Adsorption of bovine serum albumin (BSA) onto lecithin studied by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy

The adsorption of bovine serum albumin (BSA) to lecithin was investigated by ATR-FTIR spectroscopy. Lecithin films were prepared by casting aliquots of 3.2 μg lecithin in methanol onto ZnSe ATR prisms. Surface morphology and the thickness of the films were investigated by laser scanning confocal electron microscopy and scanning electron microscopy and the thickness of the films used for adsorption studies was estimated to be 40 Å. The dependency of the C{double bond, long}O peak area on the lecithin mass in the calibration curve confirms that the thickness of the film is below the penetration depth of the infrared evanescent wave. Size exclusion HPLC and fluorescence spectroscopy show that BSA conformation in up to 1 M NaCl and CaCl₂ solutions is similar to that in water with no aggregation or changes in protein conformation seen over 4 h. The kinetics of BSA adsorption on the lecithin film from water, NaCl and CaCl₂ solutions demonstrates that ions promote the protein adsorption. BSA bound more in the presence of NaCl compared to CaCl₂ at equivalent concentrations. The adsorption appeared greatest at a 0.1 M concentration for both NaCl and CaCl₂. The results are explained in terms of absorptive reactivity of BSA and lecithin surfaces upon salt addition.