This work presents a numerical and theoretical investigation of the collective dynamics of colloids in an unbounded solution but trapped in a harmonic potential. Under strict two-dimensional confinement (infinitely stiff trap) the collective colloidal diffusion is enhanced and diverges at zero wave number (like k^{-1}), due to the hydrodynamic propagation of the confining force across the layer. The analytic solution for the collective diffusion of colloids under a Gaussian trap of width δ still shows enhanced diffusion for large wavelengths kδ<1, while a gradual transition to normal diffusion for kδ>1. At intermediate and short wavelengths, we illustrate to what extent the hydrodynamic enhancement of diffusion is masked by the conservative forces between colloids. At very large wavelengths, the collective diffusion becomes faster than the solvent momentum transport and a transition from Stokesian dynamics to inertial dynamics takes place. Using our inertial coupling method code (resolving fluid inertia), we study this transition by performing simulations at small Schmidt number. Simulations confirm theoretical predictions for the k→0 limit [Phys. Rev. E 90, 062314 (2014)PLEEE81539-375510.1103/PhysRevE.90.062314] showing negative density-density time correlations. However, at finite k simulations show deviations from the theory.
The hydrophobic surfactant proteins, SP-B and SP-C, promote rapid adsorption by the surfactant lipids to the surface of the liquid that lines the alveolar air sacks of the lungs. To gain insights into the mechanisms of their function, we used X-ray diffuse scattering (XDS) and molecular dynamics (MD) simulations to determine the location of SP-B and SP-C within phospholipid bilayers. Initial samples contained the surfactant lipids from extracted calf surfactant with increasing doses of the proteins. XDS located protein density near the phospholipid headgroup and in the hydrocarbon core, presumed to be SP-B and SP-C, respectively. Measurements on dioleoylphosphatidylcholine (DOPC) with the proteins produced similar results. MD simulations of the proteins with DOPC provided molecular detail and allowed direct comparison of the experimental and simulated results. Simulations used conformations of SP-B based on other members of the saposin-like family, which form either open or closed V-shaped structures. For SP-C, the amino acid sequence suggests a partial α-helix. Simulations fit best with measurements of XDS for closed SP-B, which occurred at the membrane surface, and SP-C oriented along the hydrophobic interior. Our results provide the most definitive evidence yet concerning the location and orientation of the hydrophobic surfactant proteins.
We report on atomistic simulations of DPPC lipid monolayers using the CHARMM36 lipid force field and four-point OPC water model. The entire two-phase region where domains of the 'liquidcondensed' (LC) phase coexist with domains of the 'liquid-expanded' (LE) phase has been explored.The simulations are long enough that the complete phase-transition stage, with two domains coexisting in the monolayer, is reached in all cases. Also, system sizes used are larger than in previous works. As expected, domains of the minority phase are elongated, emphasizing the importance of anisotropic van der Waals and/or electrostatic dipolar interactions in the monolayer plane. The molecular structure is quantified in terms of distribution functions for the hydrocarbon chains and the PN dipoles. In contrast to previous work, where average distributions are calculated, distributions are here extracted for each of the coexisting phases by first identifying lipid molecules that belong to either LC or LE regions. The three-dimensional distributions show that the average tilt angle of the chains with respect to the normal outward direction is (39.0 ± 0.1) • in the LC phase.In the case of the PN dipoles the distributions indicate a tilt angle of (110.8 ± 0.5) • in the LC phase and (112.5 ± 0.5) • in the LE phase. These results are quantitatively different from previous works, which indicated a smaller normal component of the PN dipole. Also, the distributions of the monolayer-projected chains and PN dipoles have been calculated. Chain distributions peak along a particular direction in the LC domains, while they are uniform in the LE phase. Long-range ordering associated with the projected PN dipoles is absent in both phases. These results strongly suggest that LC domains do not exhibit dipolar ordering in the plane of the monolayer, the effect of these components being averaged out at short distances. Therefore, the only relevant component of the molecular dipoles, as regards both intra-and long-range interdomain interactions, is normal to the monolayer. Also, the local orientation of chain projections is almost constant in LC domains and points in the direction along which domains are elongated, suggesting that the line tension driving the phase transition is anisotropic with respect to the interfacial domain boundary. Both van der Waals interactions and interactions from normal dipolar components seem to contribute to an anisotropic line tension, with dipolar in-plane components playing a negligible role. 29 Mohammad-Aghaie, D.; Bresme, F. Force-field Dependence on the Liquid-Expanded to Liquid-Condensed Transition in DPPC Monolayers. Mol. Simul. 2016, 42, 391-397. 30 Baoukina, S.; Monticelli, L.; Marrink, S. J.; Tieleman, D. P. Pressure-Area Isotherm of a Lipid Monolayer from Molecular Dynamics Simulations. Langmuir 2007, 23, 12617-12623. 31 Choe, S.; Chang, R.; Jeon, J.; Violi, A. Molecular Dynamics Study of a Pulmonary Surfactant Film interacting with a Carbonaceous Nanoparticle. Biophys. J. 2008, 95, 4102-4114. 32 Hauser, H.; Pascher...
The collective motion of membrane lipids over hundreds of nanometers and nanoseconds plays an essential role in the formation of submicron complexes of lipids and proteins in the cell membrane. These dynamics are difficult to access experimentally and are currently poorly understood. One of the conclusions of the celebrated Saffman-Debrück (SD) theory is that lipid disturbances smaller than the Saffman length (microns) are not affected by the hydrodynamics of the embedding solvent. Using molecular dynamics and coarse-grained models with implicit hydrodynamics we show that this is not true. Hydrodynamic interactions between the membrane and the solvent strongly enhance the short-time collective diffusion of lipids at all scales. The momentum transferred between the membrane and the solvent in the normal direction (not considered by the SD theory) propagates tangentially over the membrane inducing long-ranged repulsive forces amongst lipids. As a consequence, the lipid collective diffusion coefficient increases proportionally to the disturbance wavelength. We find quantitative agreement with the predicted anomalous diffusion in quasi-two-dimensional dynamics, observed in colloids confined to a plane but embedded in a three-dimensional solvent.
We study diffusion of colloids on a fluid-fluid interface using particle simulations and fluctuating hydrodynamics. Diffusion on a two-dimensional interface with three-dimensional hydrodynamics is known to be anomalous, with the collective diffusion coefficient diverging like the inverse of the wavenumber. This unusual collective effect arises because of the compressibility of the fluid flow in the plane of the interface, and leads to a nonlinear nonlocal convolution term in the diffusion equation for the ensemble-averaged concentration. We extend the previous hydrodynamic theory to account for a species/color labeling of the particles, as necessary to model experiments based on fluorescent techniques. We study the magnitude and dynamics of density and color density fluctuations using a novel Brownian dynamics algorithm, as well as fluctuating hydrodynamics theory and simulation. We find that hydrodynamic coupling between a single tagged particle and collective density fluctuations leads to a reduction of the long-time self-diffusion coefficient, even for an ideal gas of non-interacting particles. This unexpected finding demonstrates that density functional theories that do not account for thermal fluctuations are incomplete even for ideal systems. Using linearized fluctuating hydrodynamics theory, we show that for diffusion on a fluid-fluid interface, nonequilibrium fluctuations of the total density are small compared to the equilibrium fluctuations, but fluctuations of color density are giant and exhibit a spectrum that decays as the inverse cubed power of the wavenumber. We confirm these predictions through Brownian dynamics simulations of diffusive mixing with two indistinguishable species. We also examine nonequilibrium fluctuations in systems with two-dimensional hydrodynamics, such as thin smectic films in vacuum. We find that nonequilibrium fluctuations are colossal and comparable in magnitude to the mean, and can be accurately modeled using numerical solvers for the nonlinear equations of fluctuating hydrodynamics. * Electronic address: donev@courant.nyu.edu
We present a new computational technique to quantify the solubility of planar molecules in a solvent. Solubility is calculated as the critical concentration at which solute molecules cease to stack as columns, but rather aggregate in all directions. An explicit expression for the solubility is obtained, which involves the potential of mean force between two solute molecules as a function of their center-of-mass distance in the limit of infinite dilution. This function can be easily obtained from molecular dynamics simulations involving a pair of solute molecules in a solvent using the umbrella-sampling method. As a validation of our approach, we use a generic coarse-grained molecular model to represent the molecular interactions of polycyclic-aromatic-hydrocarbon. Within that coarse-grained model, the solubility of pyrene and acenaphthene in heptane is estimated through large molecular dynamics simulations and compared to the experimental results. The umbrella-sampling method, applied to single pairs of these molecules in the solvent, provides the values of the critical cluster size in the theoretical model of molecular stacking. Umbrella-sampling simulations for the first members of the polycyclic-aromatic-hydrocarbon series then are used to predict their solubilities through our theoretical method. Within the typical uncertainty associated with theoretical solubility estimates, the agreement of our results with the experimental values is quite remarkable for most of the members of the series in a wide range of molecular masses, which confirms the general validity of the method in the case of planar molecules. Among the molecules explored, agreement with experiment fails for anthracene, for which the experimental solubility is clearly out of the general trend along the polycyclic-aromatic-hydrocarbon series, indicating that the coarse-grained representation used here is not able to capture its peculiarity. The new methodology can be applied to planar molecules to obtain relatively accurate values of solubility at a very low computational cost.
Targeting protein-protein interactions (PPI) within the cell is an appealing prospect for cancer research and therapy. Methods to inhibit PPIs have been limited though due to the ineffectiveness of small molecules for this task, and to the inability to get macromolecule cargoes -such as antibodies -into the cytosol. Because of this, targets such as RAS and MYC are sometimes considered ''undruggable'' even though they have been studied for decades. To address this, we are developing pH-sensitive pore-forming peptides to enable endosomal escape of endocytosed protein cargo. One such peptide, pHD108, is a pHtriggered gain-of-function variant of melittin, a membrane permeabilizing peptide found in bee venom. In synthetic unilamellar vesicles, pHD108 has no poration activity at pH 7; at pH % 5.5 it releases macromolecules from lipid vesicles at very low peptide concentration. In this work, we have shown that, with a low pH pulse, pHD108 allows 150 kDa proteins (IgGs) through the plasma membrane. High concentrations of peptide are needed, however, so steps to increase potency has yielded an optimal acylation strategy of a 6-carbon acyl chain at the N-terminus. To assess endosomolytic activity, a novel assay was developed using domain III of Pseudomonas Exotoxin A. When given to cells alone, the exotoxin does not induce apoptosis but when delivered to the cytosol it will induce apoptosis. pHD108 shows activity here. To further increase potency we plan to screen a focused library of peptides with rationally modified sequence variations while employing the optimal acylation strategy. Hits found there will be characterized further. Our findings here will ultimately allow researchers to target intracellular PPIs in living cells using exogenous antibodies.
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