Comparative investigation of structure characteristics of mixed β-lactoglobulin and different chain-length phophatidylcholine monolayer at the air/water interface

The dynamic adsorption and penetration characteristics of β-lactoglobulin into the monolayer of two different chain-length phosphatidylcholines (DSPC, DPPC) are systematically investigated using Brewster angle microscopy (BAM), fluorescence microscopy (FM) and film balance technique. It was found that all the two phosphatidylcholines can form stable complex films with the β-lactoglobulin. In the complex system of DPPC or DSPC/β-lactoglobulin the lipid domains were observed again owing to the penetration of the protein. With the increase of surface pressure, the stripe and block domains appeared and gradually turned brighter. It has been considered that protein is squeezed out from the lipid monolayer and aggregated at interface. The critical surface pressure of the β-lactoglobulin separated from DPPC or DSPC phase is increased when lipid has longer alkyl chain length. Therefore, the hydrophobic effect can be considered as a main factor to lead to the formation of a stabilized mixed lipid/protein film.