Theoretical Studies On Dipeptides Self-assembly And Anti-Crude-Oil-Adhesion Property Of Polymer Film

Peptide amphiphiles self-assemble into supramolecular hydrogels play important roles in biological systems and material field. Recently, the extensive applications in solar energy utilization attracted researchers’ attention. Dipeptide sodium salts can readily self-assemble into helical nanostructures as observed in experiments. But what are the primary driving forces of the organizing process at the molecular level is still worthwhile discussing. We should take the environment protection into consideration while developing science and technology. Oil spill events had serious pollution on environment. Because of the ability of absorbing water molecules onto its surface to form hydration shell, novel polymer material protected the instruments from hydrophobic materials and can be used for coating materials to purify oily wastewater.This work employed molecular dynamics simulation to study the self-assembly of dipeptide sodium salts derived from alanine base on experimental investigation. We hope that this work will contribute to the understanding and applications of molecular self-assembly. Finally, anti-fouling ability of two different polymer films was investigated. The results shown that due to the better anti-fouling ability, PAAS film can be used for purifying oily wastewater. The specific research contents are summarized as follow:1. Self-assembly of dipeptide sodium salts derived from alanine (AACs) in aqueous solution was investigated by molecular dynamics simulation. In the simulation, a united-atom description of GROMOS 45a3 force field was used. AACs molecules were dispersed into aqueous solution randomly in the initial simulation. Finally, one cylindrical nanostructure was obtained with hydrophilic groups toward water after a 200 ns simulation. The nanostructure was a helical fibril according to the simulation result at the molecular level. However, alkyl chains were partially exposed to water to form hydrophobic surface. Herein, we believe that due to the hydrophobic interaction, fibrils finally organized into larger nanofibers. To have further understanding of the formation of helical structure, special distribution functions of sodium ions and water molecules around hydrophilic groups were analyzed together with hydrophilic interactions. Hydrogen bonds and electrostatic interactions represented by the water-bridge and salt-bridge structure between dipeptide molecules are major driving forces for the hydrophilic amide groups to form the nanostructure shell. Finally, based on Yang’s experiments, suggested model was proposed through analysis of special distribution functions.2. Based on the last simulation system, we designed two dipeptides which derived from threonine and had opposite chirality. Molecular dynamics simulation and experimental techniques was combined to investigate self-assembly of the dipeptide sodium salts in aqueous solution. We found that hydrophobic interaction was major driving force of the self-assembly by analyzing non-covalent interactions. Meanwhile, strong electrostatic repulsion among hydrophilic groups makes the dipeptides unable to assemble and keep the configuration. However, water molecules and sodium ions can penetrate into hydrophilic shell to form water bridge and salt bridge structure with dipeptides. It decreases electrostatic repulsion and promotes the self-assembly. Thus, the aggregates can exist in long time simulation. In the experiments, it is found that the two molecules self-assemble into helical nanofibers with opposite handedness. The handedness of nanofiber of (L, L) is left, and that of nanofiber of (D, D) is right.3. There are many oil spill events every year in the world. That not only causes huge economic losses, but also brings serious pollution to the environment. Due to high viscosity, crude oil always causes invalidation to all instruments. To explore good anti-oil-adhesion materials, we employed molecular dynamics simulations to investigate the interactions between two different polymer films and water molecules. Based on immiscibility of oil and water, the polymer film, which is more hydrophilic, has a more stable hydration shell. Thus, it is difficult for hydrophobic molecules to penetrate into hydration shell and adhere to polymer film surface. By analyzing mean force potential, diffusion property, number of hydrogen bond and structure of hydration shell, we found that the hydration shell of the polymer film formed form PAAS is more stable. Therefore, PAAS can be used for coating materials to purify oily wastewater.