Hydrogen Bonding Sequence Directed Coil-Globule Transition in Water Soluble Thermoresponsive Polymers.

The origin of the coil-globule transition for water-soluble thermoresponsive polymers frequently used in nanomaterials remains elusive. Using polypropylene oxide as an example we demonstrate by means of atomistic molecular dynamics simulations that temperature-induced increase in the sequence length of monomers that are not hydrogen bonded to water drives the coil-globule transition. Longer chains statistically exhibit longer sequences which serve as nucleation sites for hydrophobic cluster formation, facilitating chain collapse at lower temperature in agreement with experimental data.

Aggregation-Enhanced Photoluminescence and Photoacoustics of Atomically Precise Gold Nanoclusters in Lipid Nanodiscs (NANO).

The authors designed a structurally stable nano-in-nano (NANO) system highly capable of bioimaging via an aggregation-enhanced NIR excited emission and photoacoustic response achieved based on atomically precise gold nanoclusters protected by linear thiolated ligands [Au(SC H), = 4-16] encapsulated in discoidal phospholipid bicelles through a one-pot synthesis. The detailed morphological characterization of NANO is conducted using cryogenic transmission electron microscopy, small/wide angle X-ray scattering with the support of molecular dynamics simulations, providing information on the location of Au nanoclusters in NANO. The photoluminescence observed for NANO is 20-60 times more intense than that of the free Au nanoclusters, with both excitation and emission wavelengths in the near-infrared range, and the photoacoustic signal is more than tripled.

Hydrogen-Bonded Complexes of Star Polymers.

The effect of molecular architecture, star versus linear, poly(ethylene oxide) (PEO) on the formation of hydrogen-bonded complexes with linear poly(methacrylic acid) (PMAA) is investigated experimentally and rationalized theoretically. Isothermal titration calorimetry reveals that at pH 2.5 interpolymer complexes (IPCs) of PMMA with a 6-arm star PEO (sPEO) contains ≈50% more polyacid than IPCs formed with linear PEO (lPEO).

Hydration of diblock copolymer micelles: Effects of hydrophobicity and co-solvent.

Diblock polymer micelles dispersed in an aqueous environment are being actively investigated for various applications, but there is only a qualitative understanding of the effect of the chemical structure on the micelle hydration and water dynamics as these properties are difficult to assess experimentally. Using all-atom molecular dynamics simulations, we investigate aqueous solutions of three comparable in size diblock copolymer micelles with core-forming blocks of different hydrophobicity: polybutadiene (PB), polycaprolactone (PCL), and polytetrahydrofuran (pTHF) with the same hydrophilic block, polyethylene oxide (PEO). We found that core-block hydrophobicity and ability to form hydrogen bonds with water strongly affect the water dynamics near the core: water molecules spend considerably less time in contact with the PB block than with PCL and pTHF blocks.

Growth of wormlike micelles of surfactant induced by embedded polymer: role of polymer chain length.

Incorporation of polymer chains into wormlike surfactant micelles, which find a large range of applications, offers the opportunity to modify their structure and properties. In this paper, using spectroscopic, scattering and rheological techniques and computer simulations, we study the incorporation of poly(4-vinylpyridine) of two different molecular weights (MWs) into entangled networks of wormlike surfactant micelles of potassium oleate. Using NMR-spectroscopy we show that, independent of its MW, the polymer incorporates into the core-corona interface of the surfactant micelles.

Lipid Nanodisc-Templated Self-Assembly of Gold Nanoparticles into Strings and Rings.

Gold nanoparticles (AuNPs) exhibit strong fluorescent and electromagnetic properties, which can be enhanced upon clustering and used in therapeutic, imaging, and sensing applications. A combination of gold nanoparticles with lipid nanodiscs can be attractive for AuNP self-assembly and useful in biomedical applications. Using molecular dynamics simulations we show that lipid nanodiscs can serve as templates for AuNP clustering into rings and string-like structures.

The dynamics of solvation dictates the conformation of polyethylene oxide in aqueous, isobutyric acid and binary solutions.

Polymers hydrogen-bonding with solvent represent an important broad class of polymers, properties of which depend on solvation. Using atomistic molecular dynamics simulations with the OPLS/AA force field we investigate the effect of hydrogen bonding on PEO conformation and chain mobility by comparing its behavior in isobutyric acid and aqueous solutions. In agreement with experimental data, we found that in isobutyric acid PEO forms a rather rigid extended helical structure, while in water it assumes a highly flexible coil conformation.

Spontaneous Insertion, Helix Formation, and Hydration of Polyethylene Oxide in Carbon Nanotubes.

Hydration strongly affects macromolecular conformation in solution and under nanoconfinement as encountered in nature and nanomaterials. Using atomistic molecular dynamics simulations we demonstrate that polyethylene oxide spontaneously enters single wall carbon nanotubes (CNTs) from aqueous solutions and forms rodlike, helix, and wrapped chain conformations depending on the CNT diameter. We show that water organization and the stability of the polyethylene oxide hydration shell under confinement is responsible for the helix formation, which can have significant implications for nanomaterial design.

How a viscoelastic solution of wormlike micelles transforms into a microemulsion upon absorption of hydrocarbon: new insight.

In this article, we investigate the effect of hydrocarbon addition on the rheological properties and structure of wormlike micellar solutions of potassium oleate. We show that a viscoelastic solution of entangled micellar chains is extremely responsive to hydrocarbons-the addition of only 0.5 wt % n-dodecane results in a drastic drop in viscosity by up to 5 orders of magnitude, which is due to the complete disruption of micelles and the formation of microemulsion droplets.

Selectivity of ligand-receptor interactions between nanoparticle and cell surfaces.

Selectivity of interactions between nanoparticles functionalized by tethered ligands and cell surfaces with different densities of receptors plays an essential role in biorecognition and its implementation in nanobiomedicine. We show that the onset of nanoparticle adsorption has a universal character for a range of nanoparticles: the onset receptor density decreases exponentially with the energy of ligand-receptor binding and inversely with the ligand density. We demonstrate that a bimodal tether distribution, which permits shielding ligands by longer nonfunctional tethers, leads to extra loss of entropy at the adsorption onset, enhancing the selectivity.

Monte carlo simulations of metallo-supramolecular micelles.

Using Monte Carlo simulations we show that the equilibrium properties of metallo-supramolecular micelles are determined by the competition of 2:1 and 1:1 metal-ligand complexation in the bulk and on the surface as well as steric interactions between the neighboring corona blocks attached to the surface. We predict that by increasing the association energy for the second metal-ligand bond, or decreasing the corona block length one can achieve a larger core surface coverage for metallo-supramolecular micelles. Compared to covalently bonded block copolymer micelles, we show that metallo-supramolecular micelles have smaller monomer and end group density, especially in the vicinity of the core, which may lead to experimentally observed aggregation.

Molecular dynamics simulations of supramolecular polymer rheology.

Using equilibrium and nonequilibrium molecular dynamics simulations, we studied the equilibrium and rheological properties of dilute and semidilute solutions of head-to-tail associating polymers. In our simulation model, a spontaneous complementary reversible association between the donor and the acceptor groups at the ends of oligomers was achieved by introducing a combination of truncated pseudo-Coulombic attractive potential and Lennard Jones repulsive potential between donor, acceptor, and neighboring groups. We have calculated the equilibrium properties of supramolecular polymers, such as the ring/chain equilibrium, average molecular weight, and molecular weight distribution of self-assembled chains and rings, which all agree well with previous analytical and computer modeling results.

Nanoparticle design optimization for enhanced targeting: Monte Carlo simulations.

Using computer simulations, we systematically studied the influence of different design parameters of a spherical nanoparticle tethered with monovalent ligands on its efficiency of targeting planar cell surfaces containing mobile receptors. We investigate how the nanoparticle affinity can be affected by changing the binding energy, the percent of functionalization by ligands, tether length, grafting density, and nanoparticle core size. In general, using a longer tether length or increasing the number of tethered chains without increasing the number of ligands increases the conformational penalty for tether stretching/compression near the cell surface and leads to a decrease in targeting efficiency.

Kinetics of nanoparticle targeting by dissipative particle dynamics simulations.

Dissipative particle dynamics simulations are applied to study nanoparticle targeting to a cell surface containing a high concentration of receptors. We found that the normalized number of bound ligands follows an exponential growth function 1 - exp(-t/tau), with the lifetime tau increasing as a function of the binding strength. With increasing binding energy, the shape of the adsorbed nanoparticle becomes ellipsoidal due to a large number of stably bound ligands, most of which are positioned on the nanoparticle periphery.

Cis-trans switchable metallosupramolecular polymers: computer modeling.

Using computer simulations we study linear oligomers end functionalized with ligands that can form trans- or cis-2:1 complexes with metal ions in a salt-screened good solvent. We show that trans-cis isomerization of ligand-metal complexes can significantly increase the average molecular weight as well as trigger formation of reversible metallosupramolecular network based on 3:1 ligand-metal complexes acting as cross-linkers. We predict the conditions under which the most dramatic changes in the properties of metallosupramolecular polymers, such as network formation or increase in elastic plateau modulus of the network, occur upon isomerization.

Optimization of functionalized polymer layers for specific targeting of mobile receptors on cell surfaces.

The reversible binding between a planar polymer layer functionalized by ligands and a planar cell surface containing different densities of mobile receptors has been studied by Monte Carlo simulations. Using the acceptance-ratio method, the distance-dependent profiles for the average number of ligands bound to receptors, the total free energy for the polymer layer-cell surface interaction and the interaction force were obtained. Four main design parameters for the polymer layer were considered: the degree of functionalization, chain degree of polymerization, polymer grafting density and the binding energy for the ligand-receptor interaction.

Doxorubicin and beta-lapachone release and interaction with micellar core materials: experiment and modeling.

Polymer micelles with two different core-forming blocks, poly(d,l -lactide) (PLA) and poly(epsilon-caprolactone) (PCL), but the same coronal material, poly(ethylene glycol) (PEG), were investigated in this study as nanoscopic drug carriers. The release of two different drugs, doxorubicin (DOX) and beta-lapachone (beta-lap), from PEG(5k)-b-PCL(5k) and PEG(5k)-b-PLA(5k) micelles was studied at pH 5.0 and 7.

Architectural and structural optimization of the protective polymer layer for enhanced targeting.

Using Monte Carlo simulations we study the influence of ligand architecture (valence, branching length) and structure (polydispersity) of a flat protective polymer layer on the accessibility of its functional groups and efficiency of receptor targeting. Two types of receptor surfaces were considered: the surface homogeneously covered with receptors and the surface containing a finite number of receptor sites. We found that multivalent ligands provide a larger density of targeting groups on the periphery of the layer compared to monovalent ligands for the same overall number of targeting groups per polymer layer.

Supramolecular polymer formation by metal-ligand complexation: Monte Carlo simulations and analytical modeling.

Equilibrium metal-ligand complexation leading to formation of linear or ringlike supramolecular polymers is studied by means of Monte Carlo (MC) simulations and theoretical analysis. We found that in most of the cases high-molecular-weight polymers are formed over a rather narrow composition range (near the 2:1 ligand-metal ratio). Besides the imbalance in the number of metals and ligands, the molecular weight decrease in the metal-rich area is caused by an increase in 1:1 ligand-metal complex formation.