Effect of protein penetration into phospholipid monolayers: morphology and structure

Phase transition and phase properties of dipalmitoylphosphatidylcholine (DPPC) monolayers penetrated by bovine β-lactoglobulin dissolved in a buffered aqueous subphase are experimentally studied. The phase transition during the penetration dynamics is indicated by a break point in the Π(t) transients. The condensed phase domains formed during the β-lactoglobulin penetration are visualized by Brewster angle microscopy (BAM). The lattice structure of the condensed phase is characterised by grazing incidence X-ray diffraction (GIXD). Experiments on the penetration kinetics of β-lactoglobulin into DPPC monolayers are performed, starting from different monolayer states and using different protein concentrations. The condensed phase formed after the main phase transition point, consists only of DPPC. The β-lactoglobulin penetration occurs without any specific interaction with the DPPC molecules. Number and growth of the domains depend on the area per DPPC molecule at which the β-lactoglobulin penetration takes place. A first-order main phase transition can be induced when the protein penetrates into a fluid (gaseous) DPPC monolayer. β-Lactoglobulin cannot penetrate into a condensed DPPC monolayer at a surface pressure above the equilibrium penetration pressure. Conformational changes and squeezing out of protein from the penetrated monolayer are studied by compression of penetrated monolayers in equilibrium.