Nucleoside Bisphosphates/Bisthiophosphates as Selective Zinc-Ion Chelators

Thomas Jantz, Chemistry, Bar-Ilan University, Ramat-Gan, Israel
Alon H. Sayer, Chemistry, Bar-Ilan University, Ramat-Gan, Israel
Dan T. Major, Chemistry, Bar-Ilan University, Ramat-Gan, Israel
Alexandra Vardi-Kilshtain, Chemistry, Bar-Ilan University, Ramat-Gan, Israel
Bilha Fischer, Chemistry, Bar-Ilan University, Ramat-Gan, Israel

Metal-ions like zinc, iron, and magnesium are needed in traces in the human body. An excess of metal-ions can lead to health disorders. Stable metal-ion-ligand-complexes, such as EDTA, are used to remove harmful metal-ion excess. Here, we targeted the development of new, potential, biocompatible Zn²⁺-chelators and compared to those of related adenosine analogues (ADP, ATP). We selected a nucleoside 3',5'- or 2',3'-bisphosphate scaffold, analogues 1/2 and 3/4, because two terminal phosphate groups provide more chelation sites vs. one terminal and one non-terminal phosphate group (e.g. in ADP), and higher metabolic and chemical stability, vs. nucleoside 5'-diphosphate. Therefore, we synthesized 2'-deoxyadenosine 3',5'-bis(thio)phosphate (1, (2)), and adenosine 2',3'-bis(thio)phosphate (3, (4)). Acidity constants of the ligands and stability constants of the ligand-metal-complexes were obtained from pH-potentiometric titrations, and compared to those of related adenosine analogues (ADP, ATP). The possible modes of binding were determined by ¹H and ³¹P NMR and DFT-calculations. ³¹P NMR spectrum clearly showed that both phosphate groups were involved in chelation of 1eq Zn²⁺-ions (Dd P3', -2.1 ppm; P5' -1.7 ppm) as compared to 1eq Mg²⁺-ions (Dd P3', -0.22 ppm; P5' -0.17). According to DFT-calculations, water molecules were found between the Zn²⁺-ion and both phosphate groups of ligand 1. Ligand 1 in solution, adopted a predominant Southern ribose conformer (ca. 84%), gg conformation around C4'-C5' and C5'-O5' bonds, and glycosidic angle in the anti-region (213-270°). We found for 2'-d-adenosine 3',5'-bisthiophosphate, 2, a > 100,000-fold preference for Zn²⁺ (logK 7.91) vs. Mg²⁺. Furthermore, 2'-d-adenosine 3',5'-bisthiophosphate-Zn²⁺-complex was 1000-fold more stable than the Zn²⁺-complex of the corresponding adenosine bisphosphate, 1, (logK 4.65). At physiological pH >99 % of zinc ions were chelated by the 2'-deoxyadenosine 3',5'-bisthiophosphate, 2.