Self-assembly synthetic lipid bilayer

It must be emphasised that self-assembly is very far from a unique feature of supramolecular systems - it is ubiquitous throughout life chemistry. Biological systems aside, self-assembly is also commonplace throughout chemistry. The growth of crystals, the formation of liquid crystals, the spontaneous generation of synthetic lipid bilayers, the synthesis of metal co-ordination complexes, and the alignment of molecules on existing surfaces are but a few of the many manifestations of self-assembly in chemical systems. 

Fig. 3. (a) Chemical structure of a synthetic cyclic peptide composed of an alternating sequence of D- and L-amino acids. The side chains of the amino acids have been chosen such that the peripheral functional groups of the flat rings are hydrophobic and allow insertion into lipid bilayers, (b) Proposed structure of a self-assembled transmembrane pore comprised of hydrogen bonded cyclic peptides. The channel is stabilized by hydrogen bonds between the peptide backbones of the individual molecules. These synthetic pores have been demonstrated to form ion channels in lipid bilayers (71). 

In THE PAST DECADE, IMPROVEMENTS IN infrared spectroscopic instrumentation have contributed to significant advances in the traditional analytical applications of the technique. Progress in the application of Fourier transform infrared spectroscopy to physiochemical studies of colloidal assemblies and interfaces has been more uneven, however. While much Fourier transform infrared spectroscopic work has been generated about the structure of lipid bilayers and vesicles, considerably less is available on the subjects of micelles, liquid crystals, or other structures adopted by synthetic surfactants in water. In the area of interfacial chemistry, much of the infrared spectroscopic work, both on the adsorption of polymers or proteins and on the adsorption of surfactants forming so called "self-assembled" mono- and multilayers, has transpired only in the last five years or so. 

With time and improved synthetic protocols, larger templates (fuUerenes, dendrimers, nanoparticles, colloids, micelles, lipid bilayers, self-assembled block copolymers, oligonucleotides, DNA and proteins) have been imprinted [14] and the choice of matrices has expanded to liquid crystal polysiloxanes, carbon networks, zeolites, layered aluminophosphates and colloidal crystals, though organic polymer networks remain the dominant imprint casting medium [14]. 

During the past decade, numerous studies have been undertaken to develop synthetic ionophores that might permit cations or molecules to pass through a lipid bilayer [1]. Naturally-occurring gramicidin A is known to form transmembrane channels [2] and efforts to prepare a cation-conducting channel have been reported as well [3]. In our work, we have studied the selectivity of numerous crown ethers, lariat ethers [4, 5], and multi-armed versions of the latter [6]. Much has been learned about flexible ionophores and we have now attempted to utilize the concepts of flexibility and self-assembly to permit construction of a cation- or molecule-conducting channel. 

The stractural and functional complexity of biomembranes has ehal-lenged researehers to develop simpler artificial models to mimie their properties. Amphiphilic block copolymers are of particular interest, beeause of the dual environmental affinity that is associated with covalently bound hydrophobie and hydrophilic blocks. These strive to minimize their eontaet, and therefore drive self-assembly into assemblies with different arehi-teetures. Based on their chemical specificity, as for example the hydrophilie-to hydrophobie ratio, amphiphilic block copolymers can self-assemble in dilute aqueous solutions into micelles, vesicles, tubes, wire-like structures, nanopartieles, or planar membranes at water-air interfaces. Synthetic membranes have greater mechanical stability than phospholipids because of the higher moleeular weight (Mw) of amphiphilic block copolymers, and thus are thicker and stiffer than lipid bilayers. 

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