Molecular Models of Polymer Brushes with Biotechnological Applications

Polymer brushes comprise a class of polymers bonded to a surface at one end of their chain. The proximity of these chains leads to an extended conformation, resulting in distinct properties not found in conventional polymeric films. The modulation of these properties is achievable by fine-tuning parameters like chain density per unit surface area, chain length, and chemical composition. Previously, some of the authors developed atomistic models for brushes of the polymers PDMAEMA and PMETAC, which have shown potential as gene delivery vectors, alongside PMEDSAH and PSPMA, which show antifouling properties. However, atomistic models are often limited to system sizes significantly smaller compared to those encountered in experimental conditions and can only be simulated for a small time scale. In order to overcome these limitations, we developed a simplified coarse-grain (CG) model using the MARTINI model, where monomers were represented by grains that approximate groups of atoms. Bonded parameters for the CG potentials (including reference bond lengths, angles, dihedrals, and their respective force constants) were derived interactively by matching the probability distributions of bonded degrees of freedom with those obtained from atomistic simulations of the brushes. Parameters for van der Waals (VDW) interactions are selected from the MARTINI3 force field. Preliminary results show a dependence of chain conformation with the selected MARTINI water resolution. This correlation is evident in variations observed in both brush thickness and the radius of gyration of the chains within the brush. In the case of chains free in solution, the MARTINI force field underestimates the radius of gyration by a factor of two when compared with experimental data available in the literature. While the CG model can reproduce the brush thickness observed in atomistic simulations, chains present a tangled conformation that diverges from the conformations observed in those simulations.