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Journal ArticleDOI

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.
Citations
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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


Cites background from "Entropy and dynamics of water in hy..."

  • ...5 kJ/mol at 300 K to move a single water molecule from a DPPC interface to bulk water (48)....

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Journal ArticleDOI
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

Journal ArticleDOI
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

Journal ArticleDOI
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

Journal ArticleDOI
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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Journal ArticleDOI
TL;DR: Computer simulation and a time correlation function technique are used to understand the mechanism and rate of hydrogen bond-breaking in the hydration layer surrounding an aqueous protein and the longevity of the surrounding hydrogen bond network is found to be significantly enhanced near a hydrophilic residue.
Abstract: The mechanism and the rate of hydrogen bond-breaking in the hydration layer surrounding an aqueous protein are important ingredients required to understand the various aspects of protein dynamics, its function, and stability. Here, we use computer simulation and a time correlation function technique to understand these aspects in the hydration layer of lysozyme. Water molecules in the layer are found to exhibit three distinct bond-breaking mechanisms. A large angle orientational jump of the donor water molecule is common among all of them. In the most common $(\approx80\%)$ bond-breaking event in the layer, the new acceptor water molecule comes from the first coordination shell (initially within $3.5 \AA$ of the donor), and the old acceptor water molecule remains within the first coordination shell, even after the bond-breaking. This is in contrast to that in bulk water, in which both of the acceptor molecules involve the second coordination shell. Additionally, the motion of the incoming and the outgoing acceptor molecules involved is not diffusive in the hydration layer, in contrast to their observed diffusive motion in the bulk. The difference in rotational dynamics between the bulk and the hydration layer water molecules is clearly manifested in the calculated time-dependent angular van Hove self-correlation function $(G(\theta, t))$ which has a pronounced two-peak structure in the layer, and this can be traced to the constrained translational motion in the layer. The longevity of the surrounding hydrogen bond network is found to be significantly enhanced near a hydrophilic residue.

54 citations

Journal ArticleDOI
07 Jun 2008-ACS Nano
TL;DR: Simulation of the orientational dynamics of water molecules confined in narrow carbon nanotubes and nanorings reveal that confinement leads to strong anisotropy in the orientation relaxation, which is mediated by the hopping of orientational defects.
Abstract: Molecular dynamics simulations of the orientational dynamics of water molecules confined in narrow carbon nanotubes and nanorings reveal that confinement leads to strong anisotropy in the orientational relaxation. The relaxation of the aligned dipole moments, occurring on a time scale of nanoseconds, is 3 orders of magnitude slower than that of bulk water. In contrast, the relaxation of the vector joining the two hydrogens is ten times faster compared to bulk, with a time scale of about 150 fs. The slow dipolar relaxation is mediated by the hopping of orientational defects, which are nucleated by the water molecules outside the tube, across the linear water chain.

50 citations

Journal ArticleDOI
TL;DR: It appears that the NN method offers several advantages over the more common histogram based approaches, including much faster convergence for a given amount of simulation data; an intuitive error bound that may be readily formulated without resorting to block averaging or bootstrapping; and the absence of empirically tuned parameters, which may bias the results in an uncontrolled fashion.
Abstract: Because of its fundamental importance to molecular biology, great interest has continued to persist in developing novel techniques to efficiently characterize the thermodynamic and structural features of liquid water. A particularly fruitful approach, first applied to liquid water by Lazaridis and Karplus, is to use molecular dynamics or Monte Carlo simulations to collect the required statistics to integrate the inhomogeneous solvation theory equations for the solvation enthalpy and entropy. We here suggest several technical improvements to this approach, which may facilitate faster convergence and greater accuracy. In particular, we devise a nonparametric kth nearest-neighbors (NN)-based approach to estimate the water−water correlation entropy, and we suggest an alternative factorization of the water−water correlation function that appears to more robustly describe the correlation entropy of the neat fluid. It appears that the NN method offers several advantages over the more common histogram-based appro...

50 citations

Journal ArticleDOI
TL;DR: It is estimated that for most of the water molecules that are in a strongly confined environment of the transition region between the head groups and tails, and that solvate carbonyl groups, the decay time of their reorientational correlation functions is of the order of a few tens of picoseconds.
Abstract: We studied the effects of confinement and the head group motion on the behavior of the reorientational correlation functions for water molecules at the water∕lipid bilayer interface. The correlation functions were calculated from the data obtained from two molecular dynamics simulations: one with a flexible bilayer and the other with a frozen bilayer. In our present analysis the water molecules were separated into spatial regions according to their distance from the bilayer surface and into population groups, according to the length of their stay in the corresponding regions. We estimate that for most of the water molecules that are in a strongly confined environment of the transition region between the head groups and tails, and that solvate carbonyl groups, the decay time of their reorientational correlation functions is of the order of a few tens of picoseconds. Water molecules that stay inside the transition region for long periods of time can display longer time decay (of the order of hundreds of picoseconds). This latter long time decay is determined by the dynamics of the phospholipids, it is substantially reduced when the bilayer is frozen. The decay of the correlation functions for the interfacial water molecules that are solvating the head groups is also slowed down when compared to bulk, but just by factors of 3–4.

44 citations

Journal ArticleDOI
TL;DR: Lateral proton conduction was found to occur with monolayers of all ether phospholipids examined at reduced surface pressure on subphases of low (1 mM) and high (4 M) ionic strength.
Abstract: Studies have been carried out on the lateral proton conductance properties of monolayers of the major and minor phospholipids of extremely halophilic archaebacteria, 2,3-diphytanyl-sn-glycero-1-phospho-3'-sn-glycerol 1'-phosphate (PGP) and 2,3-diphytanyl-sn-glycero-1-phospho-3'-sn-glycerol (PG), respectively, as well as on their respective deoxy analogues: 2,3-diphytanyl-sn-glycero-1-phospho-1'-propanediol 3'-phosphate (dPGP), 2,3-diphytanyl-sn-glycero-1-phospho-1'-1',3'-propanediol (dPG), and 2,3-diphytanyl-sn-glycero-1-phospho-1'-propanol (ddPG). Lateral proton conduction was found to occur with monolayers of all ether phospholipids examined at reduced surface pressure (pi greater than 25 mN/m) on subphases of low (1 mM) and high (4 M) ionic strength. Proton conduction was also detected in highly condensed monolayers (greater than 35 mN/m) of the naturally occurring phospholipids (PGP, PG) but was abruptly terminated in tightly packed monolayers (greater than 35 mN/m) of the corresponding deoxy compounds (dPGP, dPG, ddPG) on subphases with low ionic strength. conduction did occur, however, along monolayers of the deoxy compounds at high surface pressure when spread on a subphase of high ionic strength (4 M). The abrupt termination of conduction with monolayers of the deoxy compounds at low ionic strength cannot be attributed to a lipid phase transition or to changes in the lateral fluidity of the monolayers, nor was the pK of the fluorescent interfacial proton indicator affected at high surface pressures.(ABSTRACT TRUNCATED AT 250 WORDS)

43 citations