Chemiluminescence of 6-aryl-2-methylimidazo[1,2-a]pyrazin-3(7H)-ones in DMSO/TMG and in diglyme/acetate buffer: support for the chemiexcitation process to generate the singlet-excited state of neutral oxyluciferin in a high quantum yield in the Cypridina (Vargula) bioluminescence mechanism

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.

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.

Development of a Coelenterazine Derivative with Enhanced Superoxide Anion-Triggered Chemiluminescence in Aqueous Solution

Superoxide anion is a reactive oxygen species (ROS) of biological interest. More specifically, it plays a role in intra- and intercellular signaling, besides being associated with conditions such as inflammation and cancer. Given this, efforts have been made by the research community to devise new sensing strategies for this ROS species. Among them, the chemiluminescent reaction of marine Coelenterazine has been employed as a sensitive and dynamic probing approach. Nevertheless, chemiluminescent reactions are typically associated with lower emissions in aqueous solutions. Herein, here we report the synthesis of a new Coelenterazine derivative with the potential for superoxide anion sensing. Namely, this novel compound is capable of chemiluminescence in a dose-dependent manner when triggered by this ROS species. More importantly, the light-emission intensities provided by this derivative were relevantly enhanced (intensities 2.13 × 101 to 1.11 × 104 times higher) in aqueous solutions at different pH conditions when compared to native Coelenterazine. The half-life of the chemiluminescent signal is also greatly increased for the derivative. Thus, a new chemiluminescence molecule with significant potential for superoxide anion sensing was discovered and reported for the first time.

Core-Modified Coelenterazine Luciferin Analogues: Synthesis and Chemiluminescence Properties

In this work on the design and studies of luciferins related to the blue-hued coelenterazine, the synthesis of heterocyclic analogues susceptible to produce a photon, possibly at a different wavelength, is undertaken. Here, the synthesis of O-acetylated derivatives of imidazo[1,2-b]pyridazin-3(5 H)-one, imidazo[2,1-f][1,2,4]triazin-7(1 H)-one, imidazo[1,2-a]pyridin-3-ol, imidazo[1,2-a]quinoxalin-1(5 H)-one, benzo[f]imidazo[1,2-a]quinoxalin-3(11 H)-one, imidazo[1',2':1,6]pyrazino[2,3-c]quinolin-3(11 H)-one, and 5,11-dihydro-3 H-chromeno[4,3-e]imidazo[1,2-a]pyrazin-3-one is described thanks to extensive use of the Buchwald-Hartwig N-arylation reaction. The acidic hydrolysis of these derivatives then gave solutions of the corresponding luciferin analogues, which were studied. Not too unexpectedly, even if these were "dressed" with substituents found in actual substrates of the nanoKAZ/NanoLuc luciferase, no bioluminescence was observed with these compounds. However, in a phosphate buffer, all produced a light signal, by chemiluminescence, with extensive variations in their respective intensity and this could be increased by adding a quaternary ammonium salt in the buffer. This aspect was actually instrumental to determine the emission spectra of many of these luciferin analogues.

Elucidating the chemiexcitation of dioxetanones by replacing the peroxide bond with S–S, N–N and C–C bonds

Replacing the peroxide bond of dioxetanone prevents chemiluminescence by making its thermolysis energetically unfavorable and without a singlet chemiexcitation pathway.

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.

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.

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.

Perspectives on Bioluminescence Mechanisms

The molecular mechanisms of the bioluminescence systems of the firefly, bacteria and those utilizing imidazopyrazinone luciferins such as coelenterazine are gradually being uncovered using modern biophysical methods such as dynamic (ns-ps) fluorescence spectroscopy, NMR, X-ray crystallography and computational chemistry. The chemical structures of all reactants are well defined, and the spatial structures of the luciferases are providing important insight into interactions within the active cavity. It is generally accepted that the firefly and coelenterazine systems, although proceeding by different chemistries, both generate a dioxetanone high-energy species that undergoes decarboxylation to form directly the product in its S₁ state, the bioluminescence emitter. More work is still needed to establish the structure of the products completely. In spite of the bacterial system receiving the most research attention, the chemical pathway for excitation remains mysterious except that it is clearly not by a decarboxylation. Both the coelenterazine and bacterial systems have in common of being able to employ "antenna proteins," lumazine protein and the green-fluorescent protein, for tuning the color of the bioluminescence. Spatial structure information has been most valuable in informing the mechanism of the Ca²⁺ -regulated photoproteins and the antenna protein interactions.

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.