Structural changes of phospholipid monolayers caused by coupling of human serum albumin: A GIXD study at the air/water interface

Phase behavior and structural changes were studied for phospholipid monolayers coupled with human serum albumin (HSA) at the air/buffer (pH 3.8) interface by film balance and grazing incidence X-ray diffraction (GIXD) measurements. For comparison, the lipid headgroup was varied from small ionic L-α-dipalmitoyl-phosphatidic acid (DPPA) to large ionic L-α-dipalmitoyl-phosphatidyl-L-serine (DPPS) and zwitterionic L-α-distearoyl-phosphatidylcholine (DSPC). The presence of HSA leads to a more or less pronounced plateau in the pressure/area isotherms of all phospholipids. GIXD diffraction patterns illustrate that the phase sequences of DPPS and DPPA on buffer (oblique-rectangular-hexagonal) are changed because of the binding of HSA. The oblique-rectangular phase transition disappears and the rectangular-hexagonal phase transition shifts to a lower surface pressure. However, there is no effect of HSA on the structure of the DSPC monolayer (NN tilted rectangular packing). Phase diagrams of DPPA/HSA, DPPS/HSA, and DSPC/HSA have been established. A tilt angle decrease in both DPPA/HSA and DPPS/HSA monolayers is observed, but no change of the tilt angle for DSPC molecules in the mixed DSPC/HSA monolayer is found. Thus, whereas HSA binding stabilizes the liquid-expanded phase at low pressures most probably via penetration, it condenses the ordered phases of anionic monolayers via reducing the headgroup repulsion. The positional correlation lengths ζ₁ parallel and perpendicular to the tilt direction change because of the binding of HSA.