Chiral recognition and supramolecular self-assembly of adsorbed amino acids and dipeptides at the submolecular level

This thesis deals with the supramolecular organization of chiral aminoacids and dipeptides on metal surfaces. The emergence of homochirality in biomolecular systems is one of the most intriguing open questions of Nature. The self-assembly and amplification of chiral subunits into higher-order species is crucial in understanding the development of homochirality in biological function. Apart from being related to the very first organic molecules synthesized on earth, understanding the basics of chiral recognition and discrimination is of high technological relevance. In fact, the chemistry and technology of production and separation of enantiomers has evolved into a branch of materials science under the name of Chirotechnology. Even before the concept of molecule existed, Pasteur pointed out that the asymmetry of the crystals of tartaric acid he observed was related to the asymmetry of the components of which they were made. Nowadays, we can asses the landscape of these components even at the submolecular level by means of Scanning Tunneling Microscopy (STM). The aim of this thesis is twofold: primary to get an insight into the fundamental aspects of chiral recognition, under the main inspiring question: How does chiral recognition take place at the single molecule level? and secondly: How can we tune the expression of chirality on a metal surface to achieve global templation? The first part of this thesis presents a comprehensive study of the basic requirements for amino acid chiral templation of metal surfaces. By comparatively studying the adsorption behaviour of L-Phenylalanine (L-Phe) and L-Tyrosine (L-Tyr) on Cu(110) we show that conformational rigidity is a key parameter for the expression of footprint homochirality on metal surfaces. This parameter can be tuned by the appropriate choice of the amino acid residue. The molecular structure of the adsorbed molecules is studied by a combination of reflection absorption infrared spectroscopy (RAIRS), STM and molecular dynamics simulations (MD). The second part of the thesis focuses on the transfer of molecular chirality to supramolecular structures of Di-Phenylalanine (Phe-Phe) on Cu(110). We have followed the chiral recognition and discrimination among individual Phe-Phe enantiomers by STM and rationalized it using first principles and classical molecular dynamics simulations. The geometrical parameters of the final supramolecular structures are studied by near edge absorption fine structure (NEXAFS). Our results show that molecules whose static structure is not optimized for specific binding can selectively recognize each other and link through mutually induced conformational changes. The general idea of induced molecular fit in biomolecular recognition was introduced by Linus Pauling more than 50 years ago. We now report direct evidence of this dynamic scenario at the single molecule level. The last part of this thesis deals with the adsorption phase diagram of Phe-Phe on Cu(110). The encountered adsorption motifs and the chemical processes involved in the phase transitions are investigated by a combination of STM, X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and thermal programme desorption (TPD). The resulting supramolecular assemblies expose chiral cavities that can potentially be used as enantioselective adsorption sites.


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