A new class of thiolipids for the attachment of lipid bilayers on gold surfaces

Abstract

A new class of lipid mols. is synthesized, based on two dipalmitoylphosphatidic mols., 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, the authors can couple lipid bilayers to gold surfaces, with the possibility of preserving a water layer between the support and the first monolayer. The thiolipid mols. are characterized on a Langmuir film balance using fluorescence microscopy. The mol. areas of the thiolipids on the water surface are 80-90 .ANG.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 was studied by surface plasmon resonance (SPR), impedance measurements, and cyclic voltammetry. Four different supported membrane systems are studied in detail: (1) pure thiolipid layers; (2) mixed lipid bilayers contg. a first pure thiolipid monolayer and a second one of conventional phospholipids; (3) bilayers, where the first gold-attached monolayer is composed of a mixt. of thio- and conventional phospholipids with another second phospholipid layer on top; (4) monolayers of pure 1-hexadecanethiol and layers with a second phospholipid film on top of the 1-hexadecanethiol. The electrochem. expts. reveal elec. blocking layers for all lipid systems investigated with specific resistances of 104-105 W cm2. The capacitance values for pure thiolipid bilayers are in the range of 0.5-0.7 mF/cm2 for the pure thiolipid bilayers and 0.7-0.8 mF/cm2 for the mixed thiolipid/phospholipid bilayers, which is comparable to the values found for unsupported, so-called black lipid membranes. SPR measurements confirm qual. the results of the electrochem. expts. [on SciFinder (R)]