A Water-Soluble Bis(thiosemicarbazone) Ligand. A Sensitive Probe and Metal Buffer for Zinc

The first member of a water-soluble family of bis(thiosemicarbazone) ligands is reported. It forms a 1:1 complex with Zn-II that absorbs intensely in the visible region (lambda(max) = 414 nm; epsilon = 1.8(4) x 10(4) M-1 cm(-1); pH 7.3). Its affinity for Zn-II (K-D = 5.9(3) x 10(-9) M at pH 7.3) was determined by competition with ligand ethylene glycol O,O'-bis(2-aminoethyl)-N,N,N',N'-tetraacetic acid. Its potential application as a chromophoric probe was demonstrated by estimation of the Zn-II binding affinities of the soluble metal-binding domains of two plant metal-transporting proteins.