Article

A Coarse-Grained Model for Polyethylene Oxide and Polyethylene Glycol: Conformation and Hydrodynamics

Laboratory of Computational Biology, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
The Journal of Physical Chemistry B (Impact Factor: 3.38). 09/2009; 113(40):13186-94. DOI: 10.1021/jp9058966
Source: PubMed

ABSTRACT A coarse-grained (CG) model for polyethylene oxide (PEO) and polyethylene glycol (PEG) developed within the framework of the MARTINI CG force field (FF) using the distributions of bonds, angles, and dihedrals from the CHARMM all-atom FF is presented. Densities of neat low molecular weight PEO agree with experiment, and the radius of gyration R(g) = 19.1 A +/- 0.7 for 76-mers of PEO (M(w) approximately 3400), in excellent agreement with neutron scattering results for an equal sized PEG. Simulations of 9, 18, 27, 36, 44, 67, 76, 90, 112, 135, and 158-mers of the CG PEO (442 < M(w) < 6998) at low concentration in water show the experimentally observed transition from ideal chain to real chain behavior at 1600 < M(w) < 2000, in excellent agreement with the dependence of experimentally observed hydrodynamic radii of PEG. Hydrodynamic radii of PEO calculated from diffusion coefficients of the higher M(w) PEO also agree well with experiment. R(g) calculated from both all-atom and CG simulations of PEO76 at 21 and 148 mg/cm(3) are found to be nearly equal. This lack of concentration dependence implies that apparent R(g) from scattering experiments at high concentration should not be taken to be the chain dimension. Simulations of PEO grafted to a nonadsorbing surface yield a mushroom to brush transition that is well described by the Alexander-de Gennes formalism.

0 Bookmarks
 · 
168 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: Phospholipids with tethered poly(ethylene glycol) chains (PL-PEGs) offer efficient, noncovalent dispersion of carbon nanotubes (CNTs). Important questions concern the relation between micellar and CNT-assembled PL-PEG structures, and the influence of PEG length on assembly and dispersion. We explore these questions here via coarse-grained molecular dynamics simulation. Employing two representative CNT diameters and a range of PEG molecular weights, we find (i) PL-PEG aggregation number to vary inversely with PEG chain length, consistent with recent experiments, (ii) an assembled morphology to vary from micellar-like to monolayer-like, depending on PEG chain length and CNT diameter, (iii) micellar coatings to result in greater CNT dispersion ability, with a higher barrier for interparticle aggregation (84 kJ/mol) compared to monolayer coatings (60 kJ/mol), and (iv) good agreement between simulation and scaling theories of a brush-type PEG.
    Journal of Chemical & Engineering Data 05/2014; 59(10):140508125237007. DOI:10.1021/je500157b · 2.05 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Cholesteryl esters (CEs) are a form of cholesterol (CHOL) storage in the living cells, as opposed to free CHOL. CEs are major constituents of low density lipoprotein particles. Therefore, CEs are implicated in provoking atherosclerosis. Arranged into cytoplasmic lipid droplets (LDs), CEs are stored intracellularly. They can also be transported extracellularly by means of lipoproteins. In this work, large-scale molecular dynamics (MD) simulations are used to characterize molecular structure of LDs containing various fractions (10-50 mol%) of cholesteryl oleate (CO) with respect to triolein (TO) fraction. The simulated LDs were covered by a phospholipid monolayer formed by a mixture of POPC (75 mol%) and POPE (25 mol%) molecules. We report that most CO molecules are located within the hydrophobic core of LDs, whereas a small fraction (0.3-1.9 mol%) penetrates the monolayer. The solubility of CO in the phospholipid monolayer is relatively small. Due to a good miscibility with TO molecules, CO forms a liquid phase inside LD at 333 K. There is long-range order in the liquid TO-CO droplet core up to 8 nm from the phospholipid interface, resulting from the structuring of hydrophilic groups. This structuring slowly decays in the direction towards the LD center-of-mass. No sorting of TO and CO is detected, irrespective of the molar fractions simulated. The distribution of CO within the LDs is significant in determining the rate of their hydrolysis by surface-bound enzyme lipases, and thus has a subsequent impact on the levels of CO in plasma and LDLs.
    The Journal of Physical Chemistry B 09/2014; 118(38). DOI:10.1021/jp506693d · 3.38 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Microemulsions are exciting systems that are promising as tuneable self-assembling templating reaction vessels at the nanoscale. Determination of the nano-structure of microemulsions is, however, not trivial, and there are fundamental questions regarding their design. We were able to reproduce experimental data for an important microemulsion system, Sodium-AOT/n-heptane/water, using coarse-grained simulations involving relatively limited computational costs. The simulation allows visualization and deeper investigation of controversial phenomena such as bicontinuity and ion mobility.Simulations were performed using the Martini coarse-grained force field. AOT bonded parameters were fine-tuned by matching the geometry obtained from atomistic simulations. We investigated several compositions with a constant ratio of surfactant to oil while the water content was varied from 10 to 60% in weight. From mean square displacement calculation of all species, it was possible to quantify caging effects and ion mobility. Average diffusion coefficients were calculated for all charged species and trends in the diffusion coefficients were used to rationalize experimental conductivity data. Especially, the diffusion coefficient of charged species qualitatively matched the variation in conductivity as a function of water content. The scattering function was calculated for the hydrophilic species and up to 40% water content quantitatively matched the experimental data obtained from Small Angle X-ray Scattering measurements. For higher water contents, discrepancies were observed and attributed to a nearby phase separation. In particular, bicontinuity of water and oil was computationally visualized by plotting the coordinates of hydrophilic beads. Equilibrated coarse-grained simulations were reversed to atomistic models in order both to compare ion mobility and to catch finer simulation details. Especially, it was possible to capture the intimate ion pair interaction between the sodium ion and the surfactant head group.
    Soft Matter 09/2014; 10(43). DOI:10.1039/C4SM01763C · 4.15 Impact Factor

Full-text (2 Sources)

Download
32 Downloads
Available from
Jun 1, 2014