Solid-supported polymer bilayers as membrane mimics

Membranes are one of Nature’s most remarkable designs. Due to their importance in numerous cellular processes, they are prominent subjects of biochemical and biophysical fundamental research. In particular, it is crucial to understand the membrane morphology, the role of individual membrane components, and also to correlate the membrane structure to its various functions. Besides, systems inspired by natural membranes are of high interest for technological applications, such as water purification, drug screening, or sensing. However, the complexity and fragility of natural membranes often limit their direct use. For that reason, the development of membrane models is indispensable. Suitable building blocks for model systems could be lipids or amphiphilic polymers. In this thesis, robust solid-supported membrane models from amphiphilic diblock copolymers were designed by combining different methods of polymer synthesis, membrane preparation, and surface analytics. Anionic polymerization yielded a well-defined poly(butadiene)-b-poly(ethylene oxide) polymer in terms of overall molecular weight and individual block length. Through a chemical modification procedure, a sulfur-functionalized derivate of the polymer was obtained, which served for covalent immobilization of the polymer monolayers on ultrasmooth gold surfaces. For membrane preparation two different procedures were employed: on the one hand, individual polymeric monolayers were deposited on the gold supports by a combination of the well-controllable Langmuir film transfer techniques. On the other hand, in a one-step procedure, polymer superstructures were spread either on gold or on glass surfaces to yield solid-supported polymer membranes. The membranes with a covalently immobilized proximal leaflet by sulfur/gold chemistry possess high mechanical stability, and at the same time, a certain degree of mobility resulting from the non-covalent coupling of the individual sheets. The membranes were characterized by surface-sensitive techniques such as atomic force microscopy and surface plasmon resonance spectroscopy to gain insights into morphology, homogeneity, and thickness of the layers. To demonstrate the membranes’ biomimetic potential, they were incubated with peptides, polymyxin B and -haemolysin. Occurring interactions were detected by electrochemical impedance spectroscopy. In summary, this thesis might impact fundamental membrane science as well as prospective biotechnological applications

Solid-supported polymeric membranes

Biological membranes count among nature's most remarkable designs. They compartmentalize various cellular functions and serve as portals of entry to and egress from cells and organelles. It is therefore not surprising that they are prominent subjects of extensive fundamental research and serve as an inspiration for the development of novel materials. In an effort to mimic the intricacies that natural membranes display, significant attention has been directed at the development of synthetic membranes. One well-established membrane design is the "solid-supported membrane.'' Lipids, the natural constituents of membranes, have gained a firm footing as building blocks in this field, because the resulting membranes closely mimic natural membranes in both structure and function. However, they are prone to oxidation and membranes composed of lipids suffer the drawbacks of long-term instability and dehydration, thereby limiting their use, especially in applications that require stability in air. Amphiphilic block polymers are currently emerging as novel alternative building blocks for membranes. One great advantage is that they provide a choice of monomers and thus broaden the parameter space for synthetic membrane design. Polymers can therefore be tuned to exhibit diverse chemical, physical, and mechanical membrane properties. Membrane design today benefits markedly from a plethora of available chemistries inherent to polymers. With such options available, a variety of strategies to immobilize membranes on surfaces can be combined advantageously with membrane modifications that include functional groups, thereby generating highly multifunctional platforms. In sum, solid-supported polymeric membranes represent a promising, novel approach to membrane design that calls for the attention we provide them here

Amphiphilic polymers at interfaces

Self-assembly phenomena in block copolymer systems are attracting considerable interest from the scientific community and industry alike. Particularly interesting is the behavior of amphiphilic copolymers, which can self-organize to nanoscale-sized objects such as micelles, vesicles, or tubes in solution, and which form well-defined assemblies at interfaces such as air–liquid, air–solid, or liquid–solid. Depending on the polymer chemistry and architecture, various types of organization at interfaces can be expected, and further exploited for applications in nanotechnology, electronics, and biomedical sciences. In this article, we discuss the formation and characterization of Langmuir monolayers from various amphiphilic block copolymers, including chargeable and thus pH-responsive materials. Solid-supported polymer films are reviewed in the context of alteration of surface properties by ultrathin polymer layers and the possibilities for application in tissue engineering, sensors and biomaterials. Finally, we focus on how organic and polymer monolayers influence the growth of inorganic materials. This is a truly biomimetic approach since Nature uses soft interfaces to control the nucleation, growth, and morphology of biominerals such as calcium phosphate, calcium carbonate, and silica

Planar Block Copolymer Membranes by Vesicle Spreading

An easy route to planar solid-supported polymer membranes by vesicle spreading is described. Pre-organized PB-PEO assemblies were spread on two different supports, i.e. strongly hydrophilic glass surfaces and ultrasmooth gold substrates. Polymer membranes were produced on a hydrophilic support by spreading hydroxyl-functionalized polymer vesicles, while covalently immobilized polymer membranes were obtained by spreading LA-functionalized polymer vesicles on gold substrates. Covalently bound membranes were further incubated with the peptide polymyxin B. Interactions with the polymer membrane were detected by EIS. These systems are of great interest to fundamental membrane science and have potential in technological applications, such as drug screening and (bio)sensing

Formation of Viscoelastic Protein Droplets on a Chemically Functionalized Surface

Droplet formation during adsorption of the protein lactoferrin from an aqueous solution on a surface functionalized by plasma deposited poly(acrylic acid) is studied using quartz crystal microbalance and atomic force microscopy. The formation of protein droplets is particularly favored at pH values close to the isoelectric point of lactoferrin, where the molecules carry little excess charge and intermolecular attraction exceeds the molecule-surface interaction. By combining topographic data with information on the system dynamics it is possible to describe the viscoelastic properties of the adsorbate within a quantitative model for non-homogeneous layers.JRC.I.4-Nanobioscience

Membrane protein distribution in composite polymer-lipid thin films

We present a model system to demonstrate that the positioning of biomolecules (membrane proteins) in a nonnative, complex thin film environment can be regulated by the phase behavior of film components. Partial separation between an amphiphilic polymer and a lipid drives the protein to a fluid phase, mechanically more similar to a cellular bilayer

Biomimetic supported membranes from amphiphilic block copolymers

A unique combination of surface chem. and self-assembly of amphiphilic block copolymers was employed to obtain-for the first time-solid-supported biomimetic polymer bilayers. An organized monolayer from sulfur-functionalized poly(butadiene)-b-poly(ethylene oxide) was covalently attached to ultrasmooth gold upon Langmuir-Blodgett transfer. Hydrophobic interactions, on the other hand, were exploited to attach the second monolayer. As a result, we obtained a homogeneous hydrophilic-hydrophobic-hydrophilic structure, similar to supported lipid bilayers by architecture, stability and fluidity. Our polymer bilayers, however, outperform such lipid membranes with regard to tunability of thickness and stability in gaseous environments.As characterized by surface anal. tools (AFM, SPR), solid-supported polymer membranes are smooth with a thickness of ca. 11 nm, resistant to rinsing with aq. solns. and stable upon drying and rehydration. These properties could be attractive for nanotechnol. applications, such as immobilization of functional mols. or nanoparticles, sensor development or prepn. of chem. responsive functional surfaces. [on SciFinder (R)

pH-Dependent Immobilization of Proteins on Surfaces Functionalized by Plasma-Enhanced Chemical Vapor Deposition of Poly(acrylic acid)- and Poly(ethylene oxide)-like Films

The interaction of the proteins bovine serum albumin (BSA), lysozyme (Lys), lactoferrin (Lf), and fibronectin (Fn) with surfaces of protein-resistant poly(ethylene oxide) (PEO) and protein-adsorbing poly(acrylic acid) (PAA) fabricated by plasma-enhanced chemical vapor deposition has been studied with quartz crystal microbalance with dissipation monitoring (QCM-D). We focus on several parameters which are crucial for protein adsorption, i.e., the isoelectric point (pI) of the proteins, the pH of the solution, and the charge density of the sorbent surfaces, with the -potential as a measure for the latter. The measurements reveal adsorption stages characterized by different segments in the plots of the dissipation vs frequency change. PEO remains protein-repellent for BSA, Lys, and Lf at pH 4-8.5, while weak adsorption of Fn was observed. On PAA, different stages of protein adsorption processes could be distinguished under most experimental conditions. BSA, Lys, Lf, and Fn generally exhibit a rapid initial adsorption phase on PAA, often followed by slower processes. The evaluation of the adsorption kinetics also reveals different adsorption stages, whereas the number of these stages does not always correspond to the structurally different phases as revealed by the D-f plots. The results presented here, together with information obtained in previous studies by other groups on the properties of these proteins and their interaction with surfaces, allow us to develop an adsorption scenario for each of these proteins, which takes into account electrostatic protein-surface and protein-protein interaction, but also the pH-dependent properties of the proteins, such as shape and exposure of specific domains.JRC.I.4-Nanotechnology and Molecular Imagin

Highly permeable and selective pore-spanning biomimetic membrane embedded with aquaporin Z

10.1002/smll.201102120Small881185-1190SMAL