Cooperative Binding of Metal Cations to a Spiropyran-Conjugated Calix[4]arene

Cooperative binding occurs frequently in biological systems, but is less exploited in synthetic receptor molecules. We synthesized a bis(spiropyran)-conjugated calix[4]arene (bSPTC) through a 1,2,3-triazole linker and found cooperativity in bindings of a series of divalent metal cations (Ca²⁺, Mg²⁺, Cd²⁺, Co²⁺, Fe²⁺, Hg²⁺, Ni²⁺, and Zn²⁺). UV-vis absorption spectroscopy and principle component analysis were used to differentiate the recognized metal cations. The binding stoichiometry was determined by various continuation methods, which indicated that bSPTC has two binding ratios, 1:1 for Zn²⁺ and 1:2 for all other metal cations. Importantly, positive cooperativity was suggested for the 1:2 stoichiometry of bSPTC to Mg²⁺ ions from the sigmoidal saturation curve and Hill equation analysis. Density functional theory (DFT) calculations imply that the first binding of Mg²⁺ to one spiropyran destabilizes the second spiropyran, which facilitates binding of a second Mg²⁺.