Effect of Different Divalent Ions on the DNA Adsorption by Lipid Monolayers

In this study, the DNA adsorption by the cationic lipid DC-Chol in the presence of Ca2+, Mg2+, and Ba2+ ions was investigated by measuring the surface pressure-area isotherm and X-ray reflectivity of the DC-Chol/DNA LB films supported on silicon wafers. The pressure-area isotherm showed that adding divalent ions would increase the surface pressure, which indicates the interaction between the DC-Chol monolayer and the DNA is enhanced due to the presence of divalent ions. The DNA layer thickness of the DC-Chol LB films was found to increase with increasing divalent ion concentration. Calcium ions are slightly more effective than magnesium ions in enhancing the DNA adsorption. At higher ion concentrations (> 10 mM), barium ions become less effective in enhancing DNA adsorption as compared with calcium or magnesium ions. Such differences can be attributed the differences in the hydrated ion sizes. The effectiveness of enhancing the DNA adsorption is in the order of Ca2+ > Mg2+ > Ba2+. As evidenced from the X-ray SLD of the adsorbed DNA layer, the divalent ions were found to bind to the adsorbed DNA. The enhanced DNA adsorption can thus be attributed to the ion-mediated attractive interaction between the DNA strands. It is shown in this study that DC-Chol/DNA LB films of desired film thickness can be prepared by controlling the divalent ion concentrations or species.