(527d) Controlling Local Hydrophobicity in Poly(ethylene glycol) Brushes with Poly(sulfobetaine) to Mediate the Conformation of Fibronectin on Biomaterial Surfaces

Polymer brushes consisting of poly(ethylene glycol) (PEG) have been extensively studied as coatings to minimize protein adsorption and ultimately the foreign body response to biomaterials in vivo. While investigating the mechanistic connection between the properties of PEG brushes and non-specific protein adsorption and denaturation, we recently showed that increasing grafting density led to the stabilization of unfolded fibronectin (FN) on the brush surface. Based on this observation, we hypothesized that this effect was due to an increase in local regions with low hydration within the brush as grafting density increased. In this work, we investigated the extent to which the addition of poly(sulfobetaine) (PSB), a zwitterionic polymer that is highly hydrophilic, disrupted these local hydrophobic “hotspots” as means to overcome this effect. This extent was investigated using novel single-molecule methods, which permit unprecedented insight into dynamic, spatial, and population heterogeneity associated with protein-brush interactions. Such methods included combining high-throughput single-molecule fluorescence tracking with intramolecular Förster resonance energy transfer. Our results indicated that, as the fraction of PSB in the brush increased, the rate of FN adsorption decreased. However, interestingly, an optimum fraction of PSB for inhibiting the stabilization of unfolded FN as well as minimizing the denaturation of FN was observed. Additionally, in single-molecule experiments using an environmentally-sensitive fluorophore, we showed that the relative fraction of hydrophobic “hotspots” decreased with increased PSB. These results suggest that the optimum ratio of PSB-to-PEG arises from the balance between the density of hydrophobic “hotspots” and electrostatic interactions at high PSB fractions. The enhancement of electrostatic interactions between FN and the brush at high PSB fractions may also be denaturing. We are currently exploring the molecular basis for the impact of mixing PSB with PEG to prevent denaturation and the stabilization of unfolded FN via molecular dynamics simulations. Ultimately, these findings implicate the potential utility of mixed PEG/PSB brushes as a novel coating for biomaterials with improved biocompatibility over conventional polymer brushes.