Cation [M = H+, Li+, Na+, K+, Ca2+, Mg2+, NH4+, and NMe4+] interactions with the aromatic motifs of naturally occurring amino acids:: A theoretical study

Ab initio (HF, MP2, and CCSD(T)) and DFT (B3LYP) calculations were done in modeling the cation (H+, Li+, Na+, K+, Ca2+, Mg2+, NH4+, and NMe4+) interaction with aromatic side chain motifs of four amino acids (viz., phenylalanine, tyrosine, tryptophan and histidine). As the metal ion approaches the pi-framework of the model systems, they form strongly bound cation-pi complexes, where the metal ion is symmetrically disposed with respect to all ring atoms. In contrast, proton prefers to bind covalently to one of the ring carbons. The NH4+ and NMe4+ ions have shown N-H center dot center dot center dot pi interaction and C-H center dot center dot center dot pi interaction with the aromatic motifs. The interaction energies of N-H center dot center dot center dot pi and C-H center dot center dot center dot pi complexes are higher than hydrogen bonding interactions; thus, the orientation of aromatic side chains in protein is effected in the presence of ammonium ions. However, the regioselectivity of metal ion complexation is controlled by the affinity of the site of attack. In the imidazole unit of histidine the ring nitrogen has much higher metal ion (as well as proton) affinity as compared to the pi-face, facilitating the in-plane complexation of the metal ions. The interaction energies increase in the order of 1-M < 2-M < 3-M < 4-M < 5-M for all the metal ion considered. Similarly, the complexation energies with the model systems decrease in the following order: Mg2+ > Ca2+ > Li+ > Na+ > K+ congruent to NH4+ > NMe4+. The variation of the bond lengths and the extent of charge transfer upon complexation correlate well with the computed interaction energies.