Complexes of Oligopeptides of Structure-Determining Amino Acids with Phosphotungstic Acid

Peptides tend to form anisotropic structures, being both asymmetric and chiral. Structure-determining peptides include phenylalanine (Phe) and tyrosine (Tyr) amino acids as they contain aromatic rings, which are sterically demanding and prone to self-assembly via π-stacking. Pursuing the mechanisms of protein interactions with oxide nanoparticles, we used Keggin phosphotungstic acid polyoxometalate (POM) as a model. Six complexes of the POM with different peptide-based ligands were studied, including Phe, Ala (Alanine), and Tyr. Phe–Ala formed a layered structure with pockets of π-stacking involving four Phe residues. Ala–Phe formed two structures based on the rate of formation. The faster forming structure had a loose pattern of POMs in columns surrounded by chains of the ligand with alternating H-bonding and π-stacking. The slower-forming one had a denser network, also with columns of POMs, but with a more complex network of peptides, including π-stacks of three residues appearing as a "web" rather than a "chain". Ala–Ala showed mainly H-bonding connecting the molecules, with the peptide filling the spaces between POMs rather than controlling the structure. Ala seemed to mainly act as a bridge between three POMs, and it had the effect of forming a highly porous structure reminiscent of metal–organic frameworks (MOFs). Tyr formed long columns of the amino acid with both vertical and lateral H-bonding, resulting in alternating layers of POMs and parallel Tyr columns. These structures provide insights into the interactions between biomolecules and POMs, which is valuable for the design and synthesis of POM-derived composite materials.