Zwitterionic lipid (DPPC)–protein (BSA) complexes at the air–water interface

Complexation of zwitterionic lipid, dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and protein, bovine serum albumin (BSA) at the air–water interface has been studied by surface pressure (π) – mean molecular area (A) isotherms and X-ray reflectivity. Although BSA has isoelectric point nearly at pH ≈ 4.8, possibility of complex formation with lipid molecules has been investigated from low (≈4.0) to high (≈9.0) pH range in presence of divalent cation, Ca2+ in the water subphase. Both the isotherm and reflectivity analysis show that the interaction of BSA with lipid monolayer takes place from that low to high subphase pH range, i.e., complexation occurs both below and above of the isoelectric point. Only one layer of BSA forms below the lipid monolayer and the probable reasons for such complex formation have been proposed.