Oxide-dependent adsorption of a model membrane phospholipid, Dipalmitoylphosphatidylcholine: bulk adsorption isotherms
The importance of substrate chemistry and structure on supported phospholipid bilayer design and functionality is only recently being recognized. Our goal is to investigate systematically the substrate-dependence of phospholipid adsorption with an emphasis on oxide surface chemistry and to determine the dominant controlling forces. We obtained bulk adsorption isotherms at 55 °C for dipalmitoylphosphatidylcholine (DPPC) at pH values of 5.0, 7.2, and 9.0 and at two ionic strengths with and without Ca2+, on quartz (α-SiO2), rutile (α-TiO2), and corundum (α-Al2O3), which represent a wide a range of points of zero charge (PZC). Adsorption was strongly oxide- and pH-dependent. At pH 5.0, adsorption increased as quartz < rutile ≈ corundum, while at pH 7.2 and 9.0, the trend was quartz ≈ rutile < corundum. Adsorption decreased with increasing pH (increasing negative surface charge), although adsorption occurred even at pH ≥ PZC of the oxides. These trends indicate that adsorption is controlled by attractive van der Waals forces and further modified by electrostatic interactions of oxide surface sites with the negatively charged phosphate ester (−R(PO4-)R‘−) portion of the DPPC headgroup. Also, the maximum observed adsorption on negatively charged oxide surfaces corresponded to roughly two bilayers, whereas significantly higher adsorption of up to four bilayers occurred on positively charged surfaces. Calcium ions promote adsorption beyond a second bilayer, regardless of the sign of oxide surface charge. We develop a conceptual model for the structure of the electric double layer to explain these observations.