Infrared Study

Structural and functional properties of phospholipids are strongly influenced by dynamics of their hydration shells. Here, we show that local water pools as small as three water molecules around the polar headgroups in phospholipid reverse micelles (dioleoylphosphatidylcholine, DOPC) serve as efficient sinks of excess energy released during vibrational relaxation. Transient two-dimensional (2D) infrared spectra of OH stretching excitations of H2O shells demonstrate a subpicosecond buildup of a hot water ground state, in which excess energy is randomized in low-frequency modes. An analysis of center line slopes of the 2D spectra reveals kinetics of energy dissipation that are significantly faster than structural fluctuations of the water pool and remain unchanged at intermediate hydration levels between three and eight water molecules per polar headgroup. Our results suggest that confined small water pools in biomolecular systems are sufficient to dissipate excess energy originating from the decay of electronic or vibrational excitations.

Infrared spectroscopy of proteins in reverse micelles

Reverse micelles are a versatile model system for the study of crowded microenvironments containing limited water, such as those found in various tissue spaces or endosomes. They also preclude protein aggregation. Reverse micelles are amenable to study by linear and nonlinear infrared spectroscopies, which have demonstrated that the encapsulation of polypeptides and enzymatically active proteins into reverse micelles leads to conformational changes not seen in bulk solution. The potential value of this model system for understanding the folding and kinetic behavior of polypeptides and proteins in biologically important circumstances warrants increased study of reverse micelle systems by infrared spectroscopy. This article is part of a Special Issue entitled: FTIR in membrane proteins and peptide studies.