Molecular mechanisms of aqueous boundary lubrication by mucinous glycoproteins

Mucins have long been recognized as instrumental to biolubrication but the molecular details of their lubrication mechanisms have only been explored relatively recently. The glycoprotein PRG4, also known as lubricin, shares many features with mucins and appears to lubricate through similar mechanisms. A number of studies have contributed to a more in-depth understanding of mucin adsorption and layer formation on surfaces and the mechanisms by which these layers lubricate. Although mucinous glycoproteins differ in their aggregation properties, their adsorption behaviors on surfaces, and in their ability to reduce friction, they share important similarities favorable for lubrication. They are highly hydrated, they adsorb strongly to a broad range of surfaces, and the layers they form are both sterically and electrostatically repulsive, all attributes thought to contribute to boundary lubrication. They also hydrophilize hydrophobic surfaces, promoting the formation of aqueous fluid films that can lower friction at already relatively low sliding speeds. In this paper we briefly review current knowledge of mucin adsorption and lubrication, with a focus on recent advances.     

Lipids and lipid mixtures in boundary layers: From hydration lubrication to osteoarthritis

The hydration layer surrounding the phosphocholine headgroups of single-component phosphatidylcholine lipids, or of lipid-mixtures, assembled at an interface greatly modifies the interfacial properties and interactions. As water molecules within the hydration layer are held tightly by the headgroup but are nonetheless very fluid on shear, the boundary lipid layers, exposing the highly hydrated headgroup arrays, can provide efficient boundary lubrication when sliding against an opposing surface, at physiologically high contact pressures. In addition, any free lipids in the surrounding liquid can heal defects which may form during sliding on the boundary phosphatidylcholine layer. Similar boundary lipid layers contribute to the lubricating, pressure-bearing, and wear-protection functions of healthy articular joints. This review presents a survey of the relationship between the molecular composition of the interfacial complex and the lubrication behavior of the lipid-based boundary layers, which could be beneficial for designing boundary lubricants for intra-articular injection for the treatment of early osteoarthritis.     

Cation-modulated electron-transfer channel: H-atom transfer vs proton-coupled electron transfer with a variable electron-transfer channel in acylamide units

The mechanism of proton transfer (PT)/electron transfer (ET) in acylamide units was explored theoretically using density functional theory in a representative model (a cyclic coupling mode between formamide and the N-dehydrogenated formamidic radical, FF). In FF, PT/ET normally occurs via a seven-center cyclic proton-coupled electron transfer (PCET) mechanism with a N-->N PT and an O-->O ET. However, when different hydrated metal ions are bound to the two oxygen sites of FF, the PT/ET mechanism may significantly change. In addition to their inhibition of PT/ET rate, the hydrated metal ions can effectively regulate the FF PT/ET cooperative mechanism to produce a single pathway hydrogen atom transfer (HAT) or a flexible proton coupled electron transfer (PCET) mechanism by changing the ET channel. The regulation essentially originates from the change in the O...O bond strength in the transition state, subject to the binding ability of the hydrated metal ions. In general, the high valent metal ions and those with large binding energies can promote HAT, and the low valent metal ions and those with small binding energies favor PCET. Hydration may reduce the Lewis acidity of cations, and thus favor PCET. Good correlations among the binding energies, barrier heights, spin density distributions, O...O contacts, and hydrated metal ion properties have been found, which can be used to interpret the transition in the PT/ET mechanism. These findings regarding the modulation of the PT/ET pathway via hydrated metal ions may provide useful information for a greater understanding of PT/ET cooperative mechanisms, and a possible method for switching conductance in nanoelectronic devices.     

Hydration contributions to association in polyelectrolyte multilayers and complexes: visualizing hydrophobicity

Thin films of polyelectrolyte complex were assembled using the multilayering method with direct, in situ observation of all multilayer components using attenuated total internal reflectance FTIR (ATR-FTIR). Buildup and ion doping of two representative combinations of positive and negative polyelectrolytes are controlled by salt type. The internal hydration of multilayers, measured precisely by ATR-FTIR, depends on the chemical identities of the polymers and the salt ions. The efficiency of doping inversely tracks the degree of hydration: less hydrated ("hydrophobic") ions are more efficient dopants, and less hydrated polyelectrolye complexes are harder to dope. Given that polyelectrolyte complexation is essentially entropy-driven, driving forces for doping, or association (the inverse of doping), are rationalized by counting all species in the condensed polyelectrolyte phase, including water molecules. For any combination of uni-univalent salt ions and polyelectrolyte, the strength of polyelectrolyte association is described by a single universal parameter. The magnitudes of the interactions per repeat unit are not high--a few kT--and are proportional to the number of water molecules released from the polymers when they form ion pairs. Hydration within multilayers due to residual salt is extensive but may be removed by an external osmotic stressing agent.     

Approaches to hydration, old and new: Insights through Hofmeister effects

Hydration effects in colloidal interactions or problems involving electrolytes are usually taken care of by effective electrostatic potentials that subsume notions like hydrated ion size, Gurney potentials, soft and hard, chaotropic and cosmotropic ions, and inner and outer Helmholtz planes. Quantum fluctuation (dispersion) forces between ions and between ions and surfaces are missing from classical theories, at least not explicit in standard approaches to hydration. This paper outlines an evolving back-to-basics approach that allows these ion specific forces to be included in theories quantitatively. In this approach ab initio quantum mechanics is used to calculate dynamic polarisabilities of ions and to quantify bare ion radii. The ionic dispersion potentials between ions, and between ions and surfaces in water can then be given explicit analytic form from an extension of Lifshitz theory. They are included in the theory along with electrostatic potentials. In a first stage the primitive (continuum solvent) model provides a skeletal theory on which to build in hydration. Extension of the ab initio calculations to include “dressed” ions, i.e. water hydration shells for cosmotropic ions, quadrupolar and octupolar polarisability contributions and; for colloids, allowance for a surface hydration layer, permits quantification of Hofmeister effects and Gurney potentials. With these extensions, primary hydration forces (short range repulsion) arise due to an interplay between surface hydration layers and specific ion interactions. Apparent longer range “secondary hydration forces” are shown to be a consequence of ion-surface dispersion interactions and are not true “hydration forces”.     

Polyzwitterionic brushes: Extreme lubrication by design

Polymers offer the advantage that they may independently combine desirable supramolecular structure with useful local monomeric properties to yield optimal performance of different tasks. Here we utilise the remarkable lubricating properties both of dense polymer brushes, and of hydration sheaths about charges via the emerging paradigm of hydration lubrication, to design a grafted-from polyzwitterionic brush system, where each of the monomers has a structure similar to the highly-hydrated phosphorylcholine headgroups of phosphatidylcholine lipids. Such polyzwitterions are grown from a macroinitiator coating the substrate (mica) surface using atom transfer radical polymerisation (ATRP) of 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) to form exceptionally robust poly(MPC) brushes. We have characterized these brush layers via X-ray reflectometry, X-ray photoelectron spectroscopy, surface forces measurements and atomic force microscopy. Such brushes, designed to optimise their lubrication properties, are indeed found to provide state of the art boundary lubrication, achieving friction coefficients as low as 0.0004 at pressures up to 75 atmospheres over a wide range of sliding velocities. Such low friction is comparable with that of articular cartilage in healthy mammalian joints, which represents nature’s benchmark for boundary lubrication in living organisms, and suggests that hydration lubrication plays a major role in reducing friction in biological systems.     

Molecular dynamics simulations of electrolyte solutions at the (100) goethite surface

Molecular dynamics simulations of electrolyte solutions in contact with a neutral (100) goethite (alpha-FeOOH) surface were used to probe the structure of the mineral-water interface and gain insight into the adsorption properties of monovalent ions. Three electrolyte solutions were considered: NaCl, CsCl, and CsF. The electrolyte ions were chosen to cover a range of ionic sizes and affinities for the aqueous phase. The molecular dynamics simulations indicate the presence of a structured interfacial region resulting from the strong interaction of water with the mineral surface. The specific arrangement and preferred orientation of water that arise from this interaction create adsorption sites in the interfacial region, i.e., as far as 15 A away from the surface, and hence give rise to a strong correlation between the water and ion distributions. The structure of the hydrated ion, its effect on the water arrangement at the interface, and the strength of the ion-water bond are found to be key factors that determine the location and extent of ion adsorption at the interface. Additionally, in all simulations, we find a build up of positive charges near the surface due to cation adsorption, which is compensated by an accumulation of anions in the next few angstr?ms. This creates an excess of negative charges, which is in turn compensated by an excess of positive charges, and so on. As we modeled a neutral surface, the structure of the electrolyte distribution arises from the complex interplay of the interactions between the surface, water, and the electrolyte ions rather than from the need to neutralize a surface charge. In addition, our simulations indicate that the electrolyte distribution does not resemble that of a classical electrical double layer. Indeed, our calculations predict the presence of several condensed layers and oscillations in the net charge away from the surface.     

Computer Simulation of Water Clusterization on Chlorine Ions: 2. Microstructure

The molecular structure of Cl^–(H₂O) _n clusters, n = 1–60, in equilibrium with vapor, and the cluster with n = 500 was studied by the Monte Carlo method. The first hydrated layer of a cluster is formed in unsaturated water vapors. The second hydrated layer begins to be formed in saturated vapor. The position of hydrated layers is not changed with an increase in cluster size and coincides with the position of the hydrated layers of ions in aqueous solutions of weak electrolytes. Orientational order in a cluster also has the layered structure. The orientation of molecules between the layers is random. The stability of the first layer is ensured only due to direct interactions with ions, whereas the stability of subsequent layers is due to cooperative interactions between molecules and between molecules and ions. As temperature decreases, the effect of ion displacement to the cluster surface becomes stronger.     

Comprehensive 3D-RISM analysis of the hydration of small molecule binding sites in ligand-free protein structures

Hydration is a critical factor in the ligand binding process. Herein, to examine the hydration states of ligand binding sites, the three-dimensional distribution function for the water oxygen site, g_O( r ) , is computed for 3,706 ligand-free protein structures based on the corresponding small molecule–protein complexes using the 3D-RISM theory. For crystallographic waters (CWs) close to the ligand, g_O( r ) reveals that several CWs are stabilized by interaction networks formed between the ligand, CW, and protein. Based on the g_O( r ) for the crystallographic binding pose of the ligand, hydrogen bond interactions are dominant in the highly hydrated regions while weak interactions such as CH-O are dominant in the moderately hydrated regions. The polar heteroatoms of the ligand occupy the highly hydrated and moderately hydrated regions in the crystallographic (correct) and wrongly docked (incorrect) poses, respectively. Thus, the g_O( r ) of polar heteroatoms may be used to distinguish the correct binding poses.     

Theorization on ion-exchange equilibria: activity of species in 2-D phases

Ion-exchange reactions are naturally occurring at soil and sediment/water interphases, determining soil fertility and water quality. These ion-exchange reactions with inorganic and organic exchangers are applied to chemical analysis, recovery of useful ions from low-grade ores (potentially from sea water), water purification including the preparation of "ultrapure" water, production of foods and medicines, therapy, and other uses. It is important to theorize about or to model ion-exchange reactions for quantitative explanations of ion-exchange phenomena and for efficient operation of ion-exchange processes. This paper describes the modeling of ion-exchange equilibria for hydroxyl sites on metal oxides and carboxyl sites in resins with monovalent cations (alkali metal ions), a monovalent anion (nitrate ion), and divalent heavy metal ions. The procedure of modeling is as follows: the stoichiometry and material balance equations of the respective ion-exchange reactions were established based on findings here and by others. The equilibrium conditions were given by the Frumkin equation, where the mass-action relation is modified with lateral interactions between species at the interphase. The model equations were fitted to the measured data and model parameter values were determined by nonlinear regression analysis. The formation of bonds between ions and exchanger sites was evaluated by the equilibrium constant and the suppression of bond formation by electrostatic, geometric, and other lateral interactions was evaluated by the interaction constant. It was established that the properties of ions are determined by the valence, size, and hydration state of the ions. Monovalent ions (anions and cations) react with oxide surface hydroxyl and resin carboxyl sites as hydrated ions and form loose ion-site pairs by a weak electrostatic bond (nonspecific adsorption). However, the lateral interactions are large because of a large polarization of the ion-site pairs. When the monovalent cations are dehydrated to react with carboxyl sites in narrow resin nanopores, the bond formation is difficult because energy for dehydration is necessary. The suppressive lateral interactions here are small because of a small polarization of the dehydrated ion-site pairs that are in direct contact. Divalent heavy metal ions react with oxide hydroxyl sites by replacing their hydrated water molecules and form ion-site pairs in direct strong contact (specific adsorption). The bond formation becomes easier with increasing charge density of the ions evaluated by the charge/radius ratio, agreeing with the order of these ions to form hydroxo complexes in solution. The suppressive lateral interaction is, however, small for ions with large charge densities, because a strong contact bond reduces the polarization of ion-site pairs by neutralization. The properties of exchangers are functions of the molecular and pore environments around the functional groups. The acid-base nature of oxide surface-hydroxyl groups is determined by the electronegativity of surrounding lattice metal ions, and that of resin carboxyl groups by the electron-repelling effect of adjacent methyl groups. Pores in oxides have diameters sufficient to accommodate hydrated ions, and the suppression is large because of repulsion from ions adsorbed on opposite pore walls (across-pore interaction). Pores in resins differentiate ions that can access or not access sites on the internal surfaces of the pores. Narrow nanopores with diameters less than those of the hydrated ions require ions to dehydrate before they can enter. The ion-exchange reactivity here is small, as described above for dehydrated monovalent ions. In wide nanopores where hydrated ions can enter, bond formation is easier, but suppression is greater because of a larger polarization of hydrated ion-site pairs and also of the across-pore interaction. Macropores have diameters much larger than those of the hydrated ions and the bond formation is the same as that in wide nanopores, but the suppression is smaller because of the absence oe of the absence of the across-pore interaction. Finally, this paper attempts a formulation of activity coefficients of exchanging sites and adsorbed ion-site pairs and compares the proposed activity coefficients of interphase species with that of solution species given by the Debye-Hückel equation.     

Partition coefficients of methylated DNA bases obtained from free energy calculations with molecular electron density derived atomic charges

Partition coefficients serve in various areas as pharmacology and environmental sciences to predict the hydrophobicity of different substances. Recently, they have also been used to address the accuracy of force fields for various organic compounds and specifically the methylated DNA bases. In this study, atomic charges were derived by different partitioning methods (Hirshfeld and Minimal Basis Iterative Stockholder) directly from the electron density obtained by electronic structure calculations in a vacuum, with an implicit solvation model or with explicit solvation taking the dynamics of the solute and the solvent into account. To test the ability of these charges to describe electrostatic interactions in force fields for condensed phases, the original atomic charges of the AMBER99 force field were replaced with the new atomic charges and combined with different solvent models to obtain the hydration and chloroform solvation free energies by molecular dynamics simulations. Chloroform–water partition coefficients derived from the obtained free energies were compared to experimental and previously reported values obtained with the GAFF or the AMBER‐99 force field. The results show that good agreement with experimental data is obtained when the polarization of the electron density by the solvent has been taken into account, and when the energy needed to polarize the electron density of the solute has been considered in the transfer free energy. These results were further confirmed by hydration free energies of polar and aromatic amino acid side chain analogs. Comparison of the two partitioning methods, Hirshfeld‐I and Minimal Basis Iterative Stockholder (MBIS), revealed some deficiencies in the Hirshfeld‐I method related to the unstable isolated anionic nitrogen pro‐atom used in the method. Hydration free energies and partitioning coefficients obtained with atomic charges from the MBIS partitioning method accounting for polarization by the implicit solvation model are in good agreement with the experimental values. © 2018 Wiley Periodicals, Inc.     

An <Emphasis Type="Italic">ab initio</Emphasis>and AIM investigation into the hydration of 2-thioxanthine

Background  

Hydration is a universal phenomenon in nature. The interactions between biomolecules and water of hydration play a pivotal role in molecular biology. 2-Thioxanthine (2TX), a thio-modified nucleic acid base, is of significant interest as a DNA inhibitor yet its interactions with hydration water have not been investigated either computationally or experimentally. Here in, we reported an ab initio study of the hydration of 2TX, revealing water can form seven hydrated complexes.     

Macrocyclic ZnII and CuII complexes as guests of the hybrid composites based on the layered MnPS3 phase. Comparison of spectroscopic properties

A family of macrocyclic complexes [M₂LⁿCl₂] have been synthesized and characterized (M: Cu^II or Zn^II; Lⁿ: macrocyclic ligand derived from 2-hydroxy-5-methyl-1,3-benzenedicarbaldehyde and different aliphatic diamines and o-phenylenediamine). The influence of the aromaticity of the ligand and the metal center on the spectroscopic properties of the complexes (absorption and emission) has been studied. Making use of the weak interactions between hydrated potassium ions and the layers of the K_0.4Mn_0.8PS₃ precursor, the obtained macrocyclic complexes have been intercalated in the interlamellar space by a microwave assisted cationic exchange reaction. The optical properties of the obtained hybrid materials are reported. The absorption edge, recorded by solid state reflectance spectroscopy for Cu^II and the Zn^II macrocycle-based composites, is 1.67–1.76 eV, both shifted to lower energy compared with that of the pristine MnPS₃.     

Microscopic hydration of the sodium chloride ion pair

Hydration of ion pairs is an essential process in various physicochemical phenomena occurring in solutions. Isolated clusters of an ion pair solvated with finite number of waters have been considered as a model system for the critical evaluation of microscopic interactions involved in the process, and theoretical studies have contributed exclusively to the subject up to now. Here we report the first experimental characterization of structure and internal dynamics of hydrated ion pairs, NaCl-(H2O)n (n = 1-3). The measurements of their rotational spectra have proven that the clusters have cyclic forms, in which Na+ and Cl- ions are strongly interacted with the O and H atoms of the solvent molecules, respectively. The Na-Cl distance shows a pronounced increase with the successive addition of water molecules. The separation for n = 3 approaches the value predicted for the contact ion-pair state in aqueous solution by recent molecular dynamics simulations.     

DETERMINATION OF THE IN VIVO ABSORPTION AND PHOTOSYNTHETIC PROPERTIES OF THE LICHEN Acarospora schleicheri USING PHOTO ACOUSTIC SPECTROSCOPY

Photoacoustic spectroscopy was used to determine the in vivo spectra of the stratified components of the lichen Acarospora schleicheri. There were three pigmented layers observed by the photoacoustic method: an absorption band associated with the pigment rhizocarpic acid, a bulk cytochrome absorption, and the absorption of the algal chloroplast. Due to the different absorption properties of these distinct layers, the photoacoustic technique was able to monitor the physical effect of hydration on the lichen. Hydration of the lichen reduced the scatter of the sample, increasing the effective incident light intensity within the sample. Hydration also resulted in an increase in the optical absorption coefficient of the algal layer, which was interpreted as a movement of the algal chloroplast in response to low light fluence and hydrated conditions. The photoacoustic technique was also used to monitor the relative photosynthetic activity of the algae within the lichen in a hydrated and dehydrated state. The photosynthetic assay could detect photosynthetic activity in the hydrated state but not in the dehydrated state. In addition, the photosynthetic response was found to be induced and repressed with light and dark periods respectively and its detection was limited at high frequencies by the damping of the thermal wave from the algal layer.     

Evidence of hydration forces between proteins

Proteins are fundamental molecules in biology that are also involved in a wide range of industrial and biotechnological processes. Consequently, many works in the literature have been devoted to the study of protein–protein and protein–surface interactions in aqueous solutions. The results have been usually interpreted within the frame of the classical Derjaguin–Landau–Verwey–Overbeek (DLVO) theory for colloidal systems. However, against the DLVO predictions, striking evidence of repulsive forces between proteins at high salt concentrations has been observed in different works based on the analysis of the second virial coefficient or on the direct measurement of protein interaction with an atomic force microscope. Hydration forces due to the adsorption of hydrated cations onto the negatively charged protein surfaces have been invoked to rationalize this anomalous repulsion. The hydration forces between proteins provide protein-covered particles with a non-DLVO colloidal stability at high salt concentrations, as different studies in the literature has proven. This review summarizes the most relevant results published so far on the presence of hydration forces between proteins and protein-coated colloidal particles.     

Kosmotropic and chaotropic behavior of hydrated ions in aqueous solutions in terms of expansibility and compressibility parameters

Hydrated ions have fundamental applications in chemical and biological processes. Kosmotropic and chaotropic nature of hydrated ions affect the water structure in solutions depending upon their hydrophobicity or hydrophilicity nature. In present study Kosmotropic and chaotropic behavior of hydrated ions have been explained in terms of volumetric and acoustic parameters like apparent molar volume (V_ϕ), expansibility and compressibility factors for aqueous electrolytic solutions provide useful information about interactions among ions and water molecules. Results of V_ϕ showed that SO₄²⁻ ions due to stronger H-bonding with water molecules are termed as kosmotropes while Cl⁻ and HCO₃^– are chaotropes due to their weaker H-bonding with water molecules. More compressible structure of solutions in the presence of SO₄²⁻ ions indicated its kosmotropic behavior and comparatively less compressible structure of solutions in the presence of Cl⁻¹ and HCO₃^– ions renders them chaotropes. Results obtained from expansibility factor showed the dominance of electrostatic interactions over hydrophobic hydration of ions at higher temperatures. Greater values of expansibility factor for SO₄²⁻ ions as compared to Cl⁻¹ and HCO₃^– ions renders them kosmotropic ion while later are termed as chaotropes. Hence, thermo-acoustic parameters could be effectively used to describe the hydrogen bonding character of ionic solutions in terms of kosmotropic and chaotropic behavior of solutions.     

Silica surfaces lubrication by hydrated cations adsorption from electrolyte solutions

Adsorption of hydrated cations on hydrophilic surfaces has been related to a variety of phenomena associated with the short-range interaction forces and mechanisms of the adhesive contact between the surfaces. Here we have investigated the effect of the adsorption of cations on the lateral interaction. Using lateral force microscopy (LFM), we have measured the friction force between a silica particle and silica wafer in pure water and in electrolyte solutions of LiCl, NaCl, and CsCl salts. A significant lubrication effect was demonstrated for solutions of high electrolyte concentrations. It was found that the adsorbed layers of smaller and more hydrated cations have a higher lubrication capacity than the layers of larger and less hydrated cations. Additionally, we have demonstrated a characteristic dependence of the friction force on the sliding velocity of surfaces. A mechanism for the observed phenomena based on the microstructures of the adsorbed layers is proposed.