Recently, we found that an alkoxide-bridged dinuclear zinc(II) complex of 1,3-bis[bis(pyridin-2-ylmethyl) amino]propan-2-olate acts as a novel phosphate capture molecule under physiological conditions. The dizinc(II) complex forms a stable 1:1 complex with a phosphate monoester dianion in aqueous solutions. The X-ray crystal analysis of the 1:1 dizinc(II) complex with a
p-nitrophenyl phosphate dianion disclosed that each phosphate oxygen anion binds to a zinc(II) at the fifth coordination site and that the two zinc(II) ions are separated by a distance of 3.6 Å. Thus, the dizinc(II) complex having a vacancy on the two zinc(II) ions is suitable for the access of a phosphate monoester dianion. In an aqueous solution, the dizinc(II) complex strongly binds to the phenyl phosphate dianion (
Kd=2.5×10
-8 M) at a neutral pH. The anion selectivity indexes against SO
42-, CH
3COO
-, Cl
-, and the diphenyl phosphate monoanion at 25°C are 5.2×10
3, 1.6×10
4, 8.0×10
5, and >2×10
6, respectively. In addition, the 1:1 formation of the dizinc(II) complex and an inorganic phosphate dianion (HOPO
32-) was clearly detected by matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) based on a characteristic mass shift and a total-charge change of the phosphate (from -2 to +1). This finding introduced a new procedure for the analysis of phosphorylated compounds using the dizinc(II) complex as an MS probe. Therefore, we termed the dinuclear metal complex "Phos-tag" as a phosphate-binding tag molecule.
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