Turn-on fluorescent probes for detecting nitric oxide in biology

Abstract

Chapter 1. Investigating the Biological Roles of Nitric Oxide and Other Reactive Nitrogen Species Using Fluorescent Probes: This chapter presents an overview of recent progress in the field of reactive nitrogen species (RNS) sensing. Reactive nitrogen species, such as nitric oxide (NO) and its higher oxides, play important roles in cell signaling during many physiological and pathological events. Elucidation of the exact functions of these important biomolecules has been hampered by the inability to detect RNS reliably under biological conditions. A surge of research into RNS chemistry has resulted in the design of a new generation of fluorescent probes that are specific and sensitive for their respective RNS analytes. Progress in the field of nitric oxide, peroxynitrite, and nitroxyl sensing promises to advance our knowledge of important signaling events involving these species and should lead to a better understanding of oxidative biochemistry crucial to health and disease. Chapter 2. Mechanism of Nitric Oxide Reactivity and Fluorescence Enhancement of the NO-Specific Probe, CuFu1: The mechanism of the reaction of CuFu1 (FL1 = 2-{2-chloro-6-hydroxy-5-[(2- methylquinolin-8-ylamino)-methyl]-3-oxo-3H-xanthen-9-yl}benzoic acid) with NO to form FL1-NO in aqueous, buffered solutions was investigated. The reaction is first order in concentration of CuFL1, NO, and hydroxide ion. Rate saturation at high base concentrations is consistent with the fact that the protonation state of the secondary amine of the complex is crucial for reactivity. Based on this information, faster-reacting probes can be obtained by lowering the pKa of the secondary amine. The activation parameters for the reaction indicate that the mechanism is associative (ASI = -29 ± 3 cal/K-mol) and occurs with a modest thermal barrier (AHI = 9.7 ± 0.5 kcal/mol; Ea = 10.3 ± 0.5 kcal/mol). Variable pH EPR experiments indicate that as the secondary amine of CuFu1 is deprotonated, the electron density shifts yielding new spin-active species that has electron density localized on the deprotonated nitrogen atom. This result suggests that FL1-NO formation occurs when NO attacks the deprotonated secondary amine of the coordinated ligand, causing inner-sphere electron transfer to Cu(II) to form Cu(I) and subsequent FL 1-NO release from the metal. Chapter 3. Fluorescence-Based Nitric Oxide Sensing by Cu(II) Complexes that Can Be Trapped in Living Cells: A series of symmetrical, fluorescein-derived ligands appended with two derivatized 2- methyl-8-aminoquinolines were prepared and spectroscopically characterized. The ligands 2-{6-hydroxy-4,5-bis[(2-methylquinolin-8-ylamino)methyl]-3-oxo-3H-xanthen5 9-yl}benzoic acid (FL2), 2-{4,5-bis[(6-(2-ethoxy-2-oxoethoxy)-2-methylquinolin-8- ylamino)methyl]-6-hydroxy-3-oxo-3H-xanthen-9-yl}benzoic acid (FL2E), and 2,2'-{8,8'- [9-(2-Carboxyphenyl)-6-hydroxy-3-oxo-3H-xanthene-4,5-diyl]bis(methylene)bis(azanediyl) bis(2-methylquinolin-8,6-diyl)}bis(oxy)diacetic acid (FL2A) were designed to improve the dynamic range of previously described asymmetric systems, and the copper complex Cu2FL2E was constructed as a trappable NO probe that is hydrolyzed intracellularly to form Cu2FL2A. The ligands themselves are only weakly emissive and completely quenched in their Cu(II) complexes, which were generated in situ by combining each ligand with two equivalents of CuCl2 . The resulting complexes were investigated as fluorescent probes for nitric oxide. Upon introduction of excess NO under anaerobic conditions to buffered solutions of Cu2(FL2), Cu 2(FL2E), and Cu2(FL2A), the fluorescence increased by factors of 23 ± 3, 17 ± 2, and 27 ± 3, respectively. The corresponding rate constants for fluorescence turn-on were determined to be 0.006 ± 0.003 s-, 0.0058 ± 0.0009 s-4 and 0.010 ± 0.002 s4. The probes are highly specific for NO over other biologically relevant reactive oxygen and nitrogen species, as well as Zn(II), the metal ion for which structurally similar probes were designed to detect. Chapter 4. Visualization of Nitric Oxide Production in the Mouse Main Olfactory Bulb by a Cell-Trappable Copper(II) Fluorescent Probe: The visualization of NO production using fluorescence in tissue slices of the mouse main olfactory bulb is reported. This discovery was possible through the use of a novel, celltrappable probe for intracellular nitric oxide detection based on a symmetric scaffold with two NO-reactive sites. Ester moieties installed onto the fluorescent probe are cleaved by intracellular esterases to yield the corresponding negatively charged, cell-impermeable acids. The trappable ester probe Cu2(FL2E) and the membrane-impermeable acid derivative Cu2(FL2A) respond rapidly and selectively to NO in buffers that simulate biological conditions. Application of Cu2(FL2E) leads to detection of endogenously produced NO in cell cultures and olfactory bulb brain slices. Chapter 5. Dextran-Based Cell-Trappable Fluorescent Probes for Nitric Oxide Visualization in Living Cells: Two new cell-trappable fluorescent probes for nitric oxide are reported based on either incorporation of hydrolyzable esters or conjugation to aminodextran polymers. Both probes are highly selective for NO over other reactive oxygen and nitrogen species (RONS). The ability of these probes to image nitric oxide produced endogenously in Raw 264.7 cells by fluorescence is demonstrated. Chapter 6. A Cell-Trappable Fluorescent Probe for Detecting Biological Zinc: The synthesis and spectroscopic characterization of a new, cell-trappable fluorescent probe for Zn(II) is presented. This probe, 2-(4,5-bis((6-(2-ethoxy-2-oxoethoxy)quinolin- 8-yl)amino)methyl)-6-hydroxy-3-oxo-3H-xanthen-9-yl)benzoic acid (QZ2E) is poorly emissive in the off-state, but exhibits a dramatic, 120 ± 10-fold increase in fluorescence upon Zn(II) binding. This binding is selective for Zn(II) over other biologically relevant metal cations, toxic heavy metals, and most first-row transition metals, and is of appropriate affinity (Kdl = 150 ± 100 [tM, Kd2 = 3.5 ± 0.1 mM) to bind Zn(II) at physiological levels reversibly. In live cells, QZ2E localizes to the Gogli apparatus where it can detect Zn(II). It is cell membrane permeable until cleavage of its ester groups by intracellular esterases produces QZ2A, a negatively-charged acid that cannot cross the cell membrane. Appendix 1. Screening for bNOS Inhibitors in Bacillus anthracis: The incidence of anthrax infection by the Gram-positive bacterium Bacillus anthracis and the challenges of its treatment are presented. B. anthracis pathogenesis is critically dependent on NO production by the enzyme bacterial nitric oxide synthase (bNOS), a variant of the eukaryotic NOSes that does not contain a reductase domain required for catalysis. Using non-committed reductases in the cell, B. anthracis produced NO to neutralize the oxidative environment produced in macrophages as a host defense system. The fact that NO production is crucial for bacterial survival suggests that a selective bNOS inhibitor would make a good antibacterial agent against Bacillus anthracis and related pathogens. A high-throughput screen of a small-molecule library to identify potential bNOS inhibitors by fluorescence of an NO-specific probe is proposed. Optimization of fluorescence imaging in 384-well plates is presented as a first step toward this goal. Future directions to improve the screening protocol and steps for ensuring bNOS selectivity and efficacy in mice are discussed. Appendix 2. NMR Spectra.