Characterization of complexes of nucleoside-5′-phosphorothioate analogues with zinc ions

On the basis of the high affinity of Zn²⁺ to sulfur and imidazole, we targeted nucleotides such as GDP-β-S, ADP-β-S, and AP₃(β-S)A, as potential biocompatible Zn²⁺-chelators. The thiophosphate moiety enhanced the stability of the Zn²⁺- nucleotide complex by about 0.7 log units. ATP-α,β-CH ₂-γ-S formed the most stable Zn²⁺-complex studied here, log K 6.50, being ∼0.8 and ∼1.1 log units more stable than ATP-γ-S-Zn²⁺ and ATP-Zn²⁺ complexes, and was the major species, 84%, under physiological pH. Guanine nucleotides Zn²⁺ complexes were more stable by 0.3-0.4 log units than the corresponding adenine nucleotide complexes. Likewise, AP₃(β-S)A-zinc complex was ∼0.5 log units more stable than AP₃A complex. ¹H- and ³¹P NMR monitored Zn²⁺ titration showed that Zn ²⁺ coordinates with the purine nucleotide N7-nitrogen atom, the terminal phosphate, and the adjacent phosphate. In conclusion, replacement of a terminal phosphate by a thiophosphate group resulted in decrease of the acidity of the phosphate moiety by approximately one log unit, and increase of stability of Zn²⁺-complexes of the latter analogues by up to 0.7 log units. A terminal phosphorothioate contributed more to the stability of nucleotide-Zn²⁺ complexes than a bridging phosphorothioate.