(221b) Protein Adsorption At the Air-Water Interface Studied by Surface Vibrational Spectroscopy

Studies of protein structure at the air-water interface under relevant conditions represent a significant experimental challenge. Most optical techniques do not provide molecular information on liquid interfaces or a separation of the response of the interface from the intense bulk signal cannot be made unambiguously. Nonlinear optical probes such as vibrational sum-frequency generation (SFG) are, however, inherently interfacial specific and represent a promising technique to address experimental challenges associated with liquid interfaces. In order to obtain molecular level information with SFG, tunable infrared pulses are combined at the interface of interest with visible pulses of a fixed frequency. The resulting SFG spectra provide a vibrational fingerprint with information on composition and conformation of the interfacial molecular layer.

A combination of SFG spectroscopy, surface tension measurements and ellipsometry is used to study the adsorption kinetics and interfacial properties of bovine serum albumin (BSA) and b-lactoglobulin (BLGL) at the air-water interface. BSA and BLGL adsorbed at the interface give rise to pronounced vibrational bands in SFG spectra at 1650 - 1660 cm^-1 that are indicative for Amide I vibrations of the proteins directly at the interface. The amplitude of the Amid I band increases with the interfacial coverage of BSA as determined by ellipsometry, while shape and position of the vibrational band are coverage and adsorption time independent. The latter suggests that conformational changes at the water surface are negligible.

In addition to the adsorption behaviour of proteins at liquid interfaces the interaction of proteins with surfactants such as SDS (sodium dodecyl sulphate) and fatty acids can influence the properties of the molecular layer at the water surface dramatically. In fact, the introduction of surfactants in the subphase of a BSA layer at the water surface leads to substantial changes in vibrational SFG spectra and allows for kinetic studies of the protein-surfactant competitive adsorption. A replacement of interfacial proteins by surfactant molecules can be monitored by a decrease of the Amid I band (1650 cm^-1) and an increase in intensity of the surfactant related vibrational band such as the C=O stretching vibrations (1740 cm^-1) of carboxyl groups for fatty acid surfactants.