The graphite used in fusion devices as first wall material is porous and consists of granules and voids. The 1–10 µm granules are further composed of graphitic micro-crystallites (5–10 nm), which are separated by micro-voids. Understanding the hydrogen transport and trapping in such granules is an important aspect of understanding the effect of a realistic graphite structure on hydrogen recycling and hydrocarbon formation in graphite. We use Kinetic Monte Carlo (KMC) to study the diffusion of hydrogen in a typical granule of graphite. We use molecular dynamics (MD) to obtain the jump attempt frequency ωo and the migration energy Em of interstitial graphite which are inputs to the KMC. A consistent parameterization of MD within KMC is presented. The diffusion shows a non-Arrhenius behavior which can be explained with two types of different jump processes within the graphite crystal. The porous granule structure is constructed using statistical distributions for the crystallite dimensions and for crystallite orientations at a given porosity. The hydrogen trapping at inter-crystallite micro-voids are modeled by assuming that a fraction of the hydrogen atom flux transiting through the micro-void is trapped. We present a parametric study of the diffusion and trapping of hydrogen within the granule for various trapping fractions at the inter-crystallite micro-voids.
Fullerene derivative C(60)(OH)(20) inhibited microtubule polymerization at low micromolar concentrations. The inhibition was mainly attributed to the formation of hydrogen bonding between the nanoparticle and the tubulin heterodimer, the building block of the microtubule, as evidenced by docking and molecular dynamics simulations. Our circular dichroism spectroscopy measurement indicated changes in the tubulin secondary structures, while our guanosine-5'-triphosphate hydrolysis assay showed hindered release of inorganic phosphate by the nanoparticle. Isothermal titration calorimetry revealed that C(60)(OH)(20) binds to tubulin at a molar ratio of 9:1 and with a binding constant of 1.3 ± 0.16 × 10(6) M(-1), which was substantiated by the binding site and binding energy analysis using docking and molecular dynamics simulations. Our simulations further suggested that occupancy by the nanoparticles at the longitudinal contacts between tubulin dimers within a protofilament or at the lateral contacts of the M-loop and H5 and H12 helices of neighboring tubulins could also influence the polymerization process. This study offered a new molecular-level insight on how nanoparticles may reshape the assembly of cytoskeletal proteins, a topic of essential importance for illuminating cell response to engineered nanoparticles and for the advancement of nanomedicine.
Carbon-based nanomaterials possess unique structural, mechanical, and electronic properties that are exploited in numerous applications. The fate of nanomaterials in living systems and in the environment is largely unknown, though there is a reason for concern. Here it is shown how the interaction of fullerene with natural phenolic acid induces cell contraction. This phenomenon has a general applicability to carbon-based nanomaterials interacting with natural amphiphiles. Atomistic simulations reveal that the self-assembly of C(70)-gallic acid (GA) favors aggregation. Confocal fluorescence microscopy shows that C(70)-GA complexes translocate across the membranes of HT-29 cells and enter nuclear membranes. Confocal imaging further reveals the real-time uptake of C(70)-GA and the consequent contraction of the cell membranes. This contraction is attributed to the aggregation of nanoparticles into microsized particles promoted by cell surfaces, a new physical mechanism for deciphering nanotoxicity.
