Stability investigation of the mixed DPPC/protein monolayer at the air–water interface

A subphase exchange method has been used to investigate the stability of the mixed l-α-dipalmitoylphosphatidylcholine (DPPC)/β-lactoglobulin monolayer formed via the protein adsorption into the spread lipid layer at the air–water interface by Brewster angle microscopy (BAM). The BAM images demonstrated that the adsorbed protein might compress the DPPC monolayer to enter a phase transition early close to a condensed phase. When protein concentration is very high the penetration of the protein is rather fast, and a larger domain was preferentially formed without compressing the DPPC monolayer. After the mixed DPPC/β-lactoglobulin monolayer was formed, the protein solution in subphase was totally washed out by pumping in buffer water for a long time. In the fluid phase and coexistence region of DPPC monolayer with penetrated protein layer the domains have been recorded by BAM images for comparing the variation before and after the exchange of subphase. The experimental results revealed that there was not significant change of the domains in size and shape. It indicates that the adsorbed protein has a strong interaction with DPPC and is more likely to remain at the interface. A relatively stable mixed lipid–protein monolayer can be constructed by this way. Hydrophobic and electrostatic interactions between DPPC and β-lactoglobulin as well as the conformation change of β-lactoglobulin at air–water interface have been taken into account to stabilize the mixed layers.