Morphological investigation of mixed protein/phospholipid monolayers

Phase changes of the monolayer of l-α-dimyristoyl phosphatidylcholine (DMPC) caused by the penetration of protein, β-lactoglobulin or β-casein are investigated at the air/water interface by Brewster angle microscopy and fluorescence microscopy (FM). It is found that domains are formed during compressing the mixed protein/DMPC monolayer. The domains possess various ribbon shapes and higher flexibility. The results of FM measurements by labeling protein molecules reveal that the formed domains are caused by the incorporation of protein molecules into DMPC monolayer rather than by the phase transition of DMPC monolayer. To compress the mixed monolayer, we have proved by experiments the separation of β-lactoglobulin or β-casein from the DMPC monolayer. It also shows no obvious interaction between DMPC and β-lactoglobulin or β-casein.