Abstract
Large area lipid bilayers, on solid surfaces, are useful in physical studies of biological membranes. It is advantageous to minimize the interactions of these bilayers with the substrate and this can be achieved via the formation of a floating supported bilayer (FSB) upon either a surface bound phospholipid bilayer or monolayer. The FSB's independence is enabled by the continuous water layer (greater than 15 Å) that remains between the two. However, previous FSBs have had limited stability and low density. Here, we demonstrate by surface plasmon resonance and neutron reflectivity, the formation of a complete self-assembled monolayer (SAM) on gold surfaces by a synthetic phosphatidylcholine bearing a thiol group at the end of one fatty acyl chain. Furthermore, a very dense FSB (more than 96%) of saturated phosphatidylcholine can be formed on this SAM by sequential Langmuir–Blodgett and Langmuir–Schaefer procedures. Neutron reflectivity used both isotopic and magnetic contrast to enhance the accuracy of the data fits. This system offers the means to study transmembrane proteins, membrane potential effects (using the gold as an electrode) and even model bacterial outer membranes. Using unsaturated phosphatidylcholines, which have previously failed to form stable FSBs, we achieved a coverage of 73%.
Highlights
Phospholipid bilayers deposited on solid substrates are very useful systems for studying the structures, behaviours and interactions of biological membranes [1]
We describe a new approach to fabricating floating supported bilayer (FSB), where the silane-grafted phosphatidylcholine on silicon is replaced by a thiol-grafted phosphatidylcholine on a gold surface
We show that the coverage of the v-thiolipid self-assembled monolayer (SAM) is much greater than that typically seen for the silane SAMs, and the coverage of the resulting FSB is greatly improved
Summary
Phospholipid bilayers deposited on solid substrates are very useful systems for studying the structures, behaviours and interactions of biological membranes [1]. This restriction is likely to be true of bilayers directly tethered to surfaces using thiolipids with a hydrophilic spacer which have enabled the reconstitution of large integral membrane proteins [2,3,4,12,13] It has been known for many years that multilayers of lipids deposited on solid supports do retain the freedom necessary to act as realistic model systems. Hughes et al [18] replaced the lowermost layer of the system with a grafted self-assembled monolayer (SAM) of octadecyltrichlorosilane, improving the stability of the lower supporting layers during the fabrication of the floating bilayer Using this approach, it was possible to deposit DMPC which has a shorter chain length (C14) and is in the fluid phase at room temperature. We show that the dense thiophospholipid SAM allows the fabrication of FSBs containing unsaturated lipids, in this case 1-palmitoyl-2-oleoyl-snglycero-3-phosphocholine (POPC), overcoming a limitation of previous FSBs
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