The effects of metal binding on a nucleobase: the experimental charge density and electrostatic potential in 1H(+)-adeniniumtrichlorozinc(II) at 123 K and its relationship to that in adenine hydrochloride hemihydrate

The charge-density distribution in 1H(+)-adeniniumtrichlorozinc(II) has been determined from X-ray diffraction data collected to sin theta/lambda = 1.32 Angstrom(-1) at 123 K. The electrostatic potential, isolated from the crystal lattice, and the deformation density in the nucleobase have been calculated following multipole refinements based on the rigid pseudoatom model of Stewart. These and the molecular dimensions have been compared with results from the charge-density study of adenine hydrochloride hemihydrate by Cunane & Taylor [Acta Cryst. (1993). B49, 524-530] to determine the effects of metal binding on the nucleobase. The main conclusions are that while the bond lengths and angles in the pyrimidine ring are similar, those in the imidazole ring are significantly perturbed on complexation; lone-pair electron density at N7 is observed in both structures and lies significantly off the plane of the nucleobases; the positive electrostatic potential of the complexed base extends much further from the molecule than in the uncomplexed one and the regions of negative potential at N3 and N7 are depleted in the complexed base. The observed enhancement of positive electrostatic potential in the nucleobase on binding to zinc is presented in support of a model for the mechanism of reversible unwinding of DNA in the presence of zinc ions.