Two-dimensional center-of-mass diffusion of lipid-tethered poly(2-methyl-2-oxazoline) at the air-water interface studied at the single molecule level.

The two-dimensional (2D) center-of-mass diffusion, D, of end-tethered poly(2-methyl-2-oxazoline) (PMOx) lipopolymer chains was studied in a Langmuir monolayer at the air-water interface using wide-field single molecule fluorescence microscopy. In this case, tethering and stabilization of hydrophilic PMOx chains at the air-water interface is accomplished via end-tethering to lipid molecules forming a hydrophobic anchor. To explore the influence of molecular weight, M n, and surface concentration, c s, on lateral mobility, two different PMOx chain lengths of n = 30 and 50 ( n, number of monomer units) were analyzed over a wide range of c s. Using multiparticle tracking analysis of TRITC-labeled PMOx lipopolymers, we found two regimes of lipopolymer lateral mobility. At low c s, D is independent of surface concentration but increases with decreasing n. Here diffusion properties are well described by the Rouse model. In contrast, at more elevated c s, the data do not follow Rouse scaling but are in good agreement with a free area-area model of diffusion. The current study provides for the first time experimental insight into the 2D center-of-mass diffusion of end-tethered polymers at the air-water interface. The obtained results will be of importance for the understanding of diffusion processes in polymer-tethered phospholipid bilayers mimicking biomembranes at low and high tethering concentrations.

Domain registration in raft-mimicking lipid mixtures studied using polymer-tethered lipid bilayers.

The degree of domain registration in a liquid-ordered/liquid-disordered phase-separating lipid mixture consisting of 1-stearoyl-2-oleoyl-sn-3-phosphocholine, egg sphingomyelin, and cholesterol (molar mixing ratio of 1:1:1) was studied using three different planar lipid bilayer architectures distinguished by their bilayer-substrate distance d using epifluorescence microscopy. The bilayer systems, which were built layer by layer using Langmuir-Blodgett/Schaefer film depositions, included a solid-supported bilayer (d approximately 15 A) and two polymer-supported bilayers with d approximately 30 A and d approximately 58 A, respectively. Complete domain registration between Langmuir-Blodgett and Schaefer monolayer domains was observed for d approximately 58 A but not in the cases when d approximately 15 A and d approximately 30 A. Building the bilayer layer by layer guaranteed that any preexisting domains were not in registration initially; our data show that the domain registration observed was not caused by lipid flip-flop or by lateral rearrangement of preexisting large-scale domains. Instead, additional studies on bilayer systems with asymmetric lipid composition indicate that preexisting domains in the Langmuir-Blodgett monolayer induce the formation of completely registered domains in the opposite Schaefer monolayer. This study provides insight into possible biophysical mechanisms of transbilayer domain coupling. Our findings support the concept that the formation of transbilayer signaling platforms based on registered raft domains may occur without the active involvement of membrane-spanning proteins.

Lipopolymers from new 2-substituted-2-oxazolines for artificial cell membrane constructs.

We present the synthesis of novel 2-oxazoline monomers with different 2-substituents and their consecutive conversion into lipopolymers by living cationic polymerization. The side functions of these monomers were varied to realize different steric needs and hydrogen bonding interactions of the polymer side chains. 2-(2'-N-pyrrolidonyl-ethyl)-2-oxazoline, 2-(3'-methoxymonoethyleneglycol)propyl-2-oxazoline, and 2-(3'-methoxytriethyleneglycol)propyl-2-oxazoline were synthesized. All of the monomers could be converted into the corresponding lipopolymers by living cationic polymerization using 2,3-di-O-octadecyl-1-trifluormethansulfonyl-sn-glycerol as the initiator. The characterization of the 2,3-di-O-octadecyl-glycerol-poly(2-oxazoline) lipopolymers by NMR spectroscopy, IR spectroscopy, and gel permeation chromatography revealed that the targeted molar masses and compositions can be controlled by the initial initiator/monomer ([M](0)/[I](0)) ratio for all the synthesized lipopolymers. The polydispersities were found to be narrow (polydispersity indices from 1.06-1.3). The amphiphilic lipopolymers were spread at the air-water interface (Langmuir-Blodgett film balance) and the effect of the polymer side groups and chain lengths upon the Pi-area (A) isotherms of the corresponding lipopolymer monolayers were compared and analyzed. The impact of the polymer side functionalities on a 2D gel formation was examined using an interfacial rheometer operated in an oscillating stress-strain mode. Interestingly enough, none of the newly synthesized lipopolymers showed a rheological transition. This somewhat surprising result not only verified that these 2D gels are not established by hydrogen bonding among hydrophilic polymer moieties, as earlier proposed, but also supported the concept of jammed surface micelles as the more likely origin for the gelation phenomenon. [Diagram: see text]

Confinement of transmembrane cell receptors in tunable stripe micropatterns.

We report a simple method to confine transmembrane cell receptors in stripe micropatterns of a lipid/lipopolymer monolayer, which are formed as result of the transfer onto a solid substrate. The stripes are aligned perpendicular to the meniscus, whose periodicity can systematically be tuned by the transfer velocity. This strongly suggests the dominant role of the cooperative interaction between the film and substrate. Selective fluorescence labeling of lipids and lipopolymers confirms that the observed patterns coincide with the demixing of two species. Covalent coupling of polymer headgroups enables us to use the stripe patterns as a support for a lipid bilayer membrane. Spreading of lipid vesicles with platelet integrin alphaIIbbeta3 on a self-assembled membrane micropattern demonstrates that cell adhesion receptors are selectively incorporated into the lipopolymer-rich region. The method established here provides us with a tunable template for the confinement of receptor proteins to geometrically control the cell adhesion.