Synthesis and Binding studies of Receptors for Ion Recognition

Abstract

Ions recognition chemistry for anions is an important front in host-guest supramolecular chemistry for many years. Even though many synthetic receptors for anions have been investigated, further works are still necessary in order to design well selective synthetic receptors for specific anions because anion binding is a key process in many chemical and biological processes. In this study, we synthesize naphthalene-, anthracene-imidazolium, and carbazole-urea based receptors for specific anion recognition.In chapter 2, a water-soluble imidazolium-based fluorescent chemosensor senses RNA selectively through fluorescence enhancement over other biologically relevant biomolecules in aqueous solution at physiological pH 7.4. Fluorescence image detection of RNA in living cells such as onion cells, HeLa cells and animal model cells was successfully demonstrated which displays a chelation-enhanced fluorescence effect. These affinities can be attributed to the strong electrostatic (C−H)+•••A− ionic H-bonding and the aromatic moiety driven π-stacking of imidazolium-based cyclophane with the size-complementary major groove of RNA.In chapter 3, Anion detection through ionic hydrogen bonding is a challenging task in aqueous solution due to potential interference from the solvent. In the case of GTP and Iodide, it is particularly more so due to the sugar moiety and heavy atom effect, respectively. This work shows that GTP and Iodide can exhibit characteristic responses upon binding to fluorescent receptors, which make selective detection possible. A water-soluble fluorescent chemosensors function as GTP and Iodide sensors selectively through chelation-enhanced fluorescence quenching over other biologically relevant anions in aqueous solution of physiological pH 7.4. These affinities can be attributed to the strong ionic H-bonding along with additional interactions of fluorophore moieties with the nucleic base of GTP and Iˉ.In chapter 4, A new water-soluble and fluorescent imidazolium-anthracene cyclophane effectively recognizes and differentiates the biologically important GTP via excimer formation (through π-π stacking interaction between guanine moiety of GTP and anthracene ring) and ATP via fluorescence enhancement (through the vertical interaction of the H atom at the C-2 position of the adenine base with the plane of the anthracene ring (π-H interaction) in 100% aqueous solution of physiological pH 7.4. Fluorescence and 1H-NMR spectra and ab initio calculations demonstrate that excimer formation and fluorescence enhancement occur upon GTP and ATP binding, respectively, through (C−H)+•••A− hydrogen bond interactions.In chapter 5, a water-soluble and fluorescent imidazolium-anthracene cyclophane effectively recognizes the biologically important GTP and I− over other anions in 100% aqueous solution of physiological pH 7.4. Fluorescence and 1H-NMR spectra and ab initio calculations demonstrate that emission arises from the formation of an excimer state and quenching occurs upon GTP/I− binding through (C−H)+•••A− hydrogen bond interactions.In chapter 6, Cyclo-bis-(urea-3,6-dichlorocarbazole) forms a 1:2 complex with CH3CO2− and H2PO4− through hydrogen bonding with the two urea moieties, resulting in fluorescence enhancement via a combined photoinduced electron transfer (PET) and energy transfer mechanism. The binding mechanism involves a conformational change of the two urea receptors to a trans orientation after binding of the first anion, which facilitates the second interaction. These binding modes were confirmed by crystallization, theoretical calculations, fluorescence and 1H–NMR titrations.