Biologically Functionalized Block Copolymer Vesicles

During the last decade self-organization of soft materials has shown to be valuable for the creation of a wide variety of nanostructures that could be used for applications in fields ranging from materials science to biology [1].
In this context amphiphilic block copolymers are of particular interest due to their ability to self-assemble in aqueous media and their broad accessibility to different length and time scales and levels of interaction. Similar to conventional low molar mass surfactants they may form micelles, vesicles or lyotropic mesophases. These aggregates can be significantly more stable than those formed by low molar mass amphiphiles and additionally they can be further stabilized by a subsequent crosslinking polymerization. The long-term stability of these structures makes them well adapted for applications and guarantees a constant non-changing environment for embedded therapeutic or analytic molecules. Feasibility of various applications, i.e., the use of block copolymer vesicles as transfection vectors, protective shells for sensitive enzymes, or as confined reaction vessels that allow to perform (bio-) chemistry even at a single molecular level, has been demonstrated [2-4].

The walls of the block copolymer vesicles are formed by membrane-like superstructures that can be regarded as mimetics of biological membranes. In fact, we were able to show that despite their enormous thickness and stability such block copolymer membranes can be used as a matrix for functional reconstitution of membrane proteins.




Figure 2: Schematic representation of a block copolymer membrane with inserted membrane protein. The dimensional mismatch between protein and membrane causes locally considerable bilayer compression.

Generally during reconstitution experiments, the membrane proteins are randomly inserted without any preferred direction. Unfortunately many potential technical applications of such reconstituted systems depend on the correct orientation of the protein. In contrast to conventional low molar mass lipids amphiphilic block copolymers offer here a particularly interesting approach. Different water-soluble polymers are inherently incompatible and undergo phase separation in aqueous media. Hence, membranes formed by ABC triblock copolymers (with water soluble blocks A and C and a hydrophobic middle block B) are asymmetric: one side is predominantly covered by the blocks A and the other by the blocks B. This leads to containers with chemically different inner and outer surface and, simultaneously, the resulting ABC membranes represent an asymmetric matrix for the directed insertion of membrane proteins [5].

[1]	Hybrid materials from amphiphilic block copolymers Corinne Nardin, Wolfgang Meier Reviews in Molecular Biotechnology 90 (2002) 17-26
[2]	Amphiphilic block copolymer nanocontainers as bioreactors C. Nardin, J. Widmer, M. Winterhalter, W. Meier European Physical Journal E, Volume 4, Number 4, 403 - 410
[3]	Ion-carrier controlled precipitation of calcium phosphate in giant ABA triblock copolymer vesicles Marc Sauer, Thomas Haefele, Alexandra Graff, Corinne Nardin and Wolfgang Meier Chemical Communications, 2001, (23), 2452 - 2453
[4]	Virus-assisted loading of polymer nanocontainer Alexandra Graff, Marc Sauer, Patrick Van Gelder, and Wolfgang Meier Proc. Acad. Sci. (PNAS), 99, 5064 (2002)
[5]	Vesicles with asymmetric membranes from amphiphilic ABC triblock copolymers R. Stoenescu and W. Meier Chemical Communications, 2002, (24), 3016 - 3017

Contact:

Wolfgang Meier
Department of Chemistry University of Basel Switzerland

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