Biomimetic lipoglycopolymer membranes: photochemical surface attachment of supramolecular architectures with defined orientation.

The assembly of defined supramolecular architectures on a molecular scale, such as biomimetic membranes, requires the synthesis of multifunctional building blocks and a sophisticated combination of nanotechnological surface preparation techniques. A drawback of current tethered bilayer lipid membrane (tBLM) systems is their limited submembrane decoupling distance from the solid support. The necessity for large (6 nm and more) cytoplasma analogue compartments originates from several biological and biophysical requirements, such as accommodation of cytoplasmatic subunits of membrane proteins, reduction of the F rster energy transfer for the use of fluorescent or photoaffinity probes in vicinity of metal surfaces, and compensation for surface roughness of sensor surfaces often preventing good electrical tBLMs properties. Current polymer-based tBLMs on gold do not achieve membrane– surface decoupling distances of more than 5 nm, nor are polymer based tBLMs able to maintain sufficient electrical properties (resistance of at least several MWcm). To overcome these limitations, we have devised a new strategy to expand the existing scope of tBLMs using lipoglycopolymers (LGP). Special emphasis was placed on increasing the cytoskeleton analogue compartment size (decoupling distance from the sensor surface) whilst retaining tightly sealing membranes, which requires the careful design of the lipid moiety, the tethering polymer, and the reactive self-assembling monolayer (SAM) to immobilize the LGP. Furthermore, we introduced a new covalent immobilization procedure that utilizes photochemical surface attachment to assemble complex supramolecular architectures of defined orientation from aqueous solution. This procedure may have interesting additional applications in the design of new protein nanoarrays and in bionanotechnology in general. The idea of using macromolecules as a cushion to mimic the cytsol/cytoskeleton of the cell to create a hydrophilic space between membrane and solid support was first introduced by Ringsdorf and Sackmann. Macromolecular tethers impose several challenging features. They adopt coiled conformations, which strongly depend on the solvent, and have a range of molecular weights and lengths, whilst interchain interactions determine shape and stability of thin films. Lipid-functionalized small macromolecular tether systems with less than 100 repeating units n that have been used to date reached no more than 40% of their maximal theoretical thickness. Examples include poly(ethyloxazoline) (n= 50, length 3–3.5 nm) and PEG2000 (n= 45, length 4.9 nm). The requirements for a macromolecular tether can be summarized as follows: 1) There must be complete wetting between the surface and the hydrated polymer and between the membrane and the hydrated polymer; 2) the interaction between membrane and surface needs to be repulsive to prevent dewetting; and 3) nonspecific contacts by van der Waals attraction (effective up to about 3 nm) between lipids and surface must be suppressed. Of all known systems, the glycocalix of the cell is one of the most efficient systems to exert the functions mentioned above. The carbohydrate units of the glycocalix maintain a relatively high osmotic pressure, allowing a stable cell–cell distance between 10–100 nm and a high cooperative stabilization within the glycocalix by hydrogen bonding. Therefore, a carbohydrate-modified or -based macromolecule tether can be envisioned that would enable suitable stabilization within and between tether chains. Theoretical calculations show that to realize a decoupling distance of at least 6 nm, a minimum of twelve b-1,4 linked monosaccharides is required. Apart from the fact that the synthesis of dodeca[*] Dr. S. M. Schiller Freiburg Institute for Advanced Studies (FRIAS), School of Soft Matter Research, Albert-Ludwigs-Universit t Freiburg Albertstrasse 19, 79104 Freiburg (Germany) Fax: (+49)761-203-97451 E-mail: [email protected]