ATR FTIR Study of the Interaction of TiO₂ Nanoparticle Films with β-Lactoglobulin and Bile Salts

The technique of in situ particle film attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR) has been used to probe the adsorption and coadsorption (sequential) of a common food protein (β-lactoglobulin, BLG) and two representative bile salts (taurocholic acid and glycocholic acid, abbreviated as TCA and GCA) onto the surface of titanium dioxide (TiO₂) nanoparticles. Evaluating of binding interactions between commonly used (historically now, in some countries) food additives and food components, as well as the body's own digestion chemicals, is a critical step in understanding the role of colloidal phenomena in digestion and bioavailability. TCA is found to adsorb onto TiO₂ but without any significant ability to be retained when it is not present in the aqueous phase. GCA is also found to adsorb via two distinct binding mechanisms, with one type of adsorbed species being resistant to removal. BLG adsorbs, is irreversibly bound, and has altered conformation when adsorbed at pH 2 (stomach conditions) to the conformation when adsorbed at pH 6.5 (small intestine conditions). This altered conformation is not interface-dependent and is mirrored in the solution spectra of BLG. Sequential coadsorption studies indicate that TCA and GCA adsorb onto TiO₂ nanoparticle surfaces and display similar degrees of reversibility and binding in the presence or absence of preadsorbed BLG.