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: Using the theory of liquid crystallography plus oriented lipid samples, this group is the first group to obtain both material parameters associated with the fluctuations in fluid phase lipids.
289 citations
TL;DR: Water molecule rotational dynamics within a chloride anion's first hydration shell are investigated through simulations and find a labile hydration sphere, consistent with previous assessments of chloride as a weak structure breaker.
Abstract: Water molecule rotational dynamics within a chloride anion's first hydration shell are investigated through simulations. In contrast to recent suggestions that the ion's hydration shell is rigid during a water's reorientation, we find a labile hydration sphere, consistent with previous assessments of chloride as a weak structure breaker. The nondiffusive reorientation mechanism found involves a hydrogen-bond partner switch with a large amplitude angular jump and the water's departure from the anion's shell. An analytic extended jump model accounts for the simulation results, as well as available NMR and ultrafast spectroscopic data, and resolves the discrepancy between them.
283 citations
TL;DR: The two-phase thermodynamic (2PT) model for the calculation of energy and entropy of molecular fluids from the trajectory of molecular dynamics simulations is presented, making it an efficient means for extracting thermodynamic properties from MD simulations.
Abstract: Presented here is the two-phase thermodynamic (2PT) model for the calculation of energy and entropy of molecular fluids from the trajectory of molecular dynamics (MD) simulations. In this method, the density of state (DoS) functions (including the normal modes of translation, rotation, and intramolecular vibration motions) are determined from the Fourier transform of the corresponding velocity autocorrelation functions. A fluidicity parameter (f), extracted from the thermodynamic state of the system derived from the same MD, is used to partition the translation and rotation modes into a diffusive, gas-like component (with 3Nf degrees of freedom) and a nondiffusive, solid-like component. The thermodynamic properties, including the absolute value of entropy, are then obtained by applying quantum statistics to the solid component and applying hard sphere/rigid rotor thermodynamics to the gas component. The 2PT method produces exact thermodynamic properties of the system in two limiting states: the nondiffusive solid state (where the fluidicity is zero) and the ideal gas state (where the fluidicity becomes unity). We examine the 2PT entropy for various water models (F3C, SPC, SPC/E, TIP3P, and TIP4P-Ew) at ambient conditions and find good agreement with literature results obtained based on other simulation techniques. We also validate the entropy of water in the liquid and vapor phases along the vapor-liquid equilibrium curve from the triple point to the critical point. We show that this method produces converged liquid phase entropy in tens of picoseconds, making it an efficient means for extracting thermodynamic properties from MD simulations.
274 citations
TL;DR: The results demonstrate that confinement by an interface to form a nanoscopic water pool is a primary factor governing the dynamics ofnanoscopic water rather than the presence of charged groups at the interface.
Abstract: The dynamics of water confined in two different types of reverse micelles are studied using ultrafast infrared pump-probe spectroscopy of the hydroxyl OD stretch of HOD in H2O. Reverse micelles of the surfactant Aerosol-OT (ionic head group) in isooctane and the surfactant Igepal CO 520 (nonionic head group) in 50/50 wt % cyclohexane/hexane are prepared to have the same diameter water nanopools. Measurements of the IR spectra and vibrational lifetimes show that the identity of the surfactant head groups affects the local environment experienced by the water molecules inside the reverse micelles. The orientational dynamics (time-dependent anisotropy), which is a measure of the hydrogen bond network rearrangement, are very similar for the confined water in the two types of reverse micelles. The results demonstrate that confinement by an interface to form a nanoscopic water pool is a primary factor governing the dynamics of nanoscopic water rather than the presence of charged groups at the interface.
244 citations
TL;DR: The time-resolved orientational anisotropies of the OD hydroxyl stretch of dilute HOD in H(2)O confined on a nanometer length scale in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles are studied using ultrafast infrared polarization and spectrally resolved pump-probe spectroscopy, and the results are compared to the same experiments on bulk water.
Abstract: The time-resolved orientational anisotropies of the OD hydroxyl stretch of dilute HOD in H2O confined on a nanometer length scale in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles are studied using ultrafast infrared polarization and spectrally resolved pump-probe spectroscopy, and the results are compared to the same experiments on bulk water. The orientational anisotropy data for three water nanopool sizes (4.0, 2.4, and 1.7nm) can be fitted well with biexponential decays. The biexponential decays are analyzed using a wobbling-in-a-cone model that involves fast orientational diffusion within a cone followed by slower, full orientational relaxation. The data provide the cone angles, the diffusion constants for motion within the cones, and the final diffusion constants as a function of the nanopool size. The two processes can be interpreted as a local angular fluctuation of the OD and a global hydrogen bond network rearrangement process. The trend in the relative amplitudes of the long and...
222 citations