Interaction between lysozyme and humic acid in layer-by-layer assemblies: effects of pH and ionic strength

The interaction between protein and soluble organic matter is studied through layer-by-layer assembly of lysozyme (LSZ) and purified Aldrich humic acid (PAHA) at a solid surface (2-D) and in solution (3-D). By bringing a silica surface in alternating contact with solutions of LSZ and PAHA a layer-by-layer LSZ-PAHA assembly is formed. At pH 5 the negative charge density of PAHA is about 3 times that of the positive LSZ; the layers of LSZ and PAHA are stable and the adsorbed amounts decrease with increasing ionic strength. The mass ratios PAHA/LSZ in the layers depend on the ionic strength; K(+) incorporation is relatively large (∼25%) when PAHA is the outer layer of the assembly. At pH 6 and 8, and moderate ionic strength (0-100 mmol L(-1) KCl) the assembly is accompanied by partial solubilization of positive LSZ by the much more negative PAHA followed by desorption of the complex. The solubilization increases with increasing pH, and decreases with increasing KCl concentration. At 400 mmol L(-1) KCl the electrostatic interactions are so well screened that the assembly is no longer accompanied by layer erosion. Assembly of PAHA and LSZ in solution is also investigated at pH 5 and 5 mmol L(-1) KCl. The PAHA/LSZ mass ratio at the iso-electric point of the assembly depends on the order of the addition. When LSZ is added to the negative assembly K(+) is incorporated in the complex, but when PAHA is added to the positive assembly PAHA and LSZ neutralize each other.