Lower Rim 1,3-Diderivative of Calix[4]arene-Appended Salicylidene Imine (H₂L): Experimental and Computational Studies of the Selective Recognition of H₂L toward Zn²⁺ and Sensing Phosphate and Amino Acid by [ZnL]

A new 1,3-diderivative of calix[4]arene appended with hydroxymethyl salicylyl imine has been synthesized and its ion recognition toward biologically relevant Mⁿ⁺ ions studied. The receptor H₂L showed selectivity toward Zn²⁺ by switch-on fluorescence among the 12 metal ions studied with a detection limit of 192 ppb. The interaction of Zn²⁺ with H₂L has been further supported by absorption studies, and the stoichiometry of the complex formed (1:1) has been established on the basis of absorption and ESI MS. Competitive ion titrations carried out reveal that the Zn²⁺ can be detected even in the presence of other metal ions of bioimportance. The mode of interaction of Zn²⁺ with conjugate has been established by a fleet of computational calculations carried out in a cascade manner, either on the ligand or on the complex, wherein the final optimizations were carried out by the density functional theory (DFT) and found that the Zn²⁺ and Cd²⁺ indeed bind differently. In situ prepared [ZnL] complex responds to both inorganic phosphate as well as AMP, ADP, and ATP with a minimum detection limit of 426 ppb wherein the Zn²⁺ from the complex is detached and recomplexed by the added phosphate moiety. It has been possible to build an INHIBIT logic gate for the conjugate using Zn²⁺ and HPO₄^2- as inputs by monitoring the fluorescence emission band at 444 nm as output. The amino acid sensing abilities of [ZnL] have been explored by fluorescence and absorbance spectroscopy where it showed selectivity toward Cys, Asp, and His through the formation of the Zn²⁺ complex of these amino acids by chelating through their side chain moieties. Thus, while H₂L is selective for Zn²⁺ among a number of cations, the [ZnL] is selective toward phosphate among a number of anions and also toward Asp, Cys, and His among the naturally occurring amino acids.