Supramolecular structures of polypeptides

The formation of supramolecular structures is a central issue in bio- and nanotechnology and therein plays an important role for many novel developments such as biocompatible materials for integrating living cells or coating for implantable sensing devices as well as for designing smart molecular sensor surfaces for miniaturized bioanalytical techniques. The main focus of this thesis is the investigation of polypeptide layers on surfaces, in particular the formation of self-assembled monolayers (SAMs). SAMs made of long hydrocarbon chains or aromatic molecules comprising surface active and other functional groups have been deeply characterized over decades. Surprisingly, peptide SAMs did not meet the same strong interest despite their great potential in several areas. Natural (20) and artificial amino acids with different physical and chemical properties offer a plethora of novel possibilities to design coating of solid surfaces. In fact their combination in simple oligopeptide sequences provides a huge number of molecular components for the formation of SAMs. Control of SAM features such as density, flexibility, adaptability, interface properties, by tailoring the amino acid sequence opens a new route to manifold applications ranging from fundamental research to nano- and biotechnology. In this thesis different and complementary techniques have been used to investigate the self-assembling ability of polypeptides on surfaces with different physical and chemical properties. Infrared spectroscopy and circular dichroism allowed the determination of the peptide secondary structure on surfaces and in solution. Surface plasmon resonance provided the optical thicknesses of peptide SAMs on gold whereas contact angle measurements have been used to characterize their interfacial properties. In the first part of the thesis, a series of hydrophobic peptides (Ac-Cys-Alax-CONH2, x=3-6) have been self assembled on gold via the thiol group present in the cysteine. The main interest was whether these peptides formed densely packed SAMs with a defined secondary structure within the layer and how the properties of such SAMs depended on the length of the polypeptide backbone. The second part concerns the study of the interactions of an amphipathic helical peptide with various functionalized substrates. The aim was to investigate whether a proper design of the peptide could be used to predetermine its secondary structure on the substrate and whether the orientation of the peptide on the surface could be directly controlled via certain parameters of the surrounding medium. Since the hydrophilic residues of the peptide are histidines, a particular effort was spent in characterizing the peptide adsorption on substrates exposing nitrilotriacetic acid (NTA) moieties. Once established that the adsorption process did not alter its helical structure, the peptide was adsorbed on a substrate exposing two different functional groups: NTA, that could interact with the histidine residues, and methyl groups, that could interact with the hydrophobic ones. The goal was to bind the peptide in different orientation to the same substrate, exploiting the particular spatial distribution of its residues. The third part of this thesis concern the formation of nano- and micro-sized fibrils through molecular self-assembly of simple oligopeptides and protected amino acids. Such systems were chosen as models to investigate the role of the different driving forces in the formation of fibrillar structures. The aromatic protecting group being fixed, it could be shown that the protected amino acids formed fibrils with different sizes and morphologies, or did not form fibrils at all, depending on how strongly their side chains interact. The possibility of forming fibrils by using protected amino acids is of major importance not only in bio-related field but also in organic nanotechnology.