Molecular Recognition and Fluorescence Sensing of Monophosphorylated Peptides in Aqueous Solution by Bis(zinc(II)−dipicolylamine)-Based Artificial Receptors

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Abstract
The phosphorylation of proteins represents a ubiquitous mechanism for the cellular signal control of many different processes, and thus selective recognition and sensing of phosphorylated peptides and proteins in aqueous solution should be regarded as important targets in the research field of molecular recognition. We now describe the design of fluorescent chemosensors bearing two zinc ions coordinated to distinct dipicolylamine (Dpa) sites. Fluorescence titration experiments show the selective and strong binding toward phosphate derivatives in aqueous solution. On the basis of 1H NMR and 31P NMR studies, and the single-crystal X-ray structural analysis, it is clear that two Zn(Dpa) units of the binuclear receptors cooperatively act to bind a phosphate site of these derivatives. Good agreement of the binding affinity estimated by isothermal titration calorimetry with fluorescence titration measurements revealed that these two receptors can fluorometrically sense several phosphorylated peptides that have consensus sequences modified with natural kinases. These chemosensors display the following significant features: (i) clear distinction between phosphorylated and nonphosphorylated peptides, (ii) sequence-dependent recognition, and (iii) strong binding to a negatively charged phosphorylated peptide, all of which can be mainly ascribed to coordination chemistry and electrostatic interactions between the receptors and the corresponding peptides. Detailed titration experiments clarified that the phosphate anion-assisted coordination of the second Zn(II) to the binuclear receptors is crucial for the fluorescence intensification upon binding to the phosphorylated derivatives. In addition, it is demonstrated that the binuclear receptors can be useful for the convenient fluorescent detection of a natural phosphatase (PTP1B) catalyzed dephosphorylation.