A density functional study of 14N NQR parameters of imidazole derivatives and extrapolation to PfHRP2–Fe 3+-PPIX complex

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In this paper, density functional theory (DFT) was used to calculate 14N nuclear quadrupole coupling constants (NQCC), χ, and asymmetry parameters, η, for a series of imidazole derivatives: imidazole, 5-methylimidazole and histidine. These calculations were carried out with the PW91P86 method via the Gaussian 98 package. A systematic theoretical investigation of the different environmental effects on ( χ, η) values of amino 14N1 and imino 14N2 of imidazole ring of these compounds, reveals that the local surrounding of nitrogen atoms play an important role in determining their χ and η values. Our calculations in solution show that adding explicit solvent molecules to the polarizable continuum model (PCM) has a strong effect on ( χ, η) values, thereby indicating that for long-range effects, PCM, is not sufficient to describe the whole solvent effects. We also evaluate the influence of [Fe 3+ ( S = 1/2)] on the ( χ, η) values of proximal and remote nitrogens of an axial ligand and compare with those of free ligands. The results show that Fe 3+ has a strong effect on the ( χ, η) values of proximal nitrogen unlike remote nitrogen. Finally, our results predict ( χ = 1.56 MHz, η = 0.690) for proximal nitrogen and ( χ = 2.75 MHz, η = 0.169) for remote nitrogen in PfHRP2–Fe 3+-PPIX complex.