The monolayer technique as a tool to study the energetics of protein-protein interactions

Incorporation of β-lactoglobulin in a lipid/porphyrin monolayer at the air–water interface

A catanionic lipid/porphyrin monolayer was formed at the air-water interface by the tetra-anionic porphyrin, tetra-sodium-meso-tetra(4-sulfonatophenyl)porphine (TSPP), mixed with the cationic lipid dioctadecyldimethylammonium bromide (DODAB) in a 1:4 molar ratio. This binary mixture (TSPP/4DODAB) was used as the incorporation matrix of beta-lactoglobulin (betaLG). Binary and ternary systems (TSPP/4DODAB/zbetaLG, where z stands for the number of protein residues per TSPP) were characterized by surface pressure versus area (pi-A) measurements and by Brewster angle microscopy (BAM) observation at the air-water interface. Pi-A measurements and BAM images show that protein is incorporated in the expanded regime of the monolayer and is gradually expelled upon compression at high surface pressures. The successive compression-expansion cycles indicate that the protein under adsorbed to the floating film is reincorporated after the expansion of the monolayer. At low subphase pH, TSPP tends to aggregate decreasing the interaction with DODAB molecules. Electrostatic and hydrophobic interactions are responsible for the presence of betaLG at the interfacial film.

Effects of oligomerization and secondary structure on the surface behavior of pulmonary surfactant proteins SP-B and SP-C

The relationship among protein oligomerization, secondary structure at the interface, and the interfacial behavior was investigated for spread layers of native pulmonary surfactant associated proteins B and C. SP-B and SP-C were isolated either from butanol or chloroform/methanol lipid extracts that were obtained from sheep lung washings. The proteins were separated from other components by gel exclusion chromatography or by high performance liquid chromatography. SDS gel electrophoresis data indicate that the SP-B samples obtained using different solvents showed different oligomerization states of the protein. The CD and FTIR spectra of SP-B isolated from all extracts were consistent with a secondary structure dominated by alpha-helix. The CD and FTIR spectra of the first SP-C corresponded to an alpha-helical secondary structure and the spectra of the second SP-C corresponded to a mixture of alpha-helical and beta-sheet conformation. In contrast, the spectra of the third SP-C corresponded to antiparallel beta-sheets. The interfacial behavior was characterized by surface pressure/area (pi-A) isotherms. Differences in the oligomerization state of SP-B as well as in the secondary structure of SP-C all produce significant differences in the surface pressure/area isotherms. The molecular cross sections determined from the pi-A isotherms and from dynamic cycling experiments were 6 nm(2)/dimer molecule for SP-B and 1.15 nm(2)/molecule for SP-C in alpha-helical conformation and 1.05 nm(2)/molecule for SP-C in beta-sheet conformation. Both the oligomer ratio of SP-B and the secondary structure of SP-C strongly influence organization and behavior of these proteins in monolayer assemblies. In addition, alpha-helix --> beta-sheet conversion of SP-C occurs simply by an increase of the summary protein/lipid concentration in solution.