| Title
| - Cation [M = H+, Li+, Na+, K+, Ca2+, Mg2+, NH4+, and NMe4+] Interactions with theAromatic Motifs of Naturally Occurring Amino Acids: A Theoretical Study
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| Abstract
| - 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 aminoacids (viz., phenylalanine, tyrosine, tryptophan and histidine). As the metal ion approaches the π-frameworkof the model systems, they form strongly bound cation-π complexes, where the metal ion is symmetricallydisposed with respect to all ring atoms. In contrast, proton prefers to bind covalently to one of the ringcarbons. The NH4+ and NMe4+ ions have shown N−H···π interaction and C−H···π interaction with thearomatic motifs. The interaction energies of N−H···π and C−H···π complexes are higher than hydrogenbonding interactions; thus, the orientation of aromatic side chains in protein is effected in the presence ofammonium ions. However, the regioselectivity of metal ion complexation is controlled by the affinity of thesite of attack. In the imidazole unit of histidine the ring nitrogen has much higher metal ion (as well asproton) affinity as compared to the π-face, facilitating the in-plane complexation of the metal ions. Theinteraction 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+ ≅ NH4+> NMe4+. The variation of the bond lengths and the extent of charge transferupon complexation correlate well with the computed interaction energies.
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