Imino–phenolic–pyridyl conjugates of Calix[4]arene (L₁ and L₂) as primary fluorescence switch-on sensors for Zn²⁺ in solution and in HeLa Cells and the recognition of pyrophosphate and ATP by [ZnL₂]

Abstract

Pyridyl-based triazole-linked calix[4]arene conjugates, viz. L₁ and L₂, were synthesized and characterized. These two conjugates were shown to be selective and sensitive for Zn²⁺ among the 12 metal ions studied in HEPES buffer medium by fluorescence, absorption, and visual color change with the detection limit of ∼31 and ∼112 ppb, respectively, by L₁ and L₂. Moreover, the utility of the conjugates L₁ and L₂ in showing the zinc recognition in live cells has also been demonstrated using HeLa cells as monitored by fluorescence imaging. The zinc complexes of L₁ and L₂ were isolated, and the structure of [ZnL₁] has been established by single-crystal XRD and that of [ZnL₂] by DFT calculations. TDDFT calculations were performed in order to demonstrate the electronic properties of receptors and their zinc complexes. The isolated zinc complexes, viz. [ZnL₁] and [ZnL₂], have been used as molecular tools for the recognition of anions on the basis of their binding affinities toward Zn²⁺. [ZnL₂] was found to be sensitive and selective toward phosphate-bearing ions and molecules and in particular to pyrophosphate (PPi) and ATP among the other 18 anions studied; however, [ZnL₁] was not sensitive toward any of the anions studied. The selectivity has been shown on the basis of the changes observed in the emission and absorption spectral studies through the removal of Zn²⁺ from [ZnL₂] by PPi. Thus, [ZnL₂] has been shown to detect PPi up to 278 ± 10 ppb at pH 7.4 in aqueous methanolic (1/2 v/v) HEPES buffer.