Synthese von Glycoazobenzol-Konjugaten zur Verbesserung der Photoschaltbarkeit von Glyco-SAMs
The outer cell envelope of eukaryotic cells is called the glycocalyx, which is comprised of complex glycoproteins and glycolipids. Signalling between cells occurs either through substrates dissolved in the extracellular liquid and the glycocalyx or through direct cell-cell-interactions. In both cases, the so-called lectins, proteins which bind in a selective manner to carbohydrate epitopes are involved. One of these lectin-mediated processes is bacterial adhesion. Whether successful interactions between substrate and sugar ligand take place is not only dependent on the type of sugar, but also on the orientation of the carbohydrate ligand on the surface. As the glycocalyx is a highly complex and dynamic system, suitable model systems must be developed for investigation of the orientational and conformational dependence on molecular recognition. A suitable model system can be designer surfaces such as self-assembled monolayers (SAMs) composed of azobenzene based glycoconjugates (photoswitchable glyco-SAMs). After irradiation with light of a certain wavelength, as the result of the isomerisation of the N=N double bond, the orientation of the sugar head group changes drastically. In earlier work it was shown, that the bacterial adhesion to photoswitchable glyco-SAMs can be switched between an “on” and “off” state reversibly depending on the E or Z state of the glycoconjugate. In the first part of this work a photoswitchable azobenzene mannobioside for the functionalization of gold surfaces was synthesised and the E/Z isomerisation was investigated. By employing a mannobioside head group the model system should approximate the natural pattern of the glycocalyx by increasing the molecular complexity. The second part of this work is dealt with the improvement and quantification of the E-Z isomerisation of glyco-SAMs. Therefore, three different approaches, which are based on the latest research results, were followed. As it is known that E/Z isomerisation of azobenzene requires space, the first approach was based on providing the needed space. But instead of employing diluter molecules, as it was done in previous work, the azobenzene derivatives were functionalized with a bulky protection group, which was cleaved after SAM formation. The photoisomerisation of the molecules for these so called “self-diluting” SAMs was investigated in solution and on gold surfaces. In the second approach, molecules with rigid biphenyl backbones were synthesised in Suzuki cross coupling reactions and investigated to build up very densely packed SAMs. SAMs constructed of such molecules showed excellent switching properties, due to their dense packing and the resulting π-π-interactions. For the first time the E/Z isomerisation of glyco-SAMs could be quantified. Following an additional idea, space between the azobenzene derivatives on the gold surface could be provided using molecular tripods for the switching unit.
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