Mechanisms of Cd (II) binding to GMP and UMP: a combined conductometry, isothermal titration calorimetry and NMR study

Abstract

In this research, conductometry was applied as a rapid method to screen functional ligands of cadmium ion-imprinted polymers for the first time. To better understand the binding mechanisms of Cd(II) to nucleoside monophosphates (NMPs), isothermal titration calorimetry (ITC) and nuclear magnetic resonance spectroscopy (NMR) experiments were carried out. First, the magnitude of conductivity change, Δσ, was found to be proportional to the ligand’s binding capacity to cadmium, and the ΔσGMP ≈ ΔσdGMP = 103.5 μs $$\cdot $$ cm−1. Second, the ITC experiments indicated that the coordination binding of Cd(II) and guanosine monophosphate (GMP) in 4-(2-hydroxyerhyl)piperazine-1-erhanesulfonic acid (HEPES)/boric acid buffers were demonstrated as different thermodynamic process: an exothermic process (ΔH = -1.446 kcal $$\cdot $$ mol−1) in HEPES, and an exothermic process (ΔH = − 1.495 kcal $$\cdot $$ mol−1) followed by an endothermic one (ΔH = 1.383 kcal mol−1) in boric acid. In contrast, the formation of the Cd(II)–uridine monophosphate (UMP) system was an endothermic process in both buffers. NMR experiments indicated that the shift Δδ of the GMP’s proton signals were different, especially for H1: 0.37 ppm in HEPES, and 0.03 ppm in boric acid. Referring to the results of ITC experiments, the major Cd(II) binding sites of GMP in HEPES buffer was the guanosine group, and phosphate group was another site in the boric acid buffer. By contrast, the major Cd(II) binding site of UMP was the phosphate group in both buffers. In general, investigating the functional ligand–cadmium binding mechanism has provided important theoretical and experimental guidance for the subsequent preparation of cadmium ion-imprinted materials.