Adsorption of Bovine Hemoglobin onto Spherical Polyelectrolyte Brushes Monitored by Small-Angle X-ray Scattering and Fourier Transform Infrared Spectroscopy

Abstract

The adsorption of bovine hemoglobin (BHb) onto colloidal spherical polyelectrolyte brushes (SPBs) is studied by a combination of small-angle X-ray scattering (SAXS) and Fourier transform infrared spectroscopy (FTIR). The SPBs consist of a polystyrene core onto which long chains of poly(styrene sulfonic acid) are grafted. Hemoglobin is a tetrameric protein that disassembles at low pH's and high ionic strengths. The protein is embedded into the brush layer composed of strong polyacids. Thus, the protein is subjected to a pH and ionic strength that largely differs from the bulk solution. At low ionic strengths up to 650 mg of BHb per gram of SPB could be immobilized. The analysis of the particles loaded with protein by SAXS demonstrates that the protein enters deeply into the brush. A large fraction of hemoglobin is bound at the surface of the polystyrene core. We attribute this strong affinity to hydrophobic interactions between the protein and the polystyrene core. The other protein molecules are closely correlated with the polyelectrolyte chains. The secondary structure of the protein within the brush was studied by FTIR spectroscopy. The analysis revealed a significant disturbance of the secondary structure of the tetrameric protein. The content of alpha-helix is significantly lowered compared to the native conformation. Moreover, there is an increase of beta-sheet structure as compared to the native conformation. The partial loss of the structural integrity of the hydrophobic protein is due to hydrophobic interactions with the hydrophobic polystyrene core. Hydrophobic interactions with the phenyl groups of the poly(styrene sulfonate) chains influence the secondary structure as well. These findings indicate that changes of the secondary structure play a role in the uptake of hemoglobin into the poly(styrene sulfonate) brushes.