Selective recognition and detection of catecholamines : from reaction-based chemical probes to mechanically interlocked hosts

Abstract

Life is a series of well-controlled chemical reactions involving small molecules, ions and macromolecules. As an important class of neurotransmitters and hormone, catecholamines play many essential roles in different physiological processes. Catecholamines are involved in signaling, learning, motion control, immune defense and many other physiological functions, and an imbalance of catecholamine dynamics is associated with mental illnesses and neurodegenerative diseases. Although immunoassays, chromatographic, and electrochemical methods to quantify catecholamine levels in living system have been developed for a long time, they are not able to detect catecholamines with enough spatiotemporal information for a more in-depth biological study. Fluorescent probes have emerged as useful tools for visualizing target analytes at molecular level, providing invaluable information and insights on their biological roles. On the other hand, the use of a complementary host for recognizing molecule of interest is another major strategy in the detection of a biological target. In this thesis, development of reaction-based fluorescent probes and supramolecular host for catecholamines is described. In Chapter 1, an introduction on the biochemistry and neurochemistry of catecholamines, and current strategies for their detection are described. Design principles for analyte-specific fluorescent probes are also discussed, with highlights on the reaction-based and recognition-based strategies. In Chapter 2, the design, synthesis and evaluation of catecholamine probes (CAP) are described. Design of CAP is inspired by the dopamine-triggered dioxygen activation and substrate oxidation in dopamine-β-monooxygenase. CAP features a caged fluorophore conjugated to a Cu(II) complex supported by a 2-(ethylthio)-N,N-bis(pyridin-2-ylmethyl)ethanamine (N3S) ligand. Treatment with catecholamines leads to the oxidative bond cleavage and release the fluorophore in emissive form. CAPs are highly selective towards catecholamines over diols, sugars, vitamins and amino acids. This bond cleavage strategy is highly modular and a series of CAPs with different emission colors have been prepared straightforwardly. The oxidative bond cleavage reaction has been characterized by a series of experiments including LCMS, ESI-MS and fluorescence studies. Applicability of the cyan-emitting CAP, CAP488, is demonstrated in the visualization of intracellular dopamine neuronal differentiation and Parkinson’s disease models. In Chapter 3, preliminary mechanistic studies on the copper-mediated oxidation and the effects of the copper coordination environments on the catecholamine reactivity are discussed. HRMS studies suggests that the oxidation is initiated by an initial formation of a probe-catecholamine adduct. Dioxygen has been shown as the terminal oxidant for the reaction. Kinetic and electrochemical studies suggest the participation of catecholamines in the oxidative bond cleavage. Primary and secondary coordination spheres around the Cu(II) center are found to be critical to the catecholamine reactivity, suggesting that the catecholamines-triggered oxidation is highly specific to the N3S-Cu(II) coordination. In Chapter 4, rotaxane-based host for reversible catecholamine binding and sensing is investigated. The design, synthesis, characterization and evaluation of three hetero[4]rotaxanes are described. NMR and UV-Vis studies suggest no significant catecholamine binding to the rotaxanes, and future directions for rotaxane modifications to improve the binding are discussed.