Controlled deposition of lipid monolayers and bilayers onto mica and direct force measurements between galactolipid bilayers in aqueous solutions

Abstract

A description is given of the Langmuir-Blodgett deposition technique with which lipid monolayers or bilayers are deposited onto molecularly smooth mica surfaces. Results are given for the deposition of the phospholipids dipalmitoyl phosphatidylethanolamine (DPPE), dilauroyl phosphatidylcholine (DLPC), dimyristoyl phosphatidylcholine (DMPC), dipalmitoyl phosphatidylcholine (DPPC), and the galactolipids monogalactosyl diglyceride (MGDG) and digalactosyl diglyceride (DGDG). Mica covered with a deposited hydrophobic monolayer of DPPE is particularly suitable as a substrate for deposition of a second layer of a desired lipid to form a bilayer. With the help of the measured monolayer compression isotherms, the deposition of this second layer is carried out such that the deposited headgroup area of lipids in the second layer can be controlled accurately. After bilayers are deposited onto mica surfaces, the direct force measurement technique of Israelachvili is applied to measure surface interactions between two opposing bilayers. Results are given for the interactions between galactolipid bilayers. The experimental data show that doubling the number of sugar units on the bilayer surface doubles the distance range of the short-range repulsive hydration force. Below 4 nm, the distance dependence of the long-range Van der Waals interaction between DGDG bilayers in aqueous solutions obeys the nonretarded Hamaker equation. The experimental Hamaker constant in pure water is found to be 7.5 ± 1.0 × 10 −21 J which decreases to a value 3.1 ± 0.6 × 10 −21 J upon addition of 0.2 M NaCl, in accordance with the expected screening of the temperature-dependent contribution to the Van der Waals interaction.