Peptides consisting of alternated hydrophilic and hydrophobic amino acids have a natural tendency to generate ordered structures by self-assembling, a property which makes them promising materials for synthetic scaffolds in the preparation of artificial tissues and biosensors. Self-assembling oligopeptides belong to a recently discovered novel class of biomaterials. These materials are self-complementary amphiphilic oligopeptides that have regular repeating units of positively charged residues (lysine or arginine) and negatively charged residues (aspartate or glutamate) separated by hydrophobic residues (alanine or leucine). The alternation of polar and non polar pending groups and of positively and negatively charged residues makes these peptides capable of self-assembling.
The adsorption on TiO2 surface of two dipeptides AE (L-ala - L-glu) and AK (L-ala - L-lys), that are "building blocks" of self-complementary amphiphilic oligopeptides and are therefore a good model in the interpretation of the complex peptide spectra, has been investigated both theoretically and experimentally. Classical molecular dynamics simulations have been used to study the adsorption properties of H-Ala-Glu-NH2 and H-Ala-Lys-NH2 dipeptides onto rutile (110) TiO2 layers in water solution. Several peptide conformers have been considered simultaneously upon the surface. The inherent complexity of the solvated biomolecules is recognized in terms of multiple coordination, and the most probable contact points between the molecules and the surface are identified. Carbonyl oxygens as well as nitrogen atoms are all possible Ti coordination points. Local effects are responsible of adsorption and desorption events. Self-interaction effects can induce molecular reorientations giving less strongly adsorbed species.
The chemical structure and composition of thin films of both dipeptides on TiO2 were investigated by XPS, while the molecular orientation of the adsorbate was checked by angular dependent NEXAFS measurements at C-K and N-K edges. Theoretical ab-initio calculations (ΔSCF) were also performed to simulate the spectra allowing a direct comparison between experiment and theory. |