A density functional study of 14N NQR parameters of imidazole derivatives and extrapolation to PfHRP2-Fe3+-PPIX complex

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 [Fe3+ (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 Fe3+ 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-Fe3+-PPIX complex.