| Abstract |
Molecular self-assembly is a well-established bottom up approach for the construction of nano- and micro- materials. It starts from molecules that aggregate to bigger clusters and finally form nanostructures and it proceeds in the absence of external forces in a reversible way. Natural well organized assemblies have impressive properties, for example bacteriochlorophylls in green sulfur bacteria self-assemble in order to harvest solar energy efficiently and therefore are able to photosynthesize in 145m depths in the Black Sea. Inspired by nature, supramolecular chemists are investigating the molecular self-assembly of small building blocks to create materials with enhanced properties that could be exploited in various applications. In this Master thesis we investigate a variety of organic and inorganic compounds for their ability to self-assemble into organized nanostructures.
In the laboratory of bioinorganic chemistry the dipeptide diphenylalanine, famous for its self-assembling properties, has been extensively investigated through covalent attachment to chromophores such as porphyrins, BODIPYs and corroles which resulted in well-defined nanostructures. Motivated from these previous studies, in this thesis we synthesized two organic-inorganic hybrid dyads, namely POM-F and POM-FF, which consist of a Keggin type polyoxometallate (POM-COOH) and carboxyl-protected phenylalanine (NH2-F-OMe) or diphenylalanine (NH2-FF-OMe), respectively. To the best of our knowledge, this is the first example of the covalent incorporation of phenylalanine or diphenylalanine to polyoxometalate. These compounds were fully characterized by means of 1H and 13C NMR, UV-Vis, and IR spectroscopies as well as MALDI-TOF spectrometry. Moreover, we examined the tendency of the POM substituted derivatives to form supramolecular assemblies. The morphology and size of the obtained self-assembled structures were studied through Scanning Electron Microscopy (SEM) and showed that both hybrids assemble into spherical nanostructures. The main factors that determine the architecture of the resulting self-assemblies are the electrostatic interactions between the hybrid and the counter ions as shown by molecular dynamics simulations.
Is diphenylalanine the only way to induce self-assembly? In order to answer this question we explored other moieties that are capable to self-assemble and investigated whether this property is maintained after the covalent attachment to chromophores. Therefore, two other dipeptides namely Ile-Ile and Ile-Ala bearing a series of protecting groups (Fmoc-, Boc-, -Z, -OCH3) were coupled with porphyrin chromophores (TPP-NH2 and TPP-COOH). The resulting conjugates were able to form spherical and flower-like nanostructures. This is the first example where these dipeptides possess self-assembling properties and are covalently attached to porphyrins.
Small peptides are not the only bio-inspired moieties with self-assembling properties. Peptide nucleic acids are another type of compounds that combine all the Van der Waals, hydrogen bonding and/or π-π interactions that exist in peptides with Watson-Crick base pairing. Therefore, two bioconjugates were synthesized through the covalent attachment of Fmoc-PNA-G-(Bhoc)-OH with amino-tetraphenyl-porphyrin (TPPH2-NH2) and amino-boron-dipyrromethene (BDP-NH2). To the best of our knowledge, this is the first covalent coupling of such chromophores with peptide nucleic acids. For both hybrids (PNA–TPP and PNA–BDP) we were able to observe distinctive supramolecular architectures. During these studies we investigated the influence of the solvent system, the concentration and the deposition method on the morphology of the formed nanostructures. In the case of PNA–TPP under all examined conditions well-formed nanospheres were obtained. Interestingly, in the PNA–BDP hybrid by simply altering the solvent mixture, self-assemblies of two different morphologies were formed (spherical and flake shaped). Absorption and emission studies suggested the formation of J-aggregates in all the obtained nanostructures. The nano-architectures assembled by PNA conjugates are capable of light-harvesting and producing hydrogen using Pt nanoparticles as a photocatalyst.
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Bio-inspired molecular self-assembly based on organic and inorganic moieties
