A computational study on the interaction of the nitric oxide ions NO ⁺ and NO with the side groups of the aromatic amino acids

The interaction of the nitric oxide ions NO⁺ and NO^- with benzene (C₆H₆) and the aromatic R-groups of the amino acids phenylalanine (Phe), tyrosine (Tyr), histidine (His), and tryptophan (Tip) have been examined using the DFT method B3LYP and the conventional electron correlation method MP2. In particular, the structures and complexation energies of the resulting half-sandwich Ar⋯NO^+/- and sandwich [Ar⋯NO⋯Ar]^+/- complexes have been considered. For the Ar⋯NO⁺ complexes, the presence of an electron rich heteroatom within or attached to the ring is found to not preclude the cation⋯π bound complex from being the most stable. Furthermore, unlike the anionic complexes, the π⋯cation⋯π ([Ar⋯NO⋯Ar] ⁺) complexes do not correspond to a "doubling" of the parent half-sandwich.