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

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)]

Biologically addressable monolayer structures formed by templates of sulfur-bearing molecules

Claus Duschl, Martha Liley, Horst Vogel

We demonstrate that the combined application of Langmuir-Blodgett and self-assembly techniques allows the fabrication of patterns with contrasting surface properties on gold substrates. The process is monitored using fluoroscence microscopy and surface plasmon spectroscopy and microscopy. These structures are suitable for the investigation of biochem. processes at surfaces and in ultrathin films. Two examples of such processes are shown. In the first example, the structures are addressed through the binding of a monoclonal antibody to a peptide. This demonstrates the formation of self-assembled monolayers by cysteine-bearing peptides on gold, and the directed binding of proteins to the structured layers. A high contrast between specific and unspecific binding of proteins is obsd. by the patterned presentation of antigens. Such films possess considerable potential for the design of multichannel sensor devices. In the second example, a structured phospholipid layer is produced by controlled self-assembly from vesicle soln. The structures created, areas of phospholipid bilayer surrounded by a matrix of phospholipid monolayer, allow formation of a supported bilayer which is robust and strongly bound to the gold support, with small areas of free-standing bilayer which very closely resemble a phospholipid cell membrane. [on SciFinder (R)]

1994

Lipid membranes for the fabrication of functional micro- and nano-structures

Gopakumar Gopalakrishnan

The central goal of this thesis work is to fabricate novel, functional fluorescent nanostructures in confined systems offered by phospholipid membranes, which are known to have highly ordered, thermotropic and lyotropic structures. In separate approaches, we have used three different lipid systems: multilamellar planar lipid membranes, unilamellar vesicular membranes as well as lipid monolayers for the development of functional fluorescent nano-, micro- and meso-scopic structures. Techniques like fluorescence microscopy, single particle imaging, electron microscopy, electron diffraction were used to achieve fundamental understanding of the resulting structures. Multilayer stacks of phospholipid membranes have been used as an effective template for the growth of high aspect ratio fluorescent nanowires. The room temperature synthesis was achieved in the confined nanometer-sized interlamellar water space of lipid multilayers where supersaturating CdCl2 concentrations were induced by acidification leading to controlled unidirectional growth of nanowires. The possibility to render the nanowires fluorescent by doping with CdS quantum dots (QDs) and the light waveguiding along hundreds of micrometers together with the possibility of lateral manipulation make these nanowires attractive candidates for future optoelectronic applications. Novel organic-inorganic functional nanocontainers have been designed and tested by making use of vesicle forming lipid bilayers in combination with semiconductor QDs. Hydrophobic QDs can be integrated into bilayers of lipid vesicles and such lipid/QD hybrid vesicles are capable to fuse with live cells, thereby stain the cell's plasma membrane selectively with fluorescent QDs and transfer the vesicle's cargo into the cell. Modification of the membrane of such hybrid vesicles on the other hand, made them capable to enter the cytoplasm of live cells. Additionally, these hybrid vesicles were found extremely useful for long-term model membrane imaging studies. The results described in this thesis imply that cell and lipid membranes can integrate any kind of hydrophobic nanoparticle whose size matches the membrane thickness, opening novel possibilities to manipulate them as individuals or in ensemble with wide-ranging applications for nanobiotechnology. In a further step, the ability of phospholipid molecules to exhibit lamellar to non-lamellar transition using external stimuli enabled a directed self-assembly of QDs into mesoscale fluorescent structures. An easy and versatile method for the surface modification of TOPO coated CdSe QDs using 1,2-dipalmitoyl-sn-glycero-3-phosphatidic acid (DPPA) have been achieved. DPPA predominantly form non-lamellar phases when dispersed in water, for example, upon addition of Ca2+ or at a pH below 6 are known to form hexagonal II phases. This particular property of DPPA has been exploited to form mesoscale self-assemblies of QD based structures both in solution and in confined systems. Potential applications include the detection or removal of Ca2+ ions in attoliter volumes, the construction of functional devices where QDs are reversibly organized in different forms as well as use of fluorescently labeled DPPA molecules for cellular studies using fluorescence microscopy.

EPFL2006

Sulfur-bearing lipids for the covalent attachment of supported lipid bilayers to gold surfaces: a detailed characterization and analysis

Jacques Dubochet, Claus Duschl, Martha Liley, Henning Paul-Julius Stahlberg, Horst Vogel, Shaik Mohammed Zakeeruddin

In a recent paper, a new class of synthetic lipids, which self-assemble to form monolayers on gold surfaces, was introduced. The addn. of a monolayer of phospholipids to these layers results in robust supported lipid bilayers designed to reconstitute transmembrane proteins. This paper presents a detailed characterization of the monolayers and bilayers formed by these lipids and assesses their ability for the incorporation of membrane-spanning proteins. Film balance methods, electron microscopy, surface plasmon resonance, Fourier transform IR measurements and Raman spectroscopy show the feasibility of this concept. However, further modifications to improve the thiolipids are necessary. A new strategy for the formation of laterally structured supported lipid bilayers is discussed and initial results are presented. [on SciFinder (R)]

1996

Lipid membranes for the fabrication of functional micro- and nano-structures

Gopakumar Gopalakrishnan

EPFL2006