Chemiluminescence of coelenterazine analogues - structures of emitting species -

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.

Coelenterazine Analogue with Human Serum Albumin-Specific Bioluminescence

A synthetic luciferin comprising an imidazopyrazinone core, named HuLumino1, was designed to generate specific bioluminescence with human serum albumin (HSA) in real serum samples. HuLumino1 was developed by attaching a methoxy-terminated alkyl chain to C-6 of coelenterazine and by eliminating a benzyl group at C-8. HSA levels were quantified within 5% error margins of an enzyme-linked immunosorbent assay without the need for any sample pretreatments because of the high specificity of HuLumino1.

Enzymatic activity of albumin shown by coelenterazine chemiluminescence

Bioluminescence, the emission of light from live organisms, occurs in 18 phyla and is the major communication system in the deep sea. It has appeared independently many times during evolution but its origins remain unknown. Coelenterazine bioluminescence discovered in luminous jellyfish is the most common chemistry causing bioluminescence in the sea, occurring in seven phyla. Sequence similarities between coelenterazine luciferases and photoproteins from different phyla are poor (often < 5%). The aim of this study was to examine albumin that binds organic substances as a coelenterazine luciferase to test the hypothesis that the evolutionary origin of a bioluminescent protein was the result of the formation of a solvent cage containing just a few key amino acids. The results show for the first time that bovine and human albumin catalysed coelenterazine chemiluminescence consistent with a mono-oxygenase, whereas gelatin and haemoglobin, an oxygen carrier, had very weak activity. Insulin also catalysed coelenterazine chemiluminescence and was increased by Zn(2+). Albumin chemiluminescence was heat denaturable, exhibited saturable substrate characteristics and was inhibited by cations that bound these proteins and by drugs that bind to human albumin drug site I. Molecular modelling confirmed the coelenterazine binding site and identified four basic amino acids: lys195, arg222, his242 and arg257, potentially important in binding and catalysis similar to naturally occurring coelenterazine bioluminescent proteins. These results support the 'solvent cage' hypothesis for the evolutionary origin of enzymatic coelenterazine bioluminescent proteins. They also have important consequences in diseases such as diabetes, gut disorders and food intolerance where a mono-oxygenase could affect cell surface proteins.

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

In this review I will discuss chemical principles of the luminescence of imidazo[1,2-a]pyrazin-3(7H)-one compounds described to date. The review is composed of two main parts, the first dealing with the bioluminescence of coelenterate luciferin "coelenterazine" and Cypridina luciferin in marine organisms and the second with the chemiluminescence of these luciferins and their analogues. In the second section, possible applications of chemiluminescence and enhanced chemiluminescence in the area of bioassay are also discussed.

Chemiluminescence of 2-methyl-6-arylimidazo-[1,2-a]pyrazin-3(7H)-one in protic solvents: electron-donating substituent effect on the formation of the neutral singlet excited-state molecule

2-Methyl-6-arylimidazo[1,2-a]pyrazin-3(7H)-ones with a substituent such as phenyl, 4-methoxyphenyl or 4-trifluoromethoxyphenyl at the 6-position of the imidazo[1,2-a]pyrazin-3(7H)-one ring system, produced chemiluminescence emission in mixtures of water and DMF and in several mixtures of MeOH and DMF under neutral conditions. Under these protic luminescence conditions, the respective light emissions were generated from neutral singlet excited-state molecules. The electron-donating effect of the 4-methoxy substituent on the phenyl group increased the efficiency of the neutral singlet excited state formation, whereas non-substitution and a 4-trifluoromethoxy group having no electron donating ability decreased the efficiency. The compound having the electron-donating methoxy group substituent showed two chemiluminescence emitters, which generated light at lambda(max) 410-420 nm and 460 nm. It was determined that the neutral molecules in the excited state generating light emission at the shorter wavelengths are neutral singlet excited-state molecules suitable for highly efficient singlet excited-state formation. A role of the electron-donating effect of the methoxy group is postulated to be generation of the special neutral singlet excited-state molecules.