Water populations in restricted environments of lipid membrane interphases

Water at the nanoscale: From filling or dewetting hydrophobic pores and carbon nanotubes to "sliding" on graphene

In this work, we study the effect of nanoconfinement on the hydration properties of model hydrophobic pores and carbon nanotubes, determining their wetting propensity and the conditions for geometrically induced dehydration. By employing a recently introduced water structural index, we aim at two main goals: (1) to accurately quantify the local hydrophobicity and predict the drying transitions in such systems, and (2) to provide a molecular rationalization of the wetting process. In this sense, we will further discuss the number and strength of the interactions required by the water molecules to promote wetting. In the case of graphene-like surfaces, an explanation for their unexpectedly significant hydrophilicity will also be provided. On the one hand, the structural index will show that the net attraction to the dense carbon network that a water molecule experiences through several simultaneous weak interactions is sufficient to give rise to hydrophilic behavior. On the other hand, we will show that an additional effect is also at play: the hydrating water molecule is retained on the surface by a smooth exchange of such simultaneous weak interactions, as if "sliding" on graphene.

A water structure indicator suitable for generic contexts: Two-liquid behavior at hydration and nanoconfinement conditions and a molecular approach to hydrophobicity and wetting

In a recent work, we have briefly introduced a new structural index for water that, unlike previous indicators, was devised specifically for generic contexts beyond bulk conditions, making it suitable for hydration and nanoconfinement settings. In this work, we shall study this metric in detail, demonstrating its ability to reveal the existence of a fine-tuned interplay between the local structure and energetics in liquid water. This molecular principle enables the establishment of an extended hydrogen bond network, while simultaneously allowing for the existence of network defects by compensating for uncoordinated sites. By studying different water models and different temperatures encompassing both the normal liquid and the supercooled regime, this molecular mechanism will be shown to underlie the two-state behavior of bulk water. In addition, by studying functionalized self-assembled monolayers and diverse graphene-like surfaces, we shall show that this principle is also operative at hydration and nanoconfinement conditions, thus generalizing the validity of the two-liquid scenario of water to these contexts. This approach will allow us to define conditions for wettability, providing an accurate measure of hydrophobicity and a reliable predictor of filling and drying transitions. Hence, it might open the possibility of elucidating the active role of water in the broad fields of biophysics and materials science. As a preliminary step, we shall study the hydration structure and hydrophilicity of graphene-like systems (parallel graphene sheets and carbon nanotubes) as a function of the confinement dimensionality.

Atomic-scale structure of interfacial water on gel and liquid phase lipid membranes

Hydration of biological membranes is essential to a wide range of biological processes. In particular, it is intrinsically linked to lipid thermodynamic properties, which in turn influence key cell functions such as ion permeation and protein mobility. Experimental and theoretical studies of the surface of biomembranes have revealed the presence of an interfacial repulsive force, which has been linked to hydration or steric effects. Here, we directly characterise the atomic-scale structure of water near supported lipid membranes of 1,2-dimyristoyl-sn-glycero-3-phosphocholine in their gel and liquid phase through three-dimensional atomic force microscopy (3D AFM). First, we demonstrate the ability to probe the morphology of interfacial water of lipid bilayers in both phases with sub-molecular resolution by using ultrasharp tips. We then visualise the molecular arrangement of water at the lipid surface at different temperatures. Our experiments reveal that water is organised in multiple hydration layers on both the solid-ordered and liquid-disordered lipid phases. Furthermore, we observe a monotonic repulsive force, which becomes relevant only in the liquid phase. These results offer new insights into the water structuring near soft biological surfaces, and demonstrate the importance of investigating it with vertical and lateral sub-molecular resolution.

Influence of docosahexaenoic acid on the interfacial behavior of cholesterol-containing lipid membranes: Interactions with small amphiphiles and hydration properties

Cholesterol is known to significantly modify both the structural and the dynamical properties of lipid membranes. On one side, the presence of free cholesterol molecules has been determined to stiffen the membrane bilayer by stretching the hydrophobic tails. Additionally, recent experimental and computational findings have made evident the fact that cholesterol also alters the dynamics and the hydration properties of the polar head groups of DPPC model lipid membranes. In turn, we have recently shown that the Omega-3 fatty acid docosahexaenoic acid, DHA, counteracts the effect of cholesterol on DPPC membrane's mechanical properties by fluidizing the bilayer. However, such behavior represents in fact a global outcome dominated by the larger lipid hydrophobic tails that neither discriminates between the different parts of the membrane nor elucidates the effect on membrane hydration and binding properties. Thus, we now perform molecular dynamics simulations to scrutinize the influence of DHA on the interfacial behavior of cholesterol-containing lipid membranes by characterizing their hydration properties and their binding to amphiphiles. We find that while cholesterol destabilizes interactions with amphiphiles and slightly weakens the lipid's hydration layer, the incorporation of DHA practically restores the interfacial behavior of pure DPPC.

Correlations between defect propensity and dynamical heterogeneities in supercooled water

A salient feature of supercooled liquids consists in the dramatic dynamical slowdown they undergo as temperature decreases while no significant structural change is evident. These systems also present dynamical heterogeneities (DH): certain molecules, spatially arranged in clusters, relax various orders of magnitude faster than the others. However, again, no static quantity (such as structural or energetic measures) shows strong direct correlations with such fast-moving molecules. In turn, the dynamic propensity approach, an indirect measure that quantifies the tendency of the molecules to move in a given structural configuration, has revealed that dynamical constraints, indeed, originate from the initial structure. Nevertheless, this approach is not able to elicit which structural quantity is, in fact, responsible for such a behavior. In an effort to remove dynamics from its definition in favor of a static quantity, an energy-based propensity has also been developed for supercooled water, but it could only find positive correlations between the lowest-energy and the least-mobile molecules, while no correlations could be found for those more relevant mobile molecules involved in the DH clusters responsible for the system's structural relaxation. Thus, in this work, we shall define a defect propensity measure based on a recently introduced structural index that accurately characterizes water structural defects. We shall show that this defect propensity measure provides positive correlations with dynamic propensity, being also able to account for the fast-moving molecules responsible for the structural relaxation. Moreover, time dependent correlations will show that defect propensity represents an appropriate early-time predictor of the long-time dynamical heterogeneity.

Temporal segregation of biosynthetic processes is responsible for metabolic oscillations during the budding yeast cell cycle

Many cell biological and biochemical mechanisms controlling the fundamental process of eukaryotic cell division have been identified; however, the temporal dynamics of biosynthetic processes during the cell division cycle are still elusive. Here, we show that key biosynthetic processes are temporally segregated along the cell cycle. Using budding yeast as a model and single-cell methods to dynamically measure metabolic activity, we observe two peaks in protein synthesis, in the G1 and S/G2/M phase, whereas lipid and polysaccharide synthesis peaks only once, during the S/G2/M phase. Integrating the inferred biosynthetic rates into a thermodynamic-stoichiometric metabolic model, we find that this temporal segregation in biosynthetic processes causes flux changes in primary metabolism, with an acceleration of glucose-uptake flux in G1 and phase-shifted oscillations of oxygen and carbon dioxide exchanges. Through experimental validation of the model predictions, we demonstrate that primary metabolism oscillates with cell-cycle periodicity to satisfy the changing demands of biosynthetic processes exhibiting unexpected dynamics during the cell cycle.

A Molecular View of the Surface Pressure/Area Per Lipid Isotherms Assessed by FTIR/ATR Spectroscopy

The macroscopic behavior of a lipid monolayer in terms of packing and compressibility properties is classically obtained from surface pressure/area per molecule isotherms. Molecular interpretations trying to fit the II/A curves have been attempted by molecular dynamics. In this regard, the simulation is performed by introducing parameters accounting for the lipid-lipid interaction in the monolayer plane. However, water, as an essential component of the interfacial phenomena, is not explicitly included in terms of molecular arrays. This drawback appears to be a consequence of the lack of experimental evidence that may complement the macroscopic view with the microscopic features. In this work, we propose that II/A curves can be reproduced from microscopic molecular data obtained with FTIR/ATR spectroscopy. The changes in surface pressure, in fact, changes in the surface tension of the lipid–water interphase, can be related to the acyl regions exposed to water and evaluated by the ratio of isolated-to-connected CH2 populations. In turn, the area changes correspond to the variations in the primary and secondary hydration shells of the phosphate region. The isolated/connected CH2 ratio represents the extension of the non-polar region exposed to water and is linked to the resulting water surface tension. The area per lipid is determined by the excluded volume of the hydration shells around the phosphate groups in correlation to the carbonyl groups. The derivative of the frequencies of the -CH2 groups with respect to the water content gives an insight into the influence of water arrangements on the compressibility properties, which is important in understanding biologically relevant phenomena, such as osmotic stress in cells and the mechanical response of monolayers. It is concluded that the water population distributed around the different groups dominates, to a great extent, the physical properties of the lipid membranes.

Water as a Link between Membrane and Colloidal Theories for Cells

This review is an attempt to incorporate water as a structural and thermodynamic component of biomembranes. With this purpose, the consideration of the membrane interphase as a bidimensional hydrated polar head group solution, coupled to the hydrocarbon region allows for the reconciliation of two theories on cells in dispute today: one considering the membrane as an essential part in terms of compartmentalization, and another in which lipid membranes are not necessary and cells can be treated as a colloidal system. The criterium followed is to describe the membrane state as an open, non-autonomous and responsive system using the approach of Thermodynamic of Irreversible Processes. The concept of an open/non-autonomous membrane system allows for the visualization of the interrelationship between metabolic events and membrane polymorphic changes. Therefore, the Association Induction Hypothesis (AIH) and lipid properties interplay should consider hydration in terms of free energy modulated by water activity and surface (lateral) pressure. Water in restricted regions at the lipid interphase has thermodynamic properties that explain the role of H-bonding networks in the propagation of events between membrane and cytoplasm that appears to be relevant in the context of crowded systems.

Structure and dynamics of nanoconfined water and aqueous solutions

This review is devoted to discussing recent progress on the structure, thermodynamic, reactivity, and dynamics of water and aqueous systems confined within different types of nanopores, synthetic and biological. Currently, this is a branch of water science that has attracted enormous attention of researchers from different fields interested to extend the understanding of the anomalous properties of bulk water to the nanoscopic domain. From a fundamental perspective, the interactions of water and solutes with a confining surface dramatically modify the liquid's structure and, consequently, both its thermodynamical and dynamical behaviors, breaking the validity of the classical thermodynamic and phenomenological description of the transport properties of aqueous systems. Additionally, man-made nanopores and porous materials have emerged as promising solutions to challenging problems such as water purification, biosensing, nanofluidic logic and gating, and energy storage and conversion, while aquaporin, ion channels, and nuclear pore complex nanopores regulate many biological functions such as the conduction of water, the generation of action potentials, and the storage of genetic material. In this work, the more recent experimental and molecular simulations advances in this exciting and rapidly evolving field will be reported and critically discussed.

Breakdown of classical paradigms in relation to membrane structure and functions

Updates of the mosaic fluid membrane model implicitly sustain the paradigms that bilayers are closed systems conserving a state of fluidity and behaving as a dielectric slab. All of them are a consequence of disregarding water as part of the membrane structure and its essential role in the thermodynamics and kinetics of membrane response to bioeffectors. A correlation of the thermodynamic properties with the structural features of water makes possible to introduce the lipid membrane as a responsive structure due to the relaxation of water rearrangements in the kinetics of bioeffectors' interactions. This analysis concludes that the lipid membranes are open systems and, according to thermodynamic of irreversible formalism, bilayers and monolayers can be reasonable compared under controlled conditions. The inclusion of water in the complex structure makes feasible to reconsider the concept of dielectric slab and fluidity.

Effect of cholesterol on the hydration properties of ester and ether lipid membrane interphases

Fluorescence spectroscopy and Molecular Dynamics results show that cholesterol reduces water along the chains in ether lipids by changing the water distribution pattern between tightly and loosely bound water molecules. Water distribution was followed by emission spectra and generalized polarization of 6-dodecanoyl-2-dimethyl aminonaphthalene (Laurdan) inserted in 1,2-dimiristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-di-O-tetradecyl-sn-glycero-3-phosphocholine (14: 0 Diether PC) membranes. Molecular Dynamics simulations indicate that the action of cholesterol could be different in ether PC in comparison to ester PC. In addition, Cholesterol seems to act "per se" as an additional hydration center in ether lipids. Regardless of the phase state, cholesterol both in DMPC and 14:0 Diether PC vesicles, changed the distribution of water molecules decreasing the dipole relaxation of the lipid interphase generating an increase in the non-relaxable population. Above 10% Cholesterol/14:0 Diether PC ratio vesicles' interphase present an environment around Laurdan molecules similar to that corresponding to ester PC.

Water-lipid interactions in native bone by high-resolution solid-state NMR spectroscopy

The study of structural and dynamical properties of lipid and its associated interaction with different components of bone is essential to understand its role at a different level of bone homeostasis such as bone mineralization and bone metabolism. In this article, we present water-dependent dynamical changes observed in lipids (triglycerides) in its absolute native environment inside bone by high-resolution ¹H solid-state nuclear magnetic resonance spectroscopy (ssNMR). Relaxation measurement (T₂ measurement) ssNMR experiments were performed at different levels of water network induced by dehydration and H/D exchange in native bone. Our measurements reflect the changes in the local environment and dynamical properties of triglyceride due to different hydration levels. The present study explains the role of water in stabilizing the structural properties of triglycerides in bone hence will help understand its pathological role associated with bone physiology and bone disorders.

Modulation of Interfacial Hydration by Carbonyl Groups in Lipid Membranes

The lack of carbonyl groups and the presence of ether bonds give the lipid interphase a different water organization around the phosphate groups that affects the compressibility and electrical properties of lipid membranes. Generalized polarization of 1,2-di-O-tetradecyl-sn-glycero-3-phosphocholine (14:0 diether PC) in correlation with Fourier transform infrared (FTIR) analysis indicates a higher level of polarizability of water molecules in the membrane phase around the phosphate groups both below and above T_m. This reorganization of water promotes a different response in compressibility and dipole moment of the interphase, which is related to different H bonding of water molecules with phosphates (PO) and carbonyl (CO) groups.

Lipid-Surrounding Water Molecules Probed by Time-Resolved Emission Spectra of Laurdan

The hydration states of the interfacial region of lipid bilayers were investigated on the basis of the time-resolved emission spectra (TRES) analysis of 6-lauroyl-2-dimethylamino naphthalene (Laurdan), a common fluorescence probe used to analyze membrane hydration. TRES derived from long and short lifetime components were extracted from samples of different lipid species: 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC), 1,2-dioleoyl- sn-glycero-3-phosphocholine (DOPC), d- erythro- N-palmitoyl-sphingosylphosphorylcholine (PSM), and a DOPC/PSM binary bilayer system. Neither lifetime component (short or long) corresponded with the hydration properties; the short lifetime component of DOPC (1.97 ns) exhibited a peak at 440 nm, and the long lifetime components of DPPC and PSM (7.76 and 7.77 ns, respectively) exhibited peaks at the same wavelength. This similarity arose from the competition between the collisional quenching and the hydration effects of water molecules. Herein, this phenomenon was investigated using a plot of the lifetime τ and the peak position λ (τ vs λ plot), simultaneously visualizing both effects by deconvoluting the TRES. On the basis of collisional quenching theory, the distribution of the water population per lipid (water map) was generated. According to this theory, the τ vs λ plot was applied to the water map and the calculation of the number of water molecules per lipid, which is consistent with previous reports. This approach provides novel insights for the analysis of molecular hydration states using the fluorescence of Laurdan.

A new model for lipid monolayer and bilayers based on thermodynamics of irreversible processes

Lipid monolayers are used as experimental model systems to study the physical chemical properties of biomembranes. With this purpose, surface pressure/area per molecule isotherms provide a way to obtain information on packing and compressibility properties of the lipids. These isotherms have been interpreted considering the monolayer as a two dimensional ideal or van der Waals gas without contact with the water phase. These modelistic approaches do not fit the experimental results. Based on Thermodynamics of Irreversible Processes (TIP), the expansion/compression process is interpreted in terms of coupled phenomena between area changes and water fluxes between a bidimensional solution of hydrated head groups in the monolayer and the bulk solution. The formalism obtained can reproduce satisfactorily the surface pressure/area per lipid isotherms of monolayer in different states and also can explain the area expansion and compression produced in particles enclosed by bilayers during osmotic fluxes. This novel approach gives relevance to the lipid-water interaction in restricted media near the membrane and provides a formalism to understand the thermodynamic and kinetic response of biointerphases to biological effectors.

Correlation between the hydration of acyl chains and phosphate groups in lipid bilayers: Effect of phase state, head group, chain length, double bonds and carbonyl groups

This paper demonstrates by means of FTIR/ATR analysis that water molecules intercalate at different extents in the acyl chain region of lipid membranes in correlation with the hydration of the phosphate groups. This correlation is sensible to the chain length, the presence of double bonds and the phase state of the lipid membrane. The presence of carbonyl groups CO modifies the profile of hydration of the two regions as observed from the comparison of DMPC and 14:0 Diether PC. The different water populations in lipid interphases would give arrangements with different free energy states that could drive the interaction of biological effectors with membranes.

Solvatochromic Modeling of Laurdan for Multiple Polarity Analysis of Dihydrosphingomyelin Bilayer

The hydration properties of the interface between lipid bilayers and bulk water are important for determining membrane characteristics. Here, the emission properties of a solvent-sensitive fluorescence probe, 6-lauroyl-2-dimethylamino naphthalene (Laurdan), were evaluated in lipid bilayer systems composed of the sphingolipids D-erythro-N-palmitoyl-sphingosylphosphorylcholine (PSM) and D-erythro-N-palmitoyl-dihydrosphingomyelin (DHPSM). The glycerophospholipids 1-palmitoyl-2-palmitoyl-sn-glycero-3-phosphocholine and 1-oleoyl-2-oleoyl-sn-glycero-3-phosphocholine were used as controls. The fluorescence properties of Laurdan in sphingolipid bilayers indicated multiple excited states according to the results obtained from the emission spectra, fluorescence anisotropy, and the center-of-mass spectra during the decay time. Deconvolution of the Laurdan emission spectra into four components based on the solvent model enabled us to identify the varieties of hydration and the configurational states derived from intermolecular hydrogen bonding in sphingolipids. Sphingolipids showed specific, interfacial hydration properties stemming from their intra- and intermolecular hydrogen bonds. Particularly, the Laurdan in DHPSM revealed more hydrated properties compared to PSM, even though DHPSM has a higher T_m than PSM. Because DHPSM forms hydrogen bonds with water molecules (in 2NH configurational functional groups), the interfacial region of the DHPSM bilayer was expected to be in a highly polar environment. The careful analysis of Laurdan emission spectra through the four-component deconvolution in this study provides important insights for understanding the multiple polarity in the lipid membrane.

Functional Hydration Behavior: Interrelation between Hydration and Molecular Properties at Lipid Membrane Interfaces

Water is an abundant commodity and has various important functions. It stabilizes the structure of biological macromolecules, controls biochemical activities, and regulates interfacial/intermolecular interactions. Common aspects of interfacial water can be obtained by overviewing fundamental functions and properties at different temporal and spatial scales. It is important to understand the hydrogen bonding and structural properties of water and to evaluate the individual molecular species having different hydration properties. Water molecules form hydrogen bonds with biomolecules and contribute to the adjustment of their properties, such as surface charge, hydrophilicity, and structural flexibility. In this review, the fundamental properties of water molecules and the methods used for the analyses of water dynamics are summarized. In particular, the interrelation between the hydration properties, determined by molecules, and the properties of molecules, determined by their hydration properties, are discussed using the lipid membrane as an example. Accordingly, interesting water functions are introduced that provide beneficial information in the fields of biochemistry, medicine, and food chemistry.

Effects of Phenylalanine on the Liquid-Expanded and Liquid-Condensed States of Phosphatidylcholine Monolayers

Background:

Phenylalanine (Phe) is involved in physiological and pathological processes in cell membranes in which expanded and condensed states coexist. In this direction, it was reported that surface hydration is important for the binding affinity of the amino acid which significantly perturbs 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayer structure and morphology. A deeper insight showed that Phe inserts in DPPC monolayer defects as a monomer at pH 5 and forms aggregates that adsorb to the membrane surface generating a reconfiguration of the lipid arrangement in areas of higher packing. This new arrangement in the monolayer causes the reorientation of dipoles of lipid and water molecules which is congruent with the dehydration and surface tension changes reported above. With this background, this article studies the affinity of Phe in liquid-expanded 1,2-dimyristoyl-sn-glycero-3 phosphocholine (LE DMPC) and liquid-condensed 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (LC DPPC) monolayers and their effects on membrane properties.

Results:

The adsorption of Phe can be described by a cooperative process in non-independent sites suggesting that Phe/lipid systems reorganize to form new structures at a high degree of coverage. Compressibility modulus and Brewster angle microscopy (BAM) images allow to propose that Phe causes a new phase in 1,2-dimyristoyl-sn-glycero-3 phosphocholine (DMPC) and DPPC.

Conclusions:

Phe imposes new arrangements in the lipid phase to form new structures with different compressibility behavior than lipid binary mixtures of DMPC and DPPC. Phe interaction with the LC and LE phases gives place to a process in which a synergistic effect between non-independent sites can be produced. These features of Phe/lipid interaction would be of great importance to understand the multiple effects of Phe on cell membranes.

Influence of Cholesterol on the Dynamics of Hydration in Phospholipid Bilayers

We investigate the dynamics of interfacial waters in dipalmitoylphosphatidylcholine (DPPC) bilayers upon the addition of cholesterol, by molecular dynamics simulations. Our data reveal that the inclusion of cholesterol modifies the membrane aqueous interfacial dynamics: waters diffuse faster, their rotational decay time is shorter, and the DPPC/water hydrogen bond dynamics relaxes faster than in the pure DPPC membrane. The observed acceleration of the translational water dynamics agrees with recent experimental results, in which, by means of NMR techniques, an increment of the surface water diffusivity is measured upon the addition of cholesterol. A microscopic analysis of the lipid/water hydrogen bond network at the interfacial region suggests that the mechanism underlying the observed water mobility enhancement is given by the rupture of a fraction of interlipid water bridge hydrogen bonds connecting two different DPPC molecules, concomitant to the formation of new lipid/solvent bonds, whose dynamics is faster than that of the former. The consideration of a simple two-state model for the decay of the hydrogen bond correlation function yielded excellent results, obtaining two well-separated characteristic time scales: a slow one (∼250 ps) associated with bonds linking two DPPC molecules, and a fast one (∼15 ps), related to DPPC/solvent bonds.