Molecular Interactions and Binding Free Energy of Polydopamine and Methylene Blue: A DFT Study

Abstract

Polydopamine (PDA) is a kind of mussel-inspired material. It has been applied as a coating and an adsorbent material. As an adsorbent, the PDA microspheres were used to remove methylene blue (MB) in aqueous solution. It was reported that the efficiency of the PDA adsorbent depends on the pH of the solution. Furthermore, in a pH range of 3 - 10, the hydroxyl and amine functional groups of PDA can be either protonated or deprotonated. The change of the net charge of PDA can affect the intermolecular interactions between PDA and MB. It was proposed that both of the electrostatic and p-p interactions should be dominant in the acidic and basic solutions. Therefore, the structural properties and intermolecular interactions of the PDA-MB complex should be investigated. Such an investigation can be useful for the improvement of the PDA microspheres for the other dyes. To get insight into the roles of PDA structure and its role as an interesting adsorbent for MB, the PDA-MB complex formation was carried out at pH 7. The PDA dimers which have six possible structures were selected. The optimization of all PDA dimers and MB was performed in the gas phase at the B3LYP/6-311++G(d,p) calculations. After that, the complex formation of the optimized PDA dimers and MB was performed using the AutoDock Vina V1.1.2. The binding free energy of the PDA dimers and MB was in a range of -3.2 to -3.7 kcal/mol, which indicated that the binding of PDA dimer and MB is spontaneous. The results showed that the p-p interaction between PDA dimer and MB plays a crucial role in the complex formation. Likewise, the sandwich-like structures of the complexes are more stable than the twisted structures.