Magnetic resonance studies of copper (II) sorbitol complex, in solution, reveal a supramolecular structure compatible to the crystal structure

Abstract

Although the Cu²⁺ -sorbitol complex [Cu²⁺ -Sorb] structure in crystalline state has been determined by X rays, it is not known in solution, where most studies of this complex are performed. Therefore, the goal of this work was to obtain information about the structure of this complex in aqueous solution using nuclear magnetic resonance and electron paramagnetic resonance spectroscopies. The magnetic resonance results indicate that the complex is formed at approximately pH 12. In this pH the sorbitol ¹ H relaxation times were so short (broad line) that was not possible to use standard nuclear magnetic resonance parameters (nuclear Overhauser effect and spin-spin coupling constants values) to solve the three-dimensional structure. However, valuable structural information about the complex in solution was obtained. The relaxation results indicate that the Cu²⁺ ions are buried in the structure and not accessible to solvent; the ¹ H and ¹³ C spectra shows strong paramagnetic shift effect indicating short distance between these nuclei and Cu²⁺ in the structure. No electron paramagnetic resonance signal was observed in pH 12 indicating strong Cu²⁺ - Cu²⁺ dipolar interaction, compatible to Cu²⁺ -Cu²⁺ distances measured in crystal, from 1.148 to 1.393 Angstroms. The complex self-diffusion coefficient (D) of 1.58 × 10^-10 m² /s value, determined by Diffusion-Ordered Spectroscopy, is compatible to a molecular weight of 3-6 KDa. Therefore, these results corroborate that the [Cu²⁺ -Sorb] complex is assembled in solution, at pH 12, with several structural parameters compatible to the toroidal hexadecacuprate supramolecular structure determined in solid state.

Keywords: Cu-sorbitol complex; carbohydrate-metal complexes; electron paramagnetic resonance; nuclear magnetic resonance.