Sum frequency generation vibrational spectroscopy studies on protein adsorption

Sum frequency generation (SFG) vibrational spectroscopy has been successfully applied to detect orientation and conformation changes of adsorbed bovine serum albumin (BSA) molecules at different interfaces. These interfaces include BSA solution/air, BSA solution/fused silica, and BSA solution/deuterated polystyrene (d-PS) interfaces. A general thin film model has been used to interpret BSA SFG spectra. SFG spectra collected from BSA solution/air interfaces with different solution pH are quite different. Detailed analysis indicated that dramatic changes of these spectra are mainly due to the interferences between the C-H signals from interfacial BSA molecules and the O-H signals from water. This has been confirmed by spectra collected from BSA D2O solution/air interfaces. SFG studies on BSA solution/d-PS interfaces with different solution pH present similar pH dependent results. At BSA solution/fused silica interfaces, only water signals were detected. We believe that the absence of BSA signals was due to the inversion symmetry of hydrophobic side chains buried inside the BSA molecules. Thus, BSA molecules have a "hydrophilic" conformation. Comparison of SFG results at different interfaces shows that C-H signals are greatly varied. Different BSA spectra indicated that BSA structures could be very different while contacting various media. In the literature, research shows that interfacial BSA structures depend on the BSA solution pH. However, our SFG results indicate that these structures still generate similar SFG C-H signals, showing that the alignment and order of C-H functional groups are still similar. Our research of BSA solution/d-PS interfaces demonstrated that SFG can be applied to study protein adsorption on polymer materials in situ.