Buckminsterfullerene (C60) is one of the most important carbon-based nanoparticles (CNPs). Industrial-scale production of C60 has reached the level of tons; release to the environment has been confirmed (Tremblay, 2002; Qiao et al., 2007). The present study was devoted to study of the effect of clay minerals on the migration process of C60. Molecular dynamics (MD) simulations were used to study the interaction of CNPS with clay minerals through study of the adsorption of C60 on various surfaces of kaolinite and pyrophyllite in vacuum and aqueous environments. Two kinds of surfaces, hydrophobic siloxane surfaces and hydrophilic hydroxyl surfaces, were investigated. C60 is mainly adsorbed onto the vacancy of the six-membered ring, composed of SiO4 tetrahedra or AlO6 octahedra, on clay-mineral surfaces. A single adsorption layer consisting of C60 molecules with an ordered hexagonal arrangement is presented for all surfaces in vacuum. In aqueous environments, however, the monolayer appears on the siloxane surfaces only, while a cluster of C60 molecules is formed on the hydroxyl surfaces. Free energies prove that the attachment of two C60 molecules is stronger than the adsorption of C60 onto the hydroxyl surface in water, which is the reason for unfavorable formation of C60 monolayer. On the other hand, the adsorption free energy is more negative on the hydrophobic siloxane surface, explaining the monolayer formation. The existence of water, which forms hydration layers on the surfaces of clay minerals, produces energy barriers, and reduces the adsorption affinity to some extent. Because clay minerals act as geosorbents in the environment, the present study is significant in terms of understanding the migration and fate of CNPS in nature.

We have modeled the structure of human lymphotactin (hLpnt), by homology modeling and molecular dynamics simulations. This chemokine is unique in having a single disulfide bond and a long C-terminal tail. Because other structural classes of chemokines have two pairs of Cys residues, compared to one in Lpnt, and because it has been shown that both disulfide bonds are required for stability and function, the question arises how the Lpnt maintains its structural integrity. The initial structure of hLpnt was constructed by homology modeling. The first 63 residues in the monomer of hLpnt were modeled using the structure of the human CC chemokine, RANTES, whose sequence appeared most similar. The structure of the long C-terminal tail, missing in RANTES, was taken from the human muscle fatty-acid binding protein. In a Protein Data Bank search, this protein was found to contain a sequence that was most homologous to the long tail. Consequently, the modeled hLpnt C-terminal tail consisted of both α-helical and β-motifs. The complete model of the hLpnt monomer consisted of two α-helices located above the five-stranded β-sheet. Molecular dynamics simulations of the solvated initial model have indicated that the stability of the predicted fold is related to the geometry of Pro78. The five-stranded β-sheet appeared to be preserved only when Pro78 was modeled in the cis conformation. Simulations were also performed both for the C-terminal truncated forms of the hLpnt that contained one or two (CC chemokine-like) disulfide bonds, and for the chicken Lpnt (cLpnt). Our MD simulations indicated that the turn region (T30–G34) in hLpnt is important for the interactions with the receptor, and that the long C-terminal region stabilizes both the turn (T30–G34) and the five-stranded β-sheet. The major conclusion from our theoretical studies is that the lack of one disulfide bond and the extension of the C-terminus in hLptn are mutually complementary. It is very likely that removal of two Cys residues sufficiently destabilizes the structure of a chemokine molecule, particularly the core β-sheet, to abolish its biological function. However, this situation is rectified by the long C-terminal segment. The role of this long region is most likely to stabilize the first β-turn region and α-helix H1, explaining how this chemokine can function with a single disulfide bond.