Luminescence of imidazo[1,2-a]pyrazin-3(7H)-one compounds

Half-Curcumin-Based Chemiluminescence Probes and Their Applications in Detecting Quasi-Stable Oxidized Proteins

Numerous methods have been reported for detecting ROS/RNS in vitro and in vivo; however, detecting methods for the secondary products of the reactive oxygen species (ROS)/reactive nitrogen species (RNS) reactions, particularly quasi-stable oxidized products, have been much less explored. In this report, we observed that half-curcumins could generate chemiluminescence (CL). In contrast to other chemiluminescence scaffolds, the distinguishing feature of a half-curcumin is the formation of a carbanion intermediate of its acetylacetone moiety, opening unique avenues for applications. In this study, we designed a series of half-curcumins CRANAD-Xs and found that CRANAD-164 could be used to detect quasi-stable oxidized proteins (QSOP) in vivo and in patient serum samples. We illustrated that CRANAD-164 could be used to monitor the responses of taurine, an amino acid with newly reported anti-aging capacity, in an inflammatory mouse model. Remarkably, we further demonstrated that the QSOP levels were much higher in the disease serum samples, including Alzheimer's disease (AD), compared to the samples from healthy controls. Moreover, our results revealed that the sera chemiluminescence intensities were higher in aged healthy controls compared to young healthy subjects, suggesting that CRANAD-164 can be used to monitor the increase of QSOP during aging.

Comparative investigation into the anticancer activity of analogs of marine coelenterazine and coelenteramine

Cancer is still one of the most challenging diseases to treat, making the pursuit for novel molecules with potential anticancer activity an important research topic. Herein, we have performed a comparative investigation into the anticancer activity of analogs of marine coelenterazine and coelenteramine. The former is a well-known bioluminescent substrate, while the latter is a metabolic product of the resulting bioluminescent reaction. While both types of analogs showed anticancer activity toward lung and gastric cancer cell lines, we have obtained data that highlight relevant differences between the activity of these two types of compounds. More specifically, we observed relevant differences in structure-activity relationships between these types of compounds. Also, coelenteramine analogs showed time-dependent activity, while coelenterazine-based compounds usually present time-independent activity. Coelenterazine analogs also appear to be relatively safer toward noncancer cells than coelenteramine analogs. There was also seen a correlation between the activity of the coelenterazine-based compounds and their light-emission properties. Thus, these results further indicate the potential of the marine coelenterazine chemi-/bioluminescent system as a source of new molecules with anticancer activity, while providing more insight into their modes of action.

Cavity-Modified Chemiluminescent Reaction of Dioxetane

Chemiluminescence is a thermally activated chemical process that emits a photon of light by forming a fraction of products in the electronic excited state. A well-known example of this spectacular phenomenon is the emission of light in the firefly beetle, where the formation of a four-membered cyclic peroxide compound and subsequent dissociation produce a light-emitting product. The smallest cyclic peroxide, dioxetane, also exhibits chemiluminescence but with a low quantum yield as compared to that of firefly dioxetane. Employing the strong light-matter coupling has recently been found to be an alternative strategy to modify the chemical reactivity. In the presence of an optical cavity, the molecular degrees of freedom greatly mix with the cavity mode to form hybrid cavity-matter states called polaritons. These newly generated hybrid light-matter states manipulate the potential energy surfaces and significantly change the reaction dynamics. Here, we theoretically investigate the effects of a strong light-matter interaction on the chemiluminescent reaction of dioxetane using the extended Jaynes-Cummings model. The cavity couplings corresponding to the electronic and vibrational degrees of freedom have been included in the interaction Hamiltonian. We explore how the cavity alters the ground- and excited-state path energy barriers and reaction rates. Our results demonstrate that the formation of excited-state products in the dioxetane decomposition process can be either accelerated or suppressed, depending on the molecular orientation with respect to the cavity polarization.

Urea Hydrogen Peroxide and Ethyl Lactate, an Eco-Friendly Combo System in the Direct C(sp2)-H Bond Selenylation of Imidazo[2,1-b]thiazole and Related Structures

Herein, we describe a urea hydrogen peroxide-mediated sustainable protocol for the synthesis of selenylated imidazo[2,1-b]thiazole by using half molar equivalent diorganyl diselenides in ethyl lactate as a greener solvent. The reaction features high yields, easy performance on gram scale, metal-free conditions, as well as applicability to imidazopyridine and imidazopyrimidine.

Combined Experimental and Theoretical Investigation into the Photophysical Properties of Halogenated Coelenteramide Analogs

Marine Coelenterazine is one of the most well-known chemi-/bioluminescent systems, and in which reaction the chemi-/bioluminophore (Coelenteramide) is generated and chemiexcited to singlet excited states (leading to light emission). Recent studies have shown that the bromination of compounds associated with the marine Coelenterazine system can provide them with new properties, such as anticancer activity and enhanced emission. Given this, our objective is to characterize the photophysical properties of a previously reported brominated Coelenteramide analog, by employing a combined experimental and theoretical approach. To better analyze the potential halogen effect, we have also synthesized and characterized, for the first time, two new fluorinated and chlorinated Coelenteramide analogs. These compounds show similar emission spectra in aqueous solution, but with different fluorescence quantum yields, in a trend that can be correlated with the heavy-atom effect (F > Cl > Br). A blue shift in emission in other solvents is also verified with the F−Cl−Br trend. More relevantly, the fluorescence quantum yield of the brominated analog is particularly sensitive to changes in solvent, which indicates that this compound has potential use as a microenvironment fluorescence probe. Theoretical calculations indicate that the observed excited state transitions result from local excitations involving the pyrazine ring. The obtained information should be useful for the further exploration of halogenated Coelenteramides and their luminescent properties.

Comparative Investigation of the Chemiluminescent Properties of a Dibrominated Coelenterazine Analog

Chemi- and bioluminescence are remarkable light-emitting phenomena, in which thermal energy is converted into excitation energy due to a (bio)chemical reaction. Among a wide variety of chemi-/bioluminescent systems, one of the most well-known and studied systems is that of marine imidazopyrazinones, such as Coelenterazine and Cypridina luciferin. Due to the increasing usefulness of their chemi-/bioluminescent reactions in terms of imaging and sensing applications, among others, significant effort has been made over the years by researchers to develop new derivatives with enhanced properties. Herein, we report the synthesis and chemiluminescent characterization of a novel dibrominated Coelenterazine analog. This novel compound consistently showed superior luminescence, in terms of total light output and emission lifetime, to natural imidazopyrazinones and commercially available analogs in aprotic media, while being capable of yellow light emission. Finally, this new compound showed enhanced chemiluminescence in an aqueous solution when triggered by superoxide anion, showing potential to be used as a basis for optimized probes for reactive oxygen species. In conclusion, bromination of the imidazopyrazinone scaffold appears to be a suitable strategy for obtaining Coelenterazines with enhanced properties.

Theoretical Study of the Thermolysis Reaction and Chemiexcitation of Coelenterazine Dioxetanes

Coelenterazine and other imidazopyrazinones are important bioluminescent substrates widespread in marine species and can be found in eight phyla of luminescent organisms. Light emission from these systems is caused by the formation and subsequent thermolysis of a dioxetanone intermediate, whose decomposition allows for efficient chemiexcitation to singlet excited states. Interestingly, some studies have also reported the involvement of unexpected dioxetane intermediates in the chemi- and bioluminescent reactions of Coelenterazine, albeit with little information on the underlying mechanisms of these new species. Herein, we have employed a theoretical approach based on density functional theory to study for the first time the thermolysis reaction and chemiexcitation profile of two Coelenterazine dioxetanes. We have found that the thermolysis reactions of these species are feasible but with relevant energetic differences. More importantly, we found that the singlet chemiexcitation profiles of these dioxetanes are significantly less efficient than the corresponding dioxetanones. Furthermore, we identified triplet chemiexcitation pathways for the Coelenterazine dioxetanes. Given this, the chemiexcitation of these dioxetanes should lead only to minimal luminescence. Thus, our theoretical investigation of these systems indicates that the thermolysis of these dioxetanes should only provide "dark" pathways for the formation of nonluminescent degradation products of the chemi- and bioluminescent reactions of Coelenterazine and other imidazopyrazinones.

Near-infrared chemiluminescence imaging of superoxide anion production in kidneys with iron3+-nitrilotriacetate-induced acute renal oxidative stress in rats

Iron-catalyzed oxidative stress generates reactive oxygen species in the kidney and induces oxidative damage including lipid, protein, and DNA modifications which induces renal injury and may lead to cancer. An analysis of oxidative stress dynamics by reactive oxygen species has not been performed non-invasively in real time in intact kidneys and is a significant challenge in biology and medicine. Here, I report that MCLA-800 is a near-infrared chemiluminescent probe that visualizes the dynamics of superoxide anion (O₂^•-) production and the upstream generation of reactive oxygen species in living rat kidneys suffering acute renal oxidative stress induced by intraperitoneal administration of iron³⁺-nitrilotriacetate (Fe³⁺-NTA) as a representative Fe³⁺ chelate. MCLA-800 was intravenously injected at 250 nmol/kg body weight and immediately transported to the kidneys with the emitting light dependent on O₂^•- production. The magnitude of O₂^•- production correlated with the Fe³⁺-NTA dose. O₂^•- was continuously produced in the blood stream following Fe³⁺-NTA injection at 0.15 mmol/kg body weight, while peak production in the renal cortex occurred at 24 h, then decreased to the background level at 72 h. This study clearly revealed the dynamics of Fe³⁺-NTA-mediated O₂^•- production in the living kidney by chemiluminescent imaging of O₂^•- production using MCLA-800.

Formation of Coelenteramine from 2-peroxycoelenterazine in the Ca2+ -binding Photoprotein Aequorin

Aequorin consists of apoprotein (apoAequorin) and (S)-2-peroxycoelenterazine (CTZ-OOH) and is considered to be a transient-state complex of an enzyme (apoAequorin) and a substrate (coelenterazine and molecular oxygen) in the enzymatic reaction. The degradation process of CTZ-OOH in aequorin was characterized under various conditions of protein denaturation. By acid treatment, the major product from CTZ-OOH was coelenteramine (CTM), but not coelenteramide (CTMD), and no significant luminescence was observed. The counterparts of CTM from CTZ-OOH were identified as 4-hydroxyphenylpyruvic acid (4HPPA) and 4-hydroxyphenylacetic acid (4HPAA) by liquid chromatography/electrospray ionization-time-of-flight mass spectrometry (LC/ESI-TOF-MS). In the luminescence reaction of aequorin with Ca²⁺ , similar amounts of 4HPPA and 4HPAA were detected, indicating that CTM is formed by two pathways from CTZ-OOH through dioxetanone anion and not by hydrolysis from CTMD.

Non-invasive and accurate readout of superoxide anion in biological systems by near-infrared light

Infectious and inflammatory diseases involve superoxide anion (O₂^•-) production. Real-time and non-invasive evaluation of O₂^•- in intact biological systems has been a significant challenge in biology and medicine. Here, I report that an advanced near-infrared chemiluminescent probe, MCLA-800, enables reliable non-invasive optical readout of O₂^•-ex vivo and in vivo. MCLA-800 allowed highly selective and sensitive monitoring of O₂^•- in undiluted human whole blood ex vivo. For the first time, the use of MCLA-800 revealed two reproducible types of O₂^•- production in response to stimulation by unopsonized zymosan particles of Saccharomyces cerevisiae, that is, slow response (S-type) and fast response (F-type), specific to each individual. O₂^•- production was synchronized with myeloperoxidase (MPO) activation in the former type but not in the latter. Moreover, as new findings, MCLA-800 chemiluminescence demonstrated that the chemiluminescence intensity-time properties of formyl-methionyl-leucyl-phenylalanine (fMLP)- or phorbol 12-myristate 13-acetate (PMA)-induced O₂^•- production and MPO activity were independent of S- and F-type zymosan-induced MCLA-800 chemiluminescence whole blood and that PMA-induced MPO activation synchronized with PMA-induced O₂^•- production in S- and F-type zymosan-induced MCLA-800 chemiluminescence whole blood, but fMLP-induced MPO activation did not synchronize with fMLP-induced O₂^•- production in both of S- and F-type blood. Furthermore, MCLA-800 spatiotemporally allowed non-invasive and clear in vivo imaging of O₂^•- in animal models of acute dermatitis and focal arthritis. Therefore, MCLA-800 could be possibly applied in various advanced diagnostic techniques.

18F-Labeled dihydromethidine: positron emission tomography radiotracer for imaging of reactive oxygen species in intact brain

Dihydromethidine (DHM) labeled with 18F at the para position of the peripheral benzene ring was designed as a positron emission tomography (PET) radiotracer for non-invasive imaging of reactive oxygen species (ROS). This compound readily crosses the blood-brain barrier and is oxidized by ROS, and the oxidation product is retained intracellularly. PET imaging of ROS-producing rat brain microinfused with sodium nitroprusside identified specific brain regions with high ROS concentrations. This tracer should be useful for studies of the pathophysiological roles of ROS, and in the diagnosis of neurodegenerative diseases.

Luminescence of Cypridina Luciferin in the Presence of Human Plasma Alpha 1-Acid Glycoprotein

The enzyme Cypridina luciferase (CLase) enables Cypridina luciferin to emit light efficiently through an oxidation reaction. The catalytic mechanism on the substrate of CLase has been studied, but the details remain to be clarified. Here, we examined the luminescence of Cypridina luciferin in the presence of several proteins with drug-binding ability. Luminescence measurements showed that the mixture of human plasma alpha 1-acid glycoprotein (hAGP) and Cypridina luciferin produced light. The total value of the luminescence intensity over 60 s was over 12.6-fold higher than those in the presence of ovalbumin, human serum albumin, or bovine serum albumin. In the presence of heat-treated hAGP, the luminescence intensity of Cypridina luciferin was lower than in the presence of intact hAGP. Chlorpromazine, which binds to hAGP, showed an inhibitory effect on the luminescence of Cypridina luciferin, both in the presence of hAGP and a recombinant CLase. Furthermore, BlastP analysis showed that hAGP had partial amino acid sequence similarity to known CLases in the region including amino acid residues involved in the drug-binding ability of hAGP. These findings indicate enzymological similarity between hAGP and CLase and provide insights into both the enzymological understanding of CLase and development of a luminescence detection method for hAGP.

Coelenterazine-Dependent Luciferases as a Powerful Analytical Tool for Research and Biomedical Applications

: The functioning of bioluminescent systems in most of the known marine organisms is based on the oxidation reaction of the same substrate-coelenterazine (CTZ), catalyzed by luciferase. Despite the diversity in structures and the functioning mechanisms, these enzymes can be united into a common group called CTZ-dependent luciferases. Among these, there are two sharply different types of the system organization-Ca2+-regulated photoproteins and luciferases themselves that function in accordance with the classical enzyme-substrate kinetics. Along with deep and comprehensive fundamental research on these systems, approaches and methods of their practical use as highly sensitive reporters in analytics have been developed. The research aiming at the creation of artificial luciferases and synthetic CTZ analogues with new unique properties has led to the development of new experimental analytical methods based on them. The commercial availability of many ready-to-use assay systems based on CTZ-dependent luciferases is also important when choosing them by first-time-users. The development of analytical methods based on these bioluminescent systems is currently booming. The bioluminescent systems under consideration were successfully applied in various biological research areas, which confirms them to be a powerful analytical tool. In this review, we consider the main directions, results, and achievements in research involving these luciferases.

Turn-on chemiluminescence probes and dual-amplification of signal for detection of amyloid beta species in vivo

Turn-on fluorescence imaging is routinely studied; however, turn-on chemiluminescence has been rarely explored for in vivo imaging. Herein, we report the design and validation of chemiluminescence probe ADLumin-1 as a turn-on probe for amyloid beta (Aβ) species. Two-photon imaging indicates that ADLumin-1 can efficiently cross the blood-brain barrier and provides excellent contrast for Aβ plaques and cerebral amyloid angiopathy. In vivo brain imaging shows that the chemiluminescence signal of ADLumin-1 from 5-month-old transgenic 5xFAD mice is 1.80-fold higher than that from the age-matched wild-type mice. Moreover, we demonstrate that it is feasible to further dually-amplify signal via chemiluminescence resonance energy transfer (DAS-CRET) using two non-conjugated smart probes (ADLumin-1 and CRANAD-3) in solutions, brain homogenates, and in vivo whole brain imaging. Our results show that DAS-CRET can provide a 2.25-fold margin between 5-month-old 5xFAD mice and wild type mice. We believe that our strategy could be extended to other aggregating-prone proteins.

Single-molecule chemiluminescent photosensitizer for a self-activating and tumor-selective photodynamic therapy of cancer

While photodynamic therapy is known for significant advantages over conventional cancer therapies, its dependence on light has limited it to treating tumors on or just under the skin or on the outer lining of organs/cavities. Herein, we have developed a single-molecule photosensitizer capable of intracellular self-activation and with potential tumor-selectivity due to a chemiluminescent reaction involving only a cancer marker. Thus, the photosensitizer is directly chemiexcited to a triplet excited state capable of generating singlet oxygen, without requiring either a light source or any catalyst/co-factor. Cytotoxicity assays involving the photosensitizer show significant toxicity toward tumor cells, even better than reference drugs, while not inducing toxicity toward normal cells. This work provides a proof-of-concept for a novel type of photosensitizer that eliminates the current restrictions that photodynamic therapy presents regarding tumor size and localization.

Study of the Combination of Self-Activating Photodynamic Therapy and Chemotherapy for Cancer Treatment

Cancer is a very challenging disease to treat, both in terms of treatment efficiency and side-effects. To overcome these problems, there have been extensive studies regarding the possibility of improving treatment by employing combination therapy, and by exploring therapeutic modalities with reduced side-effects (such as photodynamic therapy (PDT)). Herein, this work has two aims: (i) to develop self-activating photosensitizers for use in light-free photodynamic therapy, which would eliminate light-related restrictions that this therapy currently possesses; (ii) to assess their co-treatment potential when combined with reference chemotherapeutic agents (Tamoxifen and Metformin). We synthesized three new photosensitizers capable of self-activation and singlet oxygen production via a chemiluminescent reaction involving only a cancer marker and without requiring a light source. Cytotoxicity assays demonstrated the cytotoxic activity of all photosensitizers for prostate and breast tumor cell lines. Analysis of co-treatment effects revealed significant improvements for breast cancer, producing better results for all combinations than just for the individual photosensitizers and even Tamoxifen. By its turn, co-treatment for prostate cancer only presented better results for one combination than for just the isolated photosensitizers and Metformin. Nevertheless, it should be noted that the cytotoxicity of the isolated photosensitizers in prostate tumor cells was already very appreciable.

Comparative study of the chemiluminescence of coelenterazine, coelenterazine-e and Cypridina luciferin with an experimental and theoretical approach

Imidazopyrazinone is a typical scaffold present in marine bioluminescence, in which thermal energy is converted into excitation energy in an enzyme-catalyzed reaction. In fact, the imidazopyrazinone scaffold is a common link among organisms of eight phyla. The characterization of the light emission mechanism is essential for the development of future applications in bioimaging, bioanalysis and biomedicine. Herein, we have studied the chemiluminescent reaction of three commercially-available imidazopyrazinones (Cypridina luciferin, Coelenterazine and Coelenterazine-e) in several aprotic solvents at different pH. We have found that at acidic pH only DMF and DMSO consistently present high light emission, while chemiluminescence in other solvents is negligible. We have attributed this to the inability of most solvents to allow for the deprotonation of the imidazopyrazinone core, thereby preventing the oxygenation step. We have also observed that increasing the pH of the solution leads to the inhibition of chemiluminescence, which we attributed to the deprotonation of the dioxetanone intermediate, as the neutral species is the one associated with efficient chemiexcitation. We have also observed that the pK_a of dioxetanone increases with the dielectric constant of the medium. Finally, our work indicated that the chemiexcitation yield increases with increasing polarity of the medium, due to a reduced transition dipole moment associated with S₀ → S₁ transition.

Catalytic Chemiluminescence Polymer Dots for Ultrasensitive In Vivo Imaging of Intrinsic Reactive Oxygen Species in Mice

Chemiluminescence (CL) is a promising bioimaging method due to no interferences of light source and autofluorescence. However, compared to fluorescent emission, most CL reactions show short emission time and wavelength and weak emission intensity, which limit their applications in in vivo imaging. Here, we report mimic-enzyme catalytic CL polymer dots (hemin-Pdots) consisting of hemin and fluorescent conjugated polymer based on chemiluminescence resonance energy transfer. Hemin-Pdots show about 700× enhanced CL and over 10 h light emission in the presence of CL substrates and H₂O₂. These properties are mainly due to high-catalytic activity of hemin-Pdots and slow-diffusion-controlled heterogeneous reaction. Hemin-Pdots also possess excellent biocompatibility, good stability, emission wavelength redshift, and ultrasensitive response to reactive oxygen species (ROS), and they were successfully used for real-time imaging ROS levels in the peritoneal cavity and normal and tumor tissues of mice. Hemin-Pdots as new CL probes have wide applications in bioassays, bioimaging, and photodynamic therapy.

Efficient Access to Imidazo[1,2- a]pyridines/pyrazines/pyrimidines via Catalyst-Free Annulation Reaction under Microwave Irradiation in Green Solvent

An expeditious catalyst-free heteroannulation reaction for imidazo[1,2- a]pyridines/pyrimidines/pyrazines was developed in green solvent under microwave irradiation. Using H₂O-IPA as the reaction medium, various substituted 2-aminopyridines/pyrazines/pyrimidines underwent annulation reaction with α-bromoketones under microwave irradiation to provide the corresponding imidazo[1,2- a]pyridines/pyrimidines/pyrazines in excellent yields. The synthetic methodology appears to be very simple and superior to the already reported procedures with the high abundance of commercial reagents and great ability in expanding the molecular diversity. The present synthetic sequence is visualized as an environmentally benign process which allows the introduction of three points of structural diversity to expand chemical space with excellent purity and yields. The anti-inflammatory and antimicrobial activities of the derivatives were evaluated. Screening results uncovered three derivatives with strong inhibition of albumin denaturation and two derivatives were active on Proteus and Klebsiella bacteria. These positive bioassay results implied that the library of potential anti-inflammatory agents could be rapidly prepared in an ecofriendly manner, and provided new insights into drug discovery for medicinal chemists.

Chemi- and Bioluminescence of Cyclic Peroxides

Bioluminescence is a phenomenon that has fascinated mankind for centuries. Today the phenomenon and its sibling, chemiluminescence, have impacted society with a number of useful applications in fields like analytical chemistry and medicine, just to mention two. In this review, a molecular-orbital perspective is adopted to explain the chemistry behind chemiexcitation in both chemi- and bioluminescence. First, the uncatalyzed thermal dissociation of 1,2-dioxetane is presented and analyzed to explain, for example, the preference for triplet excited product states and increased yield with larger nonreactive substituents. The catalyzed fragmentation reaction and related details are then exemplified with substituted 1,2-dioxetanone species. In particular, the preference for singlet excited product states in that case is explained. The review also examines the diversity of specific solutions both in Nature and in artificial systems and the difficulties in identifying the emitting species and unraveling the color modulation process. The related subject of excited-state chemistry without light absorption is finally discussed. The content of this review should be an inspiration to human design of new molecular systems expressing unique light-emitting properties. An appendix describing the state-of-the-art experimental and theoretical methods used to study the phenomena serves as a complement.

New bioluminescent coelenterazine derivatives with various C-6 substitutions

A series of new coelenterazine analogs with varying substituents at the C-6 position of the imidazopyrazinone core have been designed and synthesized for the extension of bioluminescence substrates.

Theoretical modulation of singlet/triplet chemiexcitation of chemiluminescent imidazopyrazinone dioxetanone via C8-substitution

DFT analysis of the thermolysis of C₈-substituted imidazopyrazinone dioxetanone allows the rational tuning of the activation barrier and singlet/triplet chemiexcitation.

Mechanistic insight into marine bioluminescence: photochemistry of the chemiexcited Cypridina (sea firefly) lumophore

Cypridina hilgendorfii (sea firefly) is a bioluminescent crustacean whose bioluminescence (BL) reaction is archetypal for a number of marine organisms, notably other bioluminescent crustaceans and coelenterates. Unraveling the mechanism of its BL is paramount for future applications of its strongly emissive lumophore. Cypridina produces light in a three-step reaction: First, the cypridinid luciferin is activated by an enzyme to produce a peroxide intermediate, cypridinid dioxetanone (CDO), which then decomposes to generate excited oxyluciferin (OxyCLnH*). Finally, OxyCLnH* deexcites to its ground state along with emission of bright blue light. Unfortunately, the detailed mechanism of the critical step, the thermolysis of CDO, remains unknown, and it is unclear whether the light emitter is generated from a neutral form (CDOH) or anionic form (CDO(-)) of the CDO precursor. In this work, we investigated the key step in the process by modeling the thermal decompositions of both CDOH and CDO(-). The calculated results indicate that the decomposition of CDO(-) occurs via the gradually reversible charge transfer (CT)-initiated luminescence (GRCTIL) mechanism, whereas CDOH decomposes through an entropic trapping mechanism without an obvious CT process. The thermolysis of CDO(-) is sensitive to solvent effects and is energetically favorable in polar environments compared with the thermolysis of CDOH. The thermolysis of CDO(-) produces the excited oxyluciferin anion (OxyCLn(-)*), which combines with a proton from the environment to form OxyCLnH*, the actual light emitter for the natural system.

Lighting up bioluminescence with coelenterazine: strategies and applications

Bioluminescence-based techniques, such as bioluminescence imaging, BRET and dual-luciferase reporter assay systems, have been widely used to examine a myriad of biological processes. Coelenterazine (CTZ), a luciferin or light-producing compound found in bioluminescent organisms, has sparked great curiosity and interest in searching for analogues with improved photochemical properties. This review summarizes the current development of coelenterazine analogues, their bioluminescence properties, and the rational design of caged coelenterazine towards biotargets, as well as their applications in bioassays. It should be emphasized that the design of caged luciferins can provide valuable insight into detailed molecular processes in organisms and will be a trend in the development of bioluminescent molecules.

Reliability of ROS and RNS detection in hematopoietic stem cells − potential issues with probes and target cell population

Many studies have provided evidence for the crucial role of the reactive oxygen species (ROS) and reactive nitrogen species (RNS) in the regulation of differentiation and/or self-renewal, and the balance between quiescence and proliferation of hematopoietic stem cells (HSCs). Several metabolic regulators have been implicated in the maintenance of HSC redox homeostasis; however, the mechanisms that are regulated by ROS and RNS, as well as their downstream signaling are still elusive. This is partially owing to a lack of suitable methods that allow unequivocal and specific detection of ROS and RNS. In this Opinion, we first discuss the limitations of the commonly used techniques for detection of ROS and RNS, and the problem of heterogeneity of the cell population used in redox studies, which, together, can result in inaccurate conclusions regarding the redox biology of HSCs. We then propose approaches that are based on single-cell analysis followed by a functional test to examine ROS and RNS levels specifically in HSCs, as well as methods that might be used in vivo to overcome these drawbacks, and provide a better understanding of ROS and RNS function in stem cells.

Recent advances in development of imidazo[1,2-a]pyrazines: synthesis, reactivity and their biological applications

The synthesis, reactivity and multifarious biological activities at the different positions of imidazo[1,2-a]pyrazines are concisely discussed in this review.

Effects of alcohols on fluorescence intensity and color of a discharged-obelin-based biomarker

Photoproteins are responsible for bioluminescence of marine coelenterates; bioluminescent and fluorescent biomarkers based on photoproteins are useful for monitoring of calcium-dependent processes in medical investigations. Here, we present the analysis of intensity and color of light-induced fluorescence of Ca(2+)-discharged photoprotein obelin in the presence of alcohols (ethanol and glycerol). Complex obelin spectra obtained at different concentrations of the alcohols at 350- and 280-nm excitation (corresponding to polypeptide-bound coelenteramide and tryptophan absorption regions) were deconvoluted into Gaussian components; fluorescent intensity and contributions of the components to experimental spectra were analyzed. Five Gaussian components were found in different spectral regions-ultraviolet (tryptophan emission), blue-green (coelenteramide emission), and red (hypothetical indole-coelenteramide exciplex emission). Inhibition coefficients and contributions of the components to experimental fluorescent spectra showed that presence of alcohols increased contributions of ultraviolet, violet, and red components, but decreased contributions of components in the blue-green region. The effects were related to (1) changes of proton transfer efficiency in fluorescent S*1 state of coelenteramide in the obelin active center and (2) formation of indole-coelenteramide exciplex at 280-nm photoexcitation. The data show that variation of fluorescence color and intensity in the presence of alcohols and dependence of emission spectra on excitation wavelength should be considered while applying the discharged obelin as a fluorescence biomarker.

Detection of superoxide anion and hydrogen peroxide production by cellular NADPH oxidases

BACKGROUND

The recent recognition that isoforms of the cellular NADPH-dependent oxidases, collectively known as the NOX protein family, participate in a wide range of physiologic and pathophysiologic processes in both the animal and plant kingdoms has stimulated interest in the identification, localization, and quantitation of their products in biological settings. Although several tools for measuring oxidants released extracellularly are available, the specificity and selectivity of the methods for reliable analysis of intracellular oxidants have not matched the enthusiasm for studying NOX proteins.

SCOPE OF REVIEW

Focusing exclusively on superoxide anion and hydrogen peroxide produced by NOX proteins, this review describes the ideal probe for analysis of O2(-) and H2O2 generated extracellularly and intracellularly by NOX proteins. An overview of the components, organization, and topology of NOX proteins provides a rationale for applying specific probes for use and a context in which to interpret results and thereby construct plausible models linking NOX-derived oxidants to biological responses. The merits and shortcomings of methods currently in use to assess NOX activity are highlighted, and those assays that provide quantitation of superoxide or H2O2 are contrasted with those intended to examine spatial and temporal aspects of NOX activity.

MAJOR CONCLUSIONS

Although interest in measuring the extracellular and intracellular products of the NOX protein family is great, robust analytical probes are limited.

GENERAL SIGNIFICANCE

The widespread involvement of NOX proteins in many biological processes requires rigorous approaches to the detection, localization, and quantitation of the oxidants produced. This article is part of a Special Issue entitled Current methods to study reactive oxygen species - pros and cons and biophysics of membrane proteins. Guest Editor: Christine Winterbourn.

Pyrazine derivatives: a patent review (2008 - present)

INTRODUCTION

Pyrazines derivatives are well-known and important two-nitrogen-containing six-membered ring aromatic heterocyclic compounds and can carry substituents at one or more of the four ring carbon atoms. Pyrazines are a class of compounds that occur in nature and various methods have been worked out for their synthesis. A large number of pyrazine derivatives have been found to possess diverse pharmacological properties, which has caused an increasing interest by researchers in this core.

AREA COVERED

This review provides a comprehensive review of the pyrazines derivatives patented between the years 2008 to 2012 as potential active compounds. The patent databases SciFinder and esp@cenet were used to locate patent applications that were published between 2008 to present. Information from articles published was also included.

EXPERT OPINION

The diversity of pyrazines derivatives found in organisms in nature with different applications began to arouse the interest of research in this nucleus. The pyrazines derivatives have numerous prominent pharmacological effects, such as antibacterial, antifungal, antimycobacterial, anti-inflammatory, analgesic, anticancer for different types, antidiabetic, treatment for arteriosclerosis, antiviral. It's the time to conduct further studies aimed at rationalizing the biological activities found in order to develop more effective and clinically interesting compounds.

Fluorescent indicators based on BODIPY

This critical review covers the advances made using the 4-bora-3a,4a-diaza-s-indacene (BODIPY) scaffold as a fluorophore in the design, synthesis and application of fluorescent indicators for pH, metal ions, anions, biomolecules, reactive oxygen species, reactive nitrogen species, redox potential, chemical reactions and various physical phenomena. The sections of the review describing the criteria for rational design of fluorescent indicators and the mathematical expressions for analyzing spectrophotometric and fluorometric titrations are applicable to all fluorescent probes (206 references).