Interaction of a dinuclear fluorescent Cd(II) complex of calix[4]arene conjugate with phosphates and its applicability in cell imaging

Abstract

A triazole-linked hydroxyethylimino conjugate of calix[4]arene (L) and its cadmium complex have been synthesized and characterized, and their structures have been established. In the complex, both the Cd²⁺ centers are bound by an N₂O₄ core, and one of it is a distorted octahedral, whereas the other is a trigonal anti-prism. The fluorescence intensity of the di-nuclear Cd(II) complex is quenched only in the presence of phosphates and not with other anions studied owing to their binding affinities and the nature of the interaction of the phosphates with Cd²⁺. These are evident even from their absorption spectra. Different phosphates exhibit changes in both their fluorescence as well as absorption spectra to varying extents, suggesting their differential interactions. Among the six phosphates, H₂PO₄⁴⁻ has higher fluorescence quenching even at low equivalents of this ion, whereas P₂O₇⁴⁻ shows only 50% quenching even at 10 equivalents. The fluorescence quenching is considerable even at 20 ppb (0.2 μM) of H₂PO₄⁻, whereas all other phosphates require a concentration of 50–580 ppb to exhibit the same effect on fluorescence spectra. Thus, the interaction of H₂PO₄⁻ is more effective by ∼30 fold as compared to that of P₂O₇⁴⁻. Fluorescence quenching by phosphate is due to the release of L from its original cadmium complex via the formation of a ternary species followed by the capture of Cd²⁺ by the phosphate, as delineated based on the combination of spectral techniques, such as absorption, emission, ¹H NMR and ESI MS. The relative interactive abilities of the six phosphates differ from each other. The removal of Cd²⁺ is demonstrated to be reversible by the repeated addition of the phosphate followed by Cd²⁺. The characteristics of the ternary species formed in each of these six phosphates have been computationally modeled using molecular mechanics. The computational study revealed that the coordination between cadmium and –CH₂–CH₂–OH breaks and new coordination is established through the phosphate oxygens, and as a result the Cd²⁺ center acquires a distorted octahedral geometry. The utility of the complex was demonstrated in HeLa cells.