A NEW CLASS OF THIOLIPIDS FOR THE ATTACHMENT OF LIPID BILAYERS ON GOLD SURFACES

A new class of lipid molecules is synthesized, based on two dipalmitoylphosphatidic molecules, each extended at the lipid phosphate by a hydrophilic spacer chain of ethoxy groups of variable length, which are then coupled as a bilipid via a terminal disulfide group at the hydrophilic spacer. These anchor-bearing ''thiolipids'' can attach to gold substrates by forming stable gold-sulfur bonds. In this way we can couple lipid bilayers to gold surfaces, with the possibility of preserving a water layer between the support and the first monolayer. The thiolipid molecules are characterized on a Langmuir film balance using fluorescence microscopy. The molecular areas of the thiolipids on the water surface are determined to be 80-90 Angstrom A(2) at a fully compressed state. The thiolipid monolayers show a typical first-order phase transition on the water surface with regular, starlike domains. The formation of thiolipid-attached mono- and bilayers on gold surfaces is investigated by surface plasmon resonance (SPR), impedance measurements, and cyclic voltammetry. Four different supported membrane systems are studied in detail: (i) pure thiolipid layers; (ii) mixed lipid bilayers containing a first pure thiolipid monolayer and a second one of conventional phospholipids; (iii) bilayers, where the first gold-attached monolayer is composed of a mixture of thio- and conventional phospholipids with another second phospholipid layer on top; (iv) monolayers of pure 1-hexadecanethiol and layers with a second phospholipid film on top of the 1-hexadecanethiol. The electrochemical experiments reveal electrically blocking layers for all lipid systems investigated with specific resistances of 10(4)-10(5) Omega cm(2). The capacitance values for pure thiolipid bilayers are in the range of 0.5-0.7 mu F/cm(2) for the pure thiolipid bilayers and 0.7-0.8 mu F/cm(2) for the mixed thiolipid/phospholipid bilayers, which is-comparable to the values found for unsupported, so-called black lipid membranes. SPR measurements confirm qualitatively the results of the electrochemical experiments.