A molecular dynamics simulation study of buckyballs in water: Atomistic versus coarse-grained models of C60

Effects of solvation shell relaxation on chain association mechanisms in poly(3-hexylthiophene) solutions

Using poly(3-hexylthiophene) (P3HT) as a model conjugated polymer and atomistic molecular dynamics simulations with carefully verified force fields, we performed in-depth investigations of solvation shell properties of P3HT chains (15 repeating units per chain) in two representative groups of non-polar (or aprotic) organic solvents (better solvents: ortho-dichlorobenzene, bromobenzene, and chlorobenzene; poorer solvents: chloroform, para-xylene, and toluene). We demonstrated that solvation shell relaxation properties in P3HT solutions dictate the formation of regular π-π associations and, hence, crystallinity through the initial chain association and subsequent chain sliding. In contrast, the mean features of polymer-solvent interactions, including solvation free energy and radial distribution function, present little or no difference for all solvent media investigated. Better-solvent media were revealed to bear relatively large values of the first solvation shell relaxation time (τ1 ≫ 100 ps) as well as larger ratios of relaxation times for the first two solvation shells (τ1/τ2 > 2), and vice versa for poorer-solvent media (τ1 ≪ 100 ps and τ1/τ2 < 2). The linear hexyl side-chain unit was noted to substantially enlarge both quantities while notably reducing the solvation free energy as well. As discussed herein, these findings shed new light on the mechanistic features by which solvent quality impacts the degree of π-π association crucial for modern applications with crystalline conjugated polymers.

Fullerene translocation through peroxidized lipid membranes

Recent cytotoxicity research suggests that fullerenes can enter the cell and cross the blood-brain barrier. However, the underlying toxicity mechanism behind the penetration of fullerenes through biological membranes is still not well understood. Here we perform coarse-grained molecular dynamics simulations to investigate the interactions of fullerenes and their polar derivatives (Janus) with model regular and peroxidized bilayers. We show that the translocation of fullerenes and their residence time in bulk water vary depending on the bilayer's peroxidation degree and fullerene polarity. The distribution of fullerenes inside the bilayer is mainly determined by the peroxidation degree and the saturation level of lipid acyl chains. The transport of pristine fullerenes through bilayers occurs at nano timescale while the complete diffusion may not be achieved for Janus fullerenes in micro timescale. As for the toxic response of fullerenes in terms of membrane damage, no mechanical disruption of model bilayers is observed throughout the studied simulation times.

Short solvent model for ion correlations and hydrophobic association

Coulomb interactions play a major role in determining the thermodynamics, structure, and dynamics of condensed-phase systems, but often present significant challenges. Computer simulations usually use periodic boundary conditions to minimize corrections from finite cell boundaries but the long range of the Coulomb interactions generates significant contributions from distant periodic images of the simulation cell, usually calculated by Ewald sum techniques. This can add significant overhead to computer simulations and hampers the development of intuitive local pictures and simple analytic theory. In this paper, we present a general framework based on local molecular field theory to accurately determine the contributions from long-ranged Coulomb interactions to the potential of mean force between ionic or apolar hydrophobic solutes in dilute aqueous solutions described by standard classical point charge water models. The simplest approximation leads to a short solvent (SS) model, with truncated solvent-solvent and solute-solvent Coulomb interactions and long-ranged but screened Coulomb interactions only between charged solutes. The SS model accurately describes the interplay between strong short-ranged solute core interactions, local hydrogen-bond configurations, and long-ranged dielectric screening of distant charges, competing effects that are difficult to capture in standard implicit solvent models.

Interfacial structural crossover and hydration thermodynamics of charged C60 in water

Classical molecular dynamics simulations of the hydration thermodynamics, structure, and dynamics of water in hydration shells of charged buckminsterfullerenes are presented in this study. Charging of fullerenes leads to a structural transition in the hydration shell, accompanied by creation of a significant population of dangling O-H bonds pointing toward the solute. In contrast to the well accepted structure-function paradigm, this interfacial structural transition causes nearly no effect on either the dynamics of hydration water or on the solvation thermodynamics. Linear response to the solute charge is maintained despite significant structural changes in the hydration shell, and solvation thermodynamic potentials are nearly insensitive to the altering structure. Only solvation heat capacities, which are higher thermodynamic derivatives of the solvation free energy, indicate some sensitivity to the local hydration structure. We have separated the solvation thermodynamic potentials into direct solute-solvent interactions and restructuring of the hydration shell and analyzed the relative contributions of electrostatic and nonpolar interactions to the solvation thermodynamics.

Generic Mechanism for Pattern Formation in the Solvation Shells of Buckminsterfullerene

Accurate description of solvation structure of a hydrophobic nanomaterial is of immense importance to understand protein folding, molecular recognition, drug binding, and many related phenomena. Moreover, spontaneous pattern formation through self-organization of solvent molecules around a nanoscopic solute is fascinating and useful in making template-directed nanostructures of desired morphologies. Recently, it has been shown using polarizable atomistic models that the hydration shell of a buckminsterfullerene can have atomically resolved ordered structure, in which C₆₀ atomic arrangement is imprinted. In analyzing any peculiar behavior of water, traditionally, emphasis has been placed on the long-ranged and orientation-dependent interactions in it. Here, we show through molecular dynamics simulation that the patterned solvation layer with the imprints of the hydrophobic surface atoms of the buckminsterfullerene can be obtained from a completely different mechanism arising from a spherically symmetric, short-ranged interaction having two characteristic lengthscales. The nature of the pattern can be modified by adjusting solvent density or pressure. Although solute-solvent dispersion interaction is the key to such pattern formation adjacent to the solute surface, the ordering at longer lengthscale is a consequence of mutual influence of short-range correlations among successive layers. The present study thus demonstrates that the formation of such patterned solvation shells around the buckminsterfullerene is not restricted to water, but encompasses a large class of anomalous fluids represented by two-lengthscale potential.

Thermodynamics of hydration of fullerols [C60(OH)n] and hydrogen bond dynamics in their hydration shells

Molecular dynamics simulations of fullerene and fullerols [C₆₀(OH)_n, where n = 2-30] in aqueous solutions have been performed for the purpose of obtaining a detailed understanding of the structural and dynamic properties of these nanoparticles in water. The structures, dynamics and hydration free energies of the solute molecules in water have been analysed. Radial distribution functions, spatial density distribution functions and hydrogen bond analyses are employed to characterize the solvation shells of water around the central solute molecules. We have found that water molecules form two solvation shells around the central solute molecule. Hydrogen bonding in the bulk solvent is unaffected by increasing n. The large decrease in solvation enthalpies of these solute molecules for n > 14 enhances solubilisation. The diffusion constants of solute molecules decrease with increasing n. The solvation free energy of C₆₀ in water is positive (52.8 kJ/mol), whereas its value for C₆₀(OH)₃₀ is highly negative (-427.1 kJ/mol). The effects of surface hydroxylation become more dominant once the fullerols become soluble.

The impacts of surface polarity on the solubility of nanoparticle

In order to study the dependence of water solubility and hydration behavior of nanoparticles on their surface polarity, we designed polar nanoparticles with varying surface polarity by assigning atomic partial charge to the surface of C60. The water solubility of the nanoparticle is enhanced by several orders of magnitude after the introduction of surface polarity. Nevertheless, when the atomic partial charge grows beyond a certain value (qM), the solubility continuously decreases to the level of nonpolar nanoparticle. It should be noted that such qM is comparable with atomic partial charge of a variety of functional groups. The hydration behaviors of nanoparticles were then studied to investigate the non-monotonic dependence of solubility on the surface polarity. The interaction between the polar nanoparticle and the hydration water is stronger than the nonpolar counterpart, which should facilitate the dissolution of the nanoparticles. On the other hand, the surface polarity also reduces the interaction of hydration water with the other water molecules and enhances the interaction between the nanoparticles which may hinder their dispersion. Besides, the introduction of surface polarity disturbs and even rearranges the hydration structure of nonpolar nanoparticle. Interestingly, the polar nanoparticle with less ordered hydration structure tends to have higher water solubility.

Interaction of C60 fullerenes with asymmetric and curved lipid membranes: a molecular dynamics study

Interaction of fullerenes with asymmetric and curved DOPC/DOPS bicelles is studied by means of coarse-grained molecular dynamics simulations. The effects caused by asymmetric lipid composition of the membrane leaflets and the curvature of the membrane are analyzed. It is shown that the aggregates of fullerenes prefer to penetrate into the membrane in the regions of the moderately positive mean curvature. Upon penetration into the hydrophobic core of the membrane fullerenes avoid the regions of the extreme positive or the negative curvature. Fullerenes increase the ordering of lipid tails, which are in direct contact with them, but do not influence other lipids significantly. Our data suggest that the effects of the membrane curvature should be taken into account in the studies concerning permeability of the membranes to fullerenes and fullerene-based drug delivery systems.

Functionalized fullerenes in water: a closer look

The excellent photophysical properties of C60 fullerenes have spurred much research on their application to aqueous systems for biological and environmental applications. Spontaneous aggregation of C60 in water and the consequent diminution of photoactivity present a significant challenge to aqueous applications. The mechanisms driving the reduction of photoactivity in fullerene aggregates and the effects of functionalization on these processes, however, are not well understood. Here, we take a closer look at the molecular phenomena of functionalized fullerene interactions in water utilizing simulation and experimental tools. Molecular dynamic simulations were performed to investigate time-evolved molecular interactions in systems containing fullerenes with water, oxygen, and/or neighboring fullerene molecules, complimented by physical and chemical characterizations of the fullerenes pre- and postaggregation. Aggregates with widely different photoactivities exhibit similar fullerene-water interactions as well as surface and aggregation characteristics. Photoactive fullerene aggregates had weaker fullerene-fullerene and fullerene-O2 interactions, suggesting the importance of molecular interactions in the sensitization route.

Interaction of C₆₀ with water: first-principles modeling and environmental implications

The nature of fullerene-water interactions has been the subject of much research and debate. Specifically, the presence of a stabilizing, negative surface potential on colloidal aggregates of C60 in water is unexpected, given the neutral nature of pure carbon, and is not well understood. Previous simulation efforts have focused on the C60-water interaction using molecular dynamics simulations that lacked the ability to account for charge transfer and distribution interactions. In this study, first-principles density functional theory was used to analyze the fundamental electronic interactions to elucidate the polarization and charge transfer between water and C60. Simulations show that charge is inductively transferred to the C60 from water molecules, with subsequent polarization of the C60 molecule. In a case with two neighboring C60 molecules, the charge polarization induces a charge onto the second C60. Simulation suggests that this charge transfer and polarization may contribute at least partly to the observed negative surface potential of fullerene aggregates and, combined with hydrogen bonding network formation around C60, provides a fundamental driving force for aggregate formation in water.

Effect of self-assembly of fullerene nano-particles on lipid membrane

Carbon nanoparticles can penetrate the cell membrane and cause cytotoxicity. The diffusion feature and translocation free energy of fullerene through lipid membranes is well reported. However, the knowledge on self-assembly of fullerenes and resulting effects on lipid membrane is poorly addressed. In this work, the self-assembly of fullerene nanoparticles and the resulting influence on the dioleoylphosphtidylcholine (DOPC) model membrane were studied by using all-atom molecular dynamics simulations with explicit solvents. Our simulation results confirm that gathered small fullerene cluster can invade lipid membrane. Simulations show two pathways: 1) assembly process is completely finished before penetration; 2) assembly process coincides with penetration. Simulation results also demonstrate that in the membrane interior, fullerene clusters tend to stay at the position which is 1.0 nm away from the membrane center. In addition, the diverse microscopic stacking mode (i.e., equilateral triangle, tetrahedral pentahedral, trigonal bipyramid and octahedron) of these small fullerene clusters are well characterized. Thus our simulations provide a detailed high-resolution characterization of the microscopic structures of the small fullerene clusters. Further, we found the gathered small fullerene clusters have significant adverse disturbances to the local structure of the membrane, but no great influence on the global integrity of the lipid membrane, which suggests the prerequisite of high-content fullerene for cytotoxicity.

Coarse-grained potential for interaction with a spherical colloidal particle and planar wall

An effective coarse-grained interaction potential between a point particle and a spherical colloidal particle with continuously distributed inverse power-law interaction sites is derived. The potential covers all ranges of spherical particle size, from a point particle up to an infinitely large particle forming a planar surface. In the small size limit, the point-to-point interaction is recovered, while in the limit of an infinitely large sphere the potential comes over to the known particle–wall potentials as, e.g., the 9–3 potential in the case of the Lennard–Jones interaction. Correctness and usefulness of the derived potential is exemplified by its application to SPC/E water at a graphite sphere and wall.

Computer simulation study of fullerene translocation through lipid membranes

Recent toxicology studies suggest that nanosized aggregates of fullerene molecules can enter cells and alter their functions, and also cross the blood-brain barrier. However, the mechanisms by which fullerenes penetrate and disrupt cell membranes are still poorly understood. Here we use computer simulations to explore the translocation of fullerene clusters through a model lipid membrane and the effect of high fullerene concentrations on membrane properties. The fullerene molecules rapidly aggregate in water but disaggregate after entering the membrane interior. The permeation of a solid-like fullerene aggregate into the lipid bilayer is thermodynamically favoured and occurs on the microsecond timescale. High concentrations of fullerene induce changes in the structural and elastic properties of the lipid bilayer, but these are not large enough to mechanically damage the membrane. Our results suggest that mechanical damage is an unlikely mechanism for membrane disruption and fullerene toxicity.

How hydrophobic buckminsterfullerene affects surrounding water structure

The hydrophobic hydration of fullerenes in water is of significant interest as the most common Buckminsterfullerene (C60) is a mesoscale sphere; C60 also has potential in pharmaceutical and nanomaterial applications. We use an all-atom molecular dynamics simulation lasting hundreds of nanoseconds to determine the behavior of a single molecule of C60 in a periodic box of water, and compare this to methane. A C60 molecule does not induce drying at the surface; however, unlike a hard sphere methane, a hard sphere C60 solute does. This is due to a larger number of attractive Lennard-Jones interactions between the carbon atom centers in C60 and the surrounding waters. In these simulations, water is not uniformly arranged but rather adopts a range of orientations in the first hydration shell despite the spherical symmetry of both solutes. There is a clear effect of solute size on the orientation of the first hydration shell waters. There is a large increase in hydrogen-bonding contacts between waters in the C60 first hydration shell. There is also a disruption of hydrogen bonds between waters in the first and second hydration shells. Water molecules in the first hydration shell preferentially create triangular structures that minimize the net water dipole near the surface near both the methane and C60 surface, reducing the total energy of the system. Additionally, in the first and second hydration shells, the water dipoles are ordered to a distance of 8 A from the solute surface. We conclude that, with a diameter of approximately 1 nm, C60 behaves as a large hydrophobic solute.

Structure and UV−Vis Spectrum of C60 Fullerene in Ethanol: A Sequential Molecular Dynamics/Quantum Mechanics Study

A molecular dynamics simulation combined with semiempirical quantum mechanics calculations has been performed to investigate the structure, dynamical, and electronic properties of pure C60 in liquid ethanol. The behavior of the fullerene alcoholic solution was obtained by using the NPT ensemble under ambient conditions, including one C60 fullerene immersed in 1000 ethanol molecules. Our analyzed center-of-mass pairwise radial distribution function indicated that, on average, there are 32, 72, 132, and 187 ethanol molecules around, respectively, the first, second, third, and fourth solvation shells of the C60 molecule. To investigate the UV-vis transition energies of C60 in the presence of ethanol, we have considered constituents of the time uncorrelated supramolecular structures of the first solvation shell, i.e., clusters of C60@{EtOH}32 types. The semiempirical calculations were performed at the intermediate neglect of differential overlap level with configuration interaction singles (INDO/CIS). Our results have pointed out that the characteristic C60 UV-vis absorbance peaks are slightly shifted to longer wavelengths, as compared to the isolated molecule. These findings are in connection with the weak donor-acceptor character of the interactions involving electron lone pairs of oxygen atoms on the solvent and the fullerene surface.

Dynamics of Water in the Hydration Shells of C60: Molecular Dynamics Simulation Using a Coarse-Grained Model

Dynamics of water in the solvation shells of a fullerene molecule as obtained from a coarse-grained (CG) model for the C60-water interaction has been presented and compared with the same obtained from the atomistic model. While in the CG model the interaction between a fullerene and a water has been represented by a simple two-body central potential as obtained from a coarse-graining of the interactions of a C60 molecule with water, in the atomistic description all the interactions between the atoms of a C60 and a water molecule have been explicitly taken into account. Extensive molecular dynamics simulations of a C60 molecule in water have been performed in isothermal-isobaric ensembles. Translational and reorientational mobilities as well as residence time of water in the solvation shells of a fullerene molecule have been obtained by calculating the corresponding time correlation functions from simulation trajectories. Comparison of the dynamical behaviors obtained from the CG and the atomistic models shows overall good agreement. The nature of the relaxation and the trend that the dynamics becomes slower with the decreasing solute-water distance as obtained from the atomistic model have, in general, been reproduced by the CG model.