Molecular Simulation Study Of Betaine Zwitterionic Self-assembled Monolayers On Au(111)

Proteins are amphiphilic molecules with affinity to material surfaces, thus are easily adsorbed on material surfaces, which causes biofouling issue. Protein-resistant materials have been studied for several decades, and widely used in applications such as bio-medical, ship coating, biochip and contact lens.At present, the widely-used antifouling materials are ethylene glycol polymers (poly(ethylene glycol) and oligo(ethylene glycol)) and phosphorylcholine. However, ethylene glycol polymers are susceptible to auto-oxidation in the presence of oxygen and transition metal ions; while phosphorylcholine is difficult to synthesize, which restrain their applications to some extent. Some studies show that zwitterionic polymeric materials have excellent protein resistance performance.Molecular mechanics method was adopted to study the stable structures of three zwitterionic materials: mixed sulfonate-terminated and trimethylamino-terminated decylthiolate SAMs, sulfobetaine-terminated decylthiolate SAM and carboxybetaine- terminated decylthiolate SAM. The result shows that the most stable structures of the three materials are (2×2 3)R26°, (2×4)R26°and (2×2 3)R15°, respectively, sharing the same packing density. In addition, dipoles of the sulfobetaine-terminated decylthiol and carboxybetaine-terminated decylthiol were fully considered. It was found that the direction of S→N tends to be regularly antiparallel arrangement. N-S is nearly parallel to the surface after vacuum minimization and inclines with a 63°tilt angle from the simulation results of hydrated system. On the contrary, C→N forms parallel arrangement in the carboxybetaine- terminated decylthiolate SAM, N-C is almost parallel to the surface in both vacuum and hydrated systems.Then, interactions between zwitterionic materials and water were investigated by molecular dynamic simulations. It was found that a stable water film forms on the surfaces of SAMs with excellent compactness. The analysis of radial distribution function shows that oxygen atoms in sulfonic acid group and carboxylic acid group play a decisive role in the protein resistance of the materials for its strong hydration capacity. The relationship of the hydratability for the three materials is sulfobetaine-terminated decylthiolate SAM > carboxybetaine-terminated decylthiolate SAM > mixed sulfonate-terminated and trimethylamino-terminated decylthiolate SAMs. The self-diffusion coefficient analysis is in accord with the radial distribution function result, suggesting that sulfobetaine-terminated decylthiolate SAM succeeds in forming a firmer water film, which makes it an excellent protein-resistant material.