Structure of a monolayer of poly(ethylene glycol) end-capped with a fluoroalkyl group and its relationship with protein adsorption at the aqueous interface

Abstract

Poly(ethylene glycol) (PEG, 6 kg mol(-1)) terminated at both ends by a hydrophobic fluoroalkyl segment [-(CH(2))(2)C(10)F(21)] (R(f)-PEG) has been shown to self-assemble into an essentially water insoluble hydrogel. Here, we characterize the monolayer, formed by R(f)-PEG and the subsequent adsorption of a protein, as a function of packing density at the aqueous interface by using thermodynamic methods, neutron reflectivity, and fluorescence microscopy. The pi-A isotherms showed two transitions. The first transition was attributed to the start of a conformational change of PEG chains from a pancake-like shape to a stretched brush-like shape due to intermicellar packing. The second transition was assigned to a collapse of the R(f)-PEG monolayer, submerging some of the micellar R(f)-PEG underneath the monolayer and forming a multilayered state. BSA (bovine serum albumin), a model protein, adsorption onto the R(f)-PEG monolayer from the subphase was also correlated with the transition state of the monolayer. At a lower packing density below the first transition, the R(f)-PEG did not suppress BSA adsorption at all. Conversely, an almost complete prevention of BSA adsorption by the R(f)-PEG was observed once the R(f)-PEG had formed a fully-covered monolayer state.