Conspectus Reactive oxygen species (ROS), such as hydrogen peroxide, are important products of oxygen metabolism that, when misregulated, can accumulate and cause oxidative stress inside cells. Accordingly, organisms have evolved molecular systems, including antioxidant metalloenzymes (such as superoxide dismutase and catalase) and an array of thiol-based redox couples, to neutralize this threat to the cell when it occurs. On the other hand, emerging evidence shows that the controlled generation of ROS, particularly H2O2, is necessary to maintain cellular fitness. The identification of NADPH oxidase enzymes, which generate specific ROS and reside in virtually all cell types throughout the body, is a prime example. Indeed, a growing body of work shows that H2O2 and other ROS have essential functions in healthy, physiological signaling pathways. The signal–stress dichotomy of H2O2 serves as a source of motivation for disentangling its beneficial from its detrimental effects on living systems. Molecular imaging of this oxygen metabolite with reaction-based probes is a powerful approach for real-time, noninvasive monitoring of H2O2 chemistry in biological specimens, but two key challenges to studying H2O2 in this way are chemoselectivity and bioorthogonality of probe molecules. Chemoselectivity is problematic because traditional methods for ROS detection suffer from nonspecific reactivity with other ROS. Moreover, some methods require enzymatic additives not compatible with live-cell or live-animal specimens. Additionally, bioorthogonality requires that the reactions must not compete with or disturb intrinsic cellular chemistry; this requirement is particularly critical with thiol- or metal-based couples mediating the major redox events within the cell. Chemoselective bioorthogonal reactions—such as alkyne–azide cycloadditions and related click reactions, the Staudinger–Bertozzi ligation, and the transformations used in various reaction-based molecular probes—have found widespread application in the modification, labeling, and detection of biological molecules and processes. In this Account, we summarize H2O2 studies from our laboratory using the H2O2-mediated oxidation of aryl boronates to phenols as a bioorthogonal approach to detect fluxes of this important ROS in living systems. We have installed this versatile switch onto organic and inorganic scaffolds to serve as ‘turn-on’ probes for visible and near-infrared (NIR) fluorescence, ratiometric fluorescence, time-gated lanthanide luminescence, and in vivo bioluminescence detection of H2O2 in living cells and animals. Further chemical and genetic manipulations target these probes to specific organelles and other subcellular locales and can also allow them to be trapped intracellularly, enhancing their sensitivity. These novel chemical tools have revealed fundamental new biological insights into the production, localization, trafficking, and in vivo roles of H2O2 in a wide variety of living systems, including immune, cancer, stem, and neural cell models.

Hydrogen sulfide (H(2)S) is emerging as an important mediator of human physiology and pathology but remains difficult to study, in large part because of the lack of methods for selective monitoring of this small signaling molecule in live biological specimens. We now report a pair of new reaction-based fluorescent probes for selective imaging of H(2)S in living cells that exploit the H(2)S-mediated reduction of azides to fluorescent amines. Sulfidefluor-1 (SF1) and Sulfidefluor-2 (SF2) respond to H(2)S by a turn-on fluorescence signal enhancement and display high selectivity for H(2)S over other biologically relevant reactive sulfur, oxygen, and nitrogen species. In addition, SF1 and SF2 can be used to detect H(2)S in both water and live cells, providing a potentially powerful approach for probing H(2)S chemistry in biological systems.

Carbon monoxide is a member of the gasotransmitter family, which also includes NO and H(2)S, and has been implicated in a variety of pathological and physiological conditions. Whereas exogenous therapeutic additions of CO to tissues and whole animals have been well-studied, the real-time spatial and temporal tracking of CO at the cellular level remains an open challenge. Here we report a new type of turn-on fluorescent probe for selective CO detection based on palladium-mediated carbonylation reactivity. CO Probe 1 (COP-1) is capable of detecting CO both in aqueous buffer and in live cells with high selectivity over a range of biologically relevant reactive small molecules, providing a potentially powerful approach for interrogating its chemistry in biological systems.

We report a new chemoenzymatic strategy for the rapid and sensitive detection of O-GlcNAc posttranslational modifications. The approach exploits the ability of an engineered mutant of beta-1,4-galactosyltransferase to selectively transfer an unnatural ketone functionality onto O-GlcNAc glycosylated proteins. Once transferred, the ketone moiety serves as a versatile handle for the attachment of biotin, thereby enabling chemiluminescent detection of the modified protein. Importantly, this approach permits the rapid visualization of proteins that are at the limits of detection using traditional methods. Moreover, it bypasses the need for radioactive precursors and captures the glycosylated species without perturbing metabolic pathways. We anticipate that this general chemoenzymatic strategy will have broad application to the study of posttranslational modifications.

Hydrogen sulfide (H 2 S) is a reactive small molecule generated in the body that can be beneficial or toxic owing to its potent redox activity. In living systems, disentangling the pathways responsible for H 2 S production and their physiological and pathological consequences remains a challenge in part due to a lack of methods for monitoring changes in endogenous H 2 S fluxes. The development of fluorescent probes with appropriate selectivity and sensitivity for monitoring production of H 2 S at biologically relevant signaling levels offers opportunities to explore its roles in a variety of systems. Here we report the design, synthesis, and application of a family of azide-based fluorescent H 2 S indicators, Sulfidefluor-4, Sulfidefluor-5 acetoxymethyl ester, and Sulfidefluor-7 acetoxymethyl ester, which offer the unique capability to image H 2 S generated at physiological signaling levels. These probes are optimized for cellular imaging and feature enhanced sensitivity and cellular retention compared with our previously reported molecules. In particular, Sulfidefluor-7 acetoxymethyl ester allows for direct, real-time visualization of endogenous H 2 S produced in live human umbilical vein endothelial cells upon stimulation with vascular endothelial growth factor (VEGF). Moreover, we show that H 2 S production is dependent on NADPH oxidase-derived hydrogen peroxide (H 2 O 2 ), which attenuates VEGF receptor 2 phosphorylation and establishes a link for H 2 S/H 2 O 2 crosstalk. molecular imaging | redox biology | thiol | VEGFR

Responsive 1,2-dioxetane chemiluminescent probes have been developed that display instantaneous, sensitive, and selective responses to H2S and are capable of imaging H2S in living mice.

Tissue oxygenation is a driving parameter of the tumor microenvironment and hypoxia can be a prognostic indicator of aggressiveness, metastasis, and poor response to therapy. Here we report a chemiluminescence imaging (CLI) agent based on the oxygen-dependent reduction of a nitroaromatic spiroadamantane 1,2-dioxetane scaffold. Hypoxia ChemiLuminescent Probe 2 (HyCL-2) responds to nitroreductase with ~170-fold increase in luminescence intensity, with high selectivity for enzymatic reductase versus other small molecule reductants. HyCL-2 can image exogenous nitroreductase in vitro and in vivo in living mice and total luminescent intensity is increased by ~5-fold under low oxygen conditions. HyCL-2 is demonstrated to report on tumor oxygenation during oxygen challenge in H1299 lung tumor xenografts grown in a murine model as independently confirmed using multi-spectral optoacoustic tomography (MSOT) imaging of hemoglobin oxygenation.

We report a new reaction-based approach for the detection of hydrogen peroxide (H2O2) using hyperpolarized 13C magnetic resonance imaging (13C MRI) and the H2O2-mediated oxidation of α-ketoacids to carboxylic acids. 13C-benzoylformic acid (13C-BFA) reacts selectively with H2O2 over other reactive oxygen species (ROS) to generate 13C-benzoic acid (BA) and can be hyperpolarized using dynamic nuclear polarization (DNP), providing a method for dual-frequency detection of H2O2. Phantom images collected using frequency-specific imaging sequences demonstrate the efficacy of this responsive contrast agent to monitor H2O2 at pre-clinical field strengths. The combination of reaction-based detection chemistry and hyperpolarized 13C magnetic resonance imaging (13C MRI) provides a potentially powerful new methodology for non-invasive multianalyte imaging in living systems.