Molecular Dynamics Simulations of a Hyperbranched Poly(ester amide): Statics, Dynamics, and Hydrogen Bonding

Static and Dynamic Behavior of Polymer/Graphite Oxide Nanocomposites before and after Thermal Reduction

Nanocomposites of hyperbranched polymers with graphitic materials are investigated with respect to their structure and thermal properties as well as the dynamics of the polymer probing the effect of the different intercalated or exfoliated structure. Three generations of hyperbranched polyester polyols are mixed with graphite oxide (GO) and the favorable interactions between the polymers and the solid surfaces lead to intercalated structure. The thermal transitions of the confined chains are suppressed, whereas their dynamics show similarities and differences with the dynamics of the neat polymers. The three relaxation processes observed for the neat polymers are observed in the nanohybrids as well, but with different temperature dependencies. Thermal reduction of the graphite oxide in the presence of the polymer to produce reduced graphite oxide (rGO) reveals an increase in the reduction temperature, which is accompanied by decreased thermal stability of the polymer. The de-oxygenation of the graphite oxide leads to the destruction of the intercalated structure and to the dispersion of the rGO layers within the polymeric matrix because of the modification of the interactions between the polymer chains and the surfaces. A significant increase in the conductivity of the resulting nanocomposites, in comparison to both the polymers and the intercalated nanohybrids, indicates the formation of a percolated rGO network.

Molecular dynamics simulation studies of hyperbranched polyglycerols and their encapsulation behaviors of small drug molecules

Computer simulation could disclose more details about the conformations of HPGs and their encapsulation behaviors of guest molecules.

Characterization of a graphene oxide/poly(acrylic acid) nanocomposite by means of molecular dynamics simulations

Graphene oxide/poly(acrylic acid) nanocomposite: static, dynamic, thermal properties and hydrogen bonding, as studied by molecular dynamics simulations.

Dynamics of Hyperbranched Polymers under Confinement: A Dielectric Relaxation Study

The effect of severe confinement on the dynamics of three different generations of hyperbranched polyesters of the Boltorn family is investigated by dielectric relaxation spectroscopy (DRS). The polymer chains are intercalated within the galleries of natural montmorillonite (Na+-MMT), thus forming 1 nm polymer films confined between solid walls. The structure of the nanocomposites is studied with X-ray diffraction and the thermal behavior of the polymers in bulk and under confinement is determined by differential scanning calorimetry. The glass transition temperatures of the polymers show a clear dependence on the generation whereas the transition is completely suppressed when all the polymer chains are intercalated. The dynamic investigation of the bulk polymers reveals two sub-Tg processes, with similar behavior for the three polymers with the segmental relaxation observed above the Tg of each polymer. For the nanocomposites, where all the polymer chains are severely confined, the dynamics show significant differences compared to that of the bulk polymers. The sub-Tg processes are similar for the three generations but significantly faster and with weaker temperature dependence than those in the bulk. The segmental process appears at temperatures below the bulk polymer Tg, it exhibits an Arrhenius temperature dependence and shows differences for the three generations. A slow process that appears at higher temperatures is due to interfacial polarization.

Effects of nanoscopic-confinement on polymer dynamics

The static and dynamic behavior of polymers in confinement close to interfaces can be very different from that in the bulk. Among the various geometries, intercalated nanocomposites, in which polymer films of ∼1 nm thickness reside between the parallel inorganic surfaces of layered silicates in a well-ordered multilayer, offer a unique avenue for the investigation of the effects of nanoconfinement on polymer structure and dynamics by utilizing conventional analytical techniques and macroscopic specimens. In this article, we provide a review of research activities mainly in our laboratory on polymer dynamics under severe confinement utilizing different polymer systems: polar and non-polar polymers were mixed with hydrophilic or organophilic silicates, respectively, whereas hyperbranched polymers were studied in an attempt to probe the effect of polymer-surface interactions by altering the number and the kinds of functional groups in the periphery of the branched polymers. The polymer dynamics was probed by quasielastic neutron scattering and dielectric relaxation spectroscopy and was compared with that of the polymers in the bulk. In all cases, very local sub-Tg processes related to the motion of side and/or end groups as well as the segmental α-relaxation were identified with distinct differences recorded between the bulk and the confined systems. Confinement was found not to affect the very local motion in the case of the linear chains whereas it made it easier for hyperbranched polymers due to modifications of the hydrogen bond network. The segmental relaxation in confinement becomes faster than that in the bulk, exhibits Arrhenius temperature dependence and is observed even below the bulk Tg due to reduced cooperativity in the confined systems.

Molecular dynamics simulation study and molecular docking descriptors in structure-based QSAR on acetylcholinesterase (AChE) inhibitors

In this study we present an approach for predicting the inhibitory activity of acetylcholinesterase (AChE) inhibitors by combining molecular dynamics (MD) simulation and docking studies in a structure-based quantitative structure-activity relationship (QSAR) model. The MD simulation was performed on AChE to obtain enzyme conformation in a water environment. The resulting conformation of the enzyme was used for docking with the most potent inhibitor (26a). Docking analysis revealed that hydrophobic interactions play important roles in the AChE-inhibitor complex. Then, all inhibitors that could bind simultaneously at the catalytic site and at the peripheral anionic site of AChE were docked into the enzyme and their interactions with AChE were used as new interpretable descriptors in a structure-based QSAR model. The least squares support vector regression was constructed using the four most relevant docking descriptors and one molecular structure descriptor. The Q(2) value of the model was found to be 0.790. Furthermore, to study the enzyme conformation stability, a second MD simulation was performed on AChE-inhibitor 26a complex. In MD simulation, the topological parameters of the inhibitor were derived from the PRODRG server, and partial atomic charges were modified using the B3LYP/6-31G level of theory. The radius of gyration for the complex showed that AChE conformation did not change in the presence of the inhibitors.

Self-association and complexation of the anti-cancer drug doxorubicin with PEGylated hyperbranched polyesters in an aqueous environment

Fully atomistic molecular dynamics simulations were employed in order to examine in detail the self-assembly characteristics and the complexation behavior of the anticancer drug doxorubicin with PEGylated hyperbranched polyesters in an aqueous environment. We have examined two variants of the polymeric compound by altering the length of the hydrophilic poly(ethylene glycol) arms attached to the hydrophobic hyperbranched core. By comparing the clustering properties of the drug molecules in a polymer-free system to those in the polymer-containing models, we were able to assess the effects related to the presence and to the structural features of the polymer moiety. In addition, we have distinguished the effects associated with the neutral and protonated drug molecules separately. It was found that, in the presence of the polymeric material, the drug molecules formed clusters preferentially close to the polymer's periphery, the characteristics of which depended on the structural details of the polymeric host and on the charge of the drug molecules. Hydrogen bonding was found to contribute to the polymer/drug complexation, with the nature of the prevailing donor/acceptor pairs depending on the charge of the drug. Dynamic analysis of the drugs' motion revealed that in the polymer-containing systems the drug molecules experienced a larger degree of confinement within the formed clusters compared to that describing their polymer-free analogues, while the structural coherence of the clusters was found to be more persistent in the system with the larger poly(ethylene glycol) arms. The results described in this work, through the monitoring of both static and dynamic aspects of the self-association and the complexation behavior of the neutral and charged molecules of doxorubicin with the polymeric host, may help toward the elucidation of the key parameters that are involved in the formation of effective polymer-based carriers for drug molecules of the anthracycline family used in cancer chemotherapy.

Synthesis and self-association of double-helical AADD arrays

The design and syntheses of four self-complementary oligomers that contain an underlying AADD hydrogen bond sequence are presented, and their self-association was examined in the solution and solid state. The molecular recognition between the two strands is highly sensitive to substitutions of their component heterocycles. Substitution with electron-donating and -withdrawing groups and the influence of preorganization has a large effect on the overall stabilities of the complexes studied. In particular, a wide range (>10(5) M(-1)) of stabilities with respect to substitutions at various positions in the AADD oligomers was demonstrated. In the most extreme case, the dimerization constant measured (K(dimer) ≥4.5×10(7) M(-1)) is comparable to the most stable homodimers of neutral AADD arrays reported to date.