Selective recognition of Zn2+ by a triazole linked thiophene-methylimine based derivative of calix[6]arene and the secondary sensing of biothiols by the corresponding zinc complex

Bioinorganic Laboratory, Department of Chemistry, Indian Institute of Technology Bombay, Powai,
Mumbai-400 076, India
E-mail: cprao@iitb.ac.in
Manuscript received online 01 November 2018, accepted 22 November 2018

A 1,3,5-tris-triazole linked thiophene-methylimino appended calix[6]arene conjugate, L has been shown to bind to Zn²+ selectively
in ethanol among the metal ions studied which include, alkali and alkaline earth ions, transition metal ions and lanthanides
by fluorescence, absorption and ESI-MS. The conjugate, L exhibited ~25 fold increase in its fluorescence intensity
at λem = 450 nm in presence of Zn²⁺ upon exciting at 330 nm with a limit of detection of 25±4 ppb while the K_a is
(2.31±0.1)×10^{5 M–1}. The control molecules, C₁ and C₂ which lack iminophenolic core and calix[6]arene platform respectively
have been studied. These do not show considerable changes in their fluorescence emission in presence of Zn²⁺ implying the
essential role of both the arene cavity as well as the role of the three arms to bind to Zn²⁺. Absorption studies further support
the binding of Zn²⁺ to L by exhibiting isosbestic points at 294 and 344 nm and stoichiometry of the 1:1 complex formed
between L and Zn²⁺ was confirmed by ESI-MS. The computationally optimized structure of {L+Zn²⁺} complex exhibited distorted
octahedral geometry about Zⁿ²+ with N₃O₃ core. The in situ generated complex, {L+Zn²⁺} interacts with biologically
relevant thiols by exhibiting fluorescence quenching of the complex due to the release of Zn²⁺ from its complex which is further
supported by absorption and ESI-MS studies.