Interaction of Cd and Zn with biologically important ligands characterized using solid-state NMR and ab initio calculations

For the first time, coordination geometry and structure of metal binding sites in biologically relevant systems are studied using chemical shift parameters obtained from solid-state NMR experiments and quantum chemical calculations. It is also the first extensive report looking at metal-imidazole interaction in the solid state. The principal values of the ¹¹³Cd chemical shift anisotropy (CSA) tensor in crystalline cadmium histidinate and two different cadmium formates (hydrate and anhydrate) were experimentally measured to understand the effect of coordination number and geometry on ¹¹³Cd CSA. Further, ¹³C and ¹⁵N chemical shifts have also been experimentally determined to examine the influence of cadmium on the chemical shifts of ¹⁵N and ¹³C nuclei present near the metal site in the cadmium-histidine complex. These values were then compared with the chemical shift values obtained from the isostructural bis(histidinato)zinc(II) complex as well as from the unbound histidine. The results show that the isotropic chemical shift values of the carboxyl carbons shift downfield and those of amino and imidazolic nitrogens shift upfield in the metal (Zn,Cd)-histidine complexes relative to the values of the unbound histidine sample. These shifts are in correspondence with the anticipated values based on the crystal structure. Ab initio calculations on the cadmium histidinate molecule show good agreement with the ¹¹³Cd CSA tensors determined from solid-state NMR experiments on powder samples. ¹⁵N chemical shifts for other model complexes, namely, zinc glycinate and zinc hexaimidazole chloride, are also considered to comprehend the effect of zinc binding on ¹⁵N chemical shifts.