Neutron Reflectometry Elucidates Density Profiles of Deuterated Proteins Adsorbed onto Surfaces Displaying Poly(ethylene glycol) Brushes: Evidence for Primary Adsorption

The concentration profile of deuterated myoglobin (Mb) adsorbed onto polystyrene substrates displaying poly(ethylene glycol) (PEG) brushes is characterized by neutron reflectometry (NR). The method allows to directly distinguish among primary adsorption at the grafting surface, ternary adsorption within the brush, and secondary adsorption at the brush outer edge. It complements depth-insensitive standard techniques, such as ellipsometry, radioactive labeling, and quartz crystal microbalance. The study explores the effect of the PEG polymerization degree, N, and the grafting density, sigma, on Mb adsorption. In the studied systems there is no indication of secondary or ternary adsorption, but there is evidence of primary adsorption involving a dense inner layer at the polystyrene surface. For sparsely grafted brushes the primary adsorption involves an additional dilute outer protein layer on top of the inner layer. The amount of protein adsorbed in the inner layer is independent of N but varies with a, while for the outer layer it is correlated to the amount of grafted PEG and is thus sensitive to both N and sigma. The use of deuterated proteins enhances the sensitivity of NR and enables monitoring exchange between deuterated and hydrogenated species.