Structure characterization and stability of mixed lipid/protein monolayer at the air/water interface

A mixed protein/lipid monolayer has been constructed by the protein adsorpt ion from subphase into the spread phospholipid monolayer. A precisely contr olled pump was used to exchange the protein solution with different pH valu es after the protein was ensured to reach the less condensed surface. The domains formed in the coexistence region of D-dipalmitoylphosphatidylch oline (D-DPPC) have been recorded by Brewster angle microscopy (BAM) combin ed with the film balance before and after the penetration of the protein, h uman serum albumin (HSA). The subphase was exchanged by gradually increasin g or decreasing pH value of the solution. Three isotherms of the mixed D-DP PC/HSA monolayer with the subphase of pH=4.2, pH=7.0 and pH=9.1, respective ly, were obtained. It indicated that the area per lipid molecule with prote in increased as the subphase pH value was lowed. Simultaneously, morphologi cal dynamic changes caused by the gradual changing of subphase pH were obse rved. These variations can be ascribed to the conformation change of protei n under the fluctuation of pH value. The hydrophobic and electrostatic inte ractions between the phospholipid and HSA were as considered for the interp retation of domain change based on the current experimental results. (C) 20 01 Elsevier Science B.V. All rights reserved.