Investigation of cadmium and zinc complexes using solid-state NMR and ab initio calculations.

Abstract

Cadmium-113 NMR spectroscopy has been utilized as a metallobioprobe for studying metalloproteins such as concanavalin-A, calmodulin and TroponinC since 1976. NMR experiments on cadmium nuclei not only provide information on the number and type of coordinating groups but also help understand conformational changes and nature of protein interactions with inhibitors and substrates. Unlike <super>13</super>C, <super>15</super>N and <super>1</super>H nuclei, <super> 113</super>Cd chemical shift spans about 900 ppm making it a valuable tool to differentiate various coordination numbers. Simple isotropic chemical shifts obtained from NMR of solution and solid samples can be used for the above purpose. At the same time, there were many instances when isotropic <super> 113</super>Cd chemical shift value was insufficient to understand the geometry around the central cadmium atom. To overcome this difficulty, chemical shift anisotropy tensor parameters of <super>113</super>Cd are determined using slow spinning and static solid-state NMR experiments. These experimentally determined values can be even more informative with the help of calculated <super> 113</super>Cd chemical shielding values using <italic>ab initio</italic> methods. In this thesis, a protocol using density functional theory available in Gaussian-98 for determining chemical shielding values in cadmium and zinc complexes is presented. Additionally, both theoretical and experimental NMR studies on biologically important molecules have been performed to understand the importance of <super>113</super>Cd chemical shifts in elucidating the structure. Our studies showed that increasing the size of the basis set will surely improve the performance of the <italic>ab initio</italic> calculations in estimating cadmium chemical shifts. Moreover, there is a better agreement between experimental and theoretical values when sadlej basis sets and hybrid functionals are used on molecules having limited intermolecular interactions such as hydrogen bonding or coordinate bonding. This thesis will also show the ability of solid-state NMR experiments in differentiating minor differences in the structure of cadmium molecules (such as anhydrous and hydrous cadmium formates).