Effect of protein penetration into phospholipid monolayers: morphology andstructure

Phase transition and phase properties of dipalmitoylphosphatidylcholine (DP PC) monolayers penetrated by bovine beta-lactoglobulin dissolved in a buffe red aqueous subphase are experimentally studied. The phase transition durin g the penetration dynamics is indicated by a break point in the Pi(t) trans ients. The condensed phase domains formed during the beta-lactoglobulin pen etration are visualized by Brewster angle microscopy (BAM). The lattice str ucture of the condensed phase is characterised by grazing incidence X-ray d iffraction (GIXD). Experiments on the penetration kinetics of beta-lactoglo bulin into DPPC monolayers are performed, starting from different monolayer states and using different protein concentrations. The condensed phase for med after the main phase transition point, consists only of DPPC. The beta- lactoglobulin penetration occurs without any specific interaction with the DPPC molecules. Number and growth of the domains depend on the area per DPP C molecule at which the beta-lactoglobulin penetration takes place. A first -order main phase transition can be induced when the protein penetrates int o a fluid (gaseous) DPPC monolayer. beta-Lactoglobulin cannot penetrate int o a condensed DPPC monolayer at a surface pressure above the equilibrium pe netration pressure. Conformational changes and squeezing out of protein fro m the penetrated monolayer are studied by compression of penetrated monolay ers in equilibrium. (C) 2000 Elsevier Science B.V. All rights reserved.