Rigid luminescent bis-zinc(II)-bis-cyclen complexes for the detection of phosphate anions and non-covalent protein labeling in aqueous solution

A series of water-soluble bis- and tetrakis-Zn^II-cyclen complexes with rigid structures was prepared to enhance the carboxylate and phosphate ion binding response in contrast to analogues with less-confined molecular structures. Boc-protected 6-chloro-1,3,5-triazine-bis-cyclen was coupled to different aryl and alkyl moieties in moderate to high yields and subsequently converted into the corresponding bis- or tetrakis-Zn ^II-cyclen complexes. The bis-Zn^II-cyclen moiety is known for its affinity to anions. Depending on the arene substituent some of the synthesized synthetic receptors are luminescent. They were studied by absorption and emission spectroscopy for their response to the presence of phosphate anions of biological relevance in buffered aqueous solution at neutral pH and for their affinity to the genetically encodable oligoaspartate and glutamate sequences (D₄ and E₄ tag) recently introduced. The rigid structures of the compounds enhance the electronic coupling between the metal complex binding site and the reporter dye. This leads to an increased anion binding response in homogeneous aqueous solution.