Entropy and dynamics of water in hydration layers of a bilayer.
07 Nov 2010-Journal of Chemical Physics (American Institute of Physics)-Vol. 133, Iss: 17, pp 174704-174704
TL;DR: The translational diffusion of water in the vicinity of the head groups is found to be in a subdiffusive regime and the rotational diffusion constant increases going away from the interface, supported by the slower reorientational relaxation of the dipole vector and OH bond vector of interfacial water.
Abstract: We compute the entropy and transport properties of water in the hydration layer of dipalmitoylphosphatidylcholine bilayer by using a recently developed theoretical scheme [two-phase thermodynamic model, termed as 2PT method; S.-T. Lin et al., J. Chem. Phys. 119, 11792 (2003)] based on the translational and rotational velocity autocorrelation functions and their power spectra. The weights of translational and rotational power spectra shift from higher to lower frequency as one goes from the bilayer interface to the bulk. Water molecules near the bilayer head groups have substantially lower entropy (48.36 J/mol/K) than water molecules in the intermediate region (51.36 J/mol/K), which have again lower entropy than the molecules (60.52 J/mol/K) in bulk. Thus, the entropic contribution to the free energy change (TΔS) of transferring an interface water molecule to the bulk is 3.65 kJ/mol and of transferring intermediate water to the bulk is 2.75 kJ/mol at 300 K, which is to be compared with 6.03 kJ/mol for melting of ice at 273 K. The translational diffusion of water in the vicinity of the head groups is found to be in a subdiffusive regime and the rotational diffusion constant increases going away from the interface. This behavior is supported by the slower reorientational relaxation of the dipole vector and OH bond vector of interfacial water. The ratio of reorientational relaxation time for Legendre polynomials of order 1 and 2 is approximately 2 for interface, intermediate, and bulk water, indicating the presence of jump dynamics in these water molecules.
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TL;DR: By using atomistic computer simulations, this work is able to determine not only the free energy for pore formation, but also the enthalpy and entropy, which yields what is believed to be significant new insights in the molecular driving forces behind membrane defects.
169 citations
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...5 kJ/mol at 300 K to move a single water molecule from a DPPC interface to bulk water (48)....
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TL;DR: The detailed characterization obtained here provides insight at atomic detail into processes relevant to biomass pretreatment for cellulosic ethanol production and general polymer coil-globule transition phenomena.
Abstract: Lignins are hydrophobic, branched polymers that regulate water conduction and provide protection against chemical and biological degradation in plant cell walls. Lignins also form a residual barrier to effective hydrolysis of plant biomass pretreated at elevated temperatures in cellulosic ethanol production. Here, the temperature-dependent structure and dynamics of individual softwood lignin polymers in aqueous solution are examined using extensive (17 μs) molecular dynamics simulations. With decreasing temperature the lignins are found to transition from mobile, extended to glassy, compact states. The polymers are composed of blobs, inside which the radius of gyration of a polymer segment is a power-law function of the number of monomers comprising it. In the low temperature states the blobs are interpermeable, the polymer does not conform to Zimm/Stockmayer theory, and branching does not lead to reduction of the polymer size, the radius of gyration being instead determined by shape anisotropy. At high temperatures the blobs become spatially separated leading to a fractal crumpled globule form. The low-temperature collapse is thermodynamically driven by the increase of the translational entropy and density fluctuations of water molecules removed from the hydration shell, thus distinguishing lignin collapse from enthalpically driven coil-globule polymer transitions and providing a thermodynamic role of hydration water density fluctuations in driving hydrophobic polymer collapse. Although hydrophobic, lignin is wetted, leading to locally enhanced chain dynamics of solvent-exposed monomers. The detailed characterization obtained here provides insight at atomic detail into processes relevant to biomass pretreatment for cellulosic ethanol production and general polymer coil-globule transition phenomena.
126 citations
TL;DR: The interaction of pure water, and also of aqueous ionic solutions, with model membranes is described, showing that a symbiosis of experimental and computational work over the past few years has resulted in substantial progress in the field.
Abstract: In a sense, life is defined by membranes, because they
delineate the barrier between the living cell and its
surroundings. Membranes are also essential for regulating the
machinery of life throughout many interfaces within the cell's
interior. A large number of experimental, computational, and
theoretical studies have demonstrated how the properties of
water and ionic aqueous solutions change due to the vicinity of
membranes and, in turn, how the properties of membranes depend
on the presence of aqueous solutions. Consequently,
understanding the character of aqueous solutions at their
interface with biological membranes is critical to research
progress on many fronts. The importance of incorporating a
molecular-level description of water into the study of
biomembrane surfaces was demonstrated by an examination of the
interaction between phospholipid bilayers that can serve as
model biological membranes. The results showed that, in
addition to well-known forces, such as van der Waals and
screened Coulomb, one has to consider a repulsion force due to
the removal of water between surfaces. It was also known that
physicochemical properties of biological membranes are strongly
influenced by the specific character of the ions in the
surrounding aqueous solutions because of the observation that
different anions produce different effects on muscle twitch
tension. In this Account, we describe the interaction of pure
water, and also of aqueous ionic solutions, with model
membranes. We show that a symbiosis of experimental and
computational work over the past few years has resulted in
substantial progress in the field. We now better understand the
origin of the hydration force, the structural properties of
water at the interface with phospholipid bilayers, and the
influence of phospholipid headgroups on the dynamics of water.
We also improved our knowledge of the ion-specific effect,
which is observed at the interface of the phospholipid bilayer
and aqueous solution, and its connection with the Hofmeister
series.
102 citations
TL;DR: This review focuses on three topics that highlight the latest findings on MPC polymers, that is, specific recognition of C-reactive protein (CRP), cell-membrane-penetration abilities, and lubrication properties.
Abstract: 2-Methacryloyloxyethyl phosphorylcholine (MPC) is a custom methacrylate with a zwitterionic phosphorylcholine moiety on the side chain. In the past 25 years, MPC has been used as a building block for a wide range of polymeric biomaterials because of its excellent resistance to nonspecific protein adsorption, cell adhesion, and blood coagulation. Recently, MPC polymers with specific features have been used in bioengineering and nanomedicine. This review focuses on three topics that highlight the latest findings on MPC polymers, that is, specific recognition of C-reactive protein (CRP), cell-membrane-penetration abilities, and lubrication properties. These developments will extend the applications of this biomimetic material from bioinert polymers to biosensing, CRP inhibitors, prodrug carriers, subcellular bioimaging, cell manipulation, and joint replacement. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41766.
99 citations
TL;DR: The recently developed two phase thermodynamics method is used to compute translational and rotational entropies of confined water molecules inside single-walled carbon nanotubes and shows that the increase in energy of a water molecule inside the nanotube is compensated by the gain in its rotational entropy.
Abstract: Experiments and computer simulations demonstrate that water spontaneously fills the hydrophobic cavity of a carbon nanotube. To gain a quantitative thermody- namic understanding of this phenomenon, we use the recently developed Two Phase Thermodynamics (2PT) method to compute translational and rotational entropies of confined water molecules inside single-walled carbon nanotubes and show that the increase in energy of a water molecule inside the nanotube is compensated by the gain in its rotational entropy. The confined water is in equilibrium with the bulk wa- ter and the Helmholtz free energy per water molecule of confined water is the same as that in the bulk within the accuracy of the simulation results. A comparison of translational and rotational spectra of water molecules confined in carbon nanotubes with that of bulk water shows significant shifts in the positions of the spectral peaks that are directly related to the tube radius.
88 citations
References
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TL;DR: This work used molecular dynamics simulations to study the reorientational dynamics of water molecules confined inside narrow carbon nanotubes immersed in a bath of water and showed that the confined water molecules exhibit bistability in their re orientational relaxation, which proceeds by angular jumps between the two stable states.
Abstract: We used molecular dynamics (MD) simulations to study the reorientational dynamics of water molecules confined inside narrow carbon nanotubes immersed in a bath of water. Our simulations show that the confined water molecules exhibit bistability in their reorientational relaxation, which proceeds by angular jumps between the two stable states. The angular jump of a water molecule in the bulk involves the breaking of a hydrogen bond with one of its neighbors and the formation of a hydrogen bond with a different neighbor. In contrast, the angular jump of a confined water molecule corresponds to an interchange of the two hydrogen atoms that can form a hydrogen bond with the same neighbor. The free energy barrier between these two states is a few kBT. The analytic solution of a simplified two-state jump model that qualitatively explains the reorientational behavior observed in simulations is also presented.
41 citations
TL;DR: It is found that the first hydration shell around the water molecule participates roughly in 70% of the total orientational entropy of water, and this rate is roughly temperature independent.
Abstract: Entropies of simple point charge (SPC) water were calculated over the temperature range 278-363 K using the two-particle correlation function approximation. Then, the total two-particle contribution to the entropy of the system was divided into three parts, which we call translational, configurational, and orientational. The configurational term describes the contribution to entropy, which originates from spatial distribution of surrounding water molecules (treated as points, represented by the center of mass) around the central one. It has been shown that this term can serve as the metric of the overall orientational ordering in liquid water. Analyzing each of these three terms as a function of intermolecular distance, r, we also find a rational definition of the hydration shell around the water molecule; the estimated radii of the first and second hydration shells are 0.35 nm and 0.58 nm, respectively. We find, moreover, that the first hydration shell around the water molecule participates roughly in 70% of the total orientational entropy of water, and this rate is roughly temperature independent.
32 citations
TL;DR: The effect of hydrocarbon branching on aqueous dynamics was found to be small, a result similar to the effect on the interfacial water structure, and the hydrocarbon phase shows a larger variation for all dynamical probes, a trend consistent with their interfacial structure.
Abstract: Water/hydrocarbon interfaces are studied using molecular dynamics simulations in order to understand the effect of hydrocarbon branching on the dynamics of the system at and away from the interface. A recently proposed procedure for studying the intrinsic structure of the interface in such systems is utilized, and dynamics are probed in the usual laboratory frame as well as the intrinsic frame. The use of these two frames of reference leads to insight into the effect of capillary waves at the interface on dynamics. The systems were partitioned into zones with a width of 5 A, and a number of quantities of dynamical relevance, namely, the residence times, mean squared displacements, the velocity auto correlation functions, and orientational time correlations for molecules of both phases, were calculated in the laboratory and intrinsic frames at and away from the interface. For the aqueous phase, translational motion is found to be (a) diffusive at long times and not anomalous as in proteins or micelles, (b)...
31 citations
TL;DR: It is found that the time scale of the hydrogen bond fluctuations of the hydration shell of Br- is independent of the nature of the cation and the concentration.
Abstract: We study the hydrogen bond dynamics of solutions of LiBr and NaBr in isotopically diluted water (2% HDO:D2O) with femtosecond spectral hole-burning spectroscopy. We study the frequency fluctuations of the O-H stretch vibrations of the HDO molecules and observe spectral dynamics with time constants of 0.8 +/- 0.1 ps and 4.3 +/- 0.3 ps. The slow process we assign to the hydrogen bond fluctuations of the O-H center dot center dot Br- hydrogen bonds of the hydration shell of the Br- anion. We find that the time scale of the hydrogen bond fluctuations of the hydration shell of Br- is independent of the nature of the cation and the concentration.
29 citations
TL;DR: The single molecule harmonic oscillator approximation for water, although not exact, provides a rapid, insightful, and useful means to evaluate the thermodynamic properties of water from a computer simulation in a way that can account for the quantization of water's energy levels.
Abstract: A method to calculate the classical and quantum free energy of a liquid from a computer simulation by using cell theory [J. Chem. Phys. 2007, 126, 064504] is tested for liquid water and ice Ih against experiment as a function of temperature. This fast and efficient method reproduces reasonably well the experimental values of entropy, enthalpy, and free energy of a liquid across the supercooled, stable, and superheated range of temperatures considered. There are small differences between classical and quantum results of water at 298 K, necessitating a small correction term to reproduce water’s enthalpy of vaporisation. Only at higher temperatures is entropy underestimated by up to 9 J K−1 mol−1 as verified by thermodynamic integration calculations. Satisfactory agreement for ice, however, is only obtained by using the quantum formulation. Even then, at higher temperatures, the entropies exceed experiment by up to 15 J K−1 mol−1. Further insight into the quantum nature of water is provided by inspecting the...
19 citations