Study of the Cation···p Interaction in the Ternary Systems Involving Aromatic Units of Amino Acids

Non-covalent interactions play a key role in many areas of modern chemistry, especially in the field of supramolecular chemistry. In particular, interactions involving aromatic rings are key processes in both chemical and biological recognition. Interactions involving p systems are widely used in supramolecular chemistry and they are very important binding forces that determine the packing of organic molecules in crystals. On the other hand, the presence of cation···p interactions is also crucial for the characteristics and stability of proteins. As already known, certain amino acids have aromatic groups in their side chains, which would allow their interaction with cations present in the environment. Furthermore, considering that other amino acids have protonated groups in their side chains, is more than likely for cation···p contacts to occur in a protein involving side chains of amino acids. Aromatic chains of phenylalanine, tyrosine and tryptophan may act therefore as important binding sites for cations. In this work a study of the interaction in M⁺···p···p systems formed by the combination of two aromatic rings and a cation has been carried out. In order to gather more information about the nature of the interaction an energy decomposition analysis has been carried out by means of the Local Molecular Orbital- Energy Decomposition Analysis (LMO-EDA) approach, the energy being divided in electrostatic, repulsion, induction and dispersion contributions. The interacting units have been chosen as to represent the side chain of different amino acids. Therefore, benzene, phenol and indole have been considered as aromatic moieties, whereas the cationic unit is guanidinium in order to model the side chain of arginine.