Binding and ratiometric dual ion recognition of Zn²⁺ and Cu²⁺ by 1, 3, 5-tris-amidoquinoline conjugate of calix[6]arene by spectroscopy and its supramolecular features by microscopy

Abstract

Lower rim amide linked 8-amino quinoline and 8-amino naphthalene moiety 1,3,5-triderivatives of calix[6]arene L₁ and L₂ have been synthesized and characterized. While the L1 acts as a receptor molecule, the L₂ acts as a control molecule. The complexation between L₁ and Cu²⁺ or Zn²⁺ was delineated by the absorption and electrospray ionization (ESI) MS spectra. The binding ability of these molecules toward biologically important metal ions was studied by fluorescence and absorption spectroscopy. The derivative L₁ detects Zn²⁺ by bringing ratiometric change in the fluorescence signals at 390 and 490 nm, but in the case of Cu²⁺, it is only the fluorescence quenching of 390 nm band that is observed, while no new band is observed at 390 nm. The stoichiometry of both the complexes is 1:1 and was confirmed in both the cases by measuring the ESI mass spectra. The isotopic peak pattern observed in the ESI MS confirmed the presence of Zn²⁺ or Cu²⁺ present in the corresponding complex formed with L₁. Among these two ions, the Cu²⁺ exhibits higher sensitivity. The density-functional theory (DFT) studies revealed the conformational changes in the arms and also revealed the coordination features in the case of the metal complexes. The arm conformational changes upon Zn²⁺ binding were supported by nuclear Overhauser effect spectrometry (NOESY) studies. The stronger binding of Cu²⁺ over that of Zn²⁺ observed from the absorption study was further supported by the complexational energies computed from the computational data. While the L₁ exhibited spherical particles, upon complexation with Cu²⁺, it exhibits chain like morphological features in scanning electron microscopy (SEM) but only small aggregates in the case of Zn²⁺. Thus, even the microscopy data can differentiate the complex formed between L₁ and Cu²⁺ from that formed with Zn²⁺.