Investigations of lysozyme adsorption at the air/water and quartz/water interfaces by vibrational sum frequency spectroscopy

We used vibrational sum frequency spectroscopy (VSFS) to investigate lysozyme adsorption at the quartz/water and air/water interfaces. These investigations found that water structure depended intimately on bulk pH at both the hydrophobic, air, and hydrophilic, quartz, interfaces. Changes in the interfacial potential caused by protonation/deprotonation of titratable groups on the protein (as well as on quartz in the case of the solid/liquid studies) played a major role in this. Furthermore, intensity in the CH stretch range from lysozyme was observed under all conditions investigated at the air/water interface. Because only moieties that are well-aligned at the interface can be observed in VSFS, this was taken to be an indication that at least some lysozyme residues must be organized at the hydrophobic air surface. By contrast, CH stretches were not observed from protein adsorbed at the hydrophilic quartz/water interface unless the pH was raised above 8.0, causing the interface to take on a net negative charge. Even under very basic conditions, however, the CH stretch modes were much weaker in this case than those observed at the hydrophobic air/water interface.