ATR-FTIR study of lipopolysaccharides at mineral surfaces

Lipopolysaccharides (LPS) are ubiquitous in natural aqueous systems because of bacterial cell turnover and lysis. LPS sorption and conformation at the mineral/water interface are strongly influenced by both solution and surface chemistry. In this study, the interaction of LPS with various surfaces (ZnSe, GeO₂, α-Fe₂O₃, α-Al₂O₃) that vary in surface charge and hydrophobicity was investigated using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The presence of Ca²⁺ (versus Na⁺) in LPS solutions resulted in aggregate reorientation and increased sorptive retention. ATR-FTIR spectra of Na-LPS systems are consistent with reduced surface affinity and are similar to those of solution phase LPS. Ca-LPS spectra reveal hydrophobic interactions of the lipid A region at the ZnSe internal reflection element (IRE). However, pH-dependent charge controls Ca-LPS sorption to hydrophilic surfaces (GeO₂, α-Fe₂O₃, and α-Al₂O₃), where bonding occurs principally via O-antigen functional groups. As a result of accumulation at the solid-liquid interface, spectra of Ca-LPS represent primarily surface-bound LPS. Variable-angle ATR-FTIR spectra of Ca-LPS systems show depth-dependent trends that occur at the spatial scale of LPS aggregates, consistent with the formation of vesicular structures.