Substituent effects on fluorescence properties of thiazolo[4,5-b]pyrazine derivatives.

Based on spectroscopic measurements and DFT calculations, fluorescence properties of thiazolo[4,5-b]pyrazine (TPy) derivatives with the phenyl group at the C2 position were studied. TPys were readily prepared from the corresponding amidopyrazines, which have a similar fluorescent core to a bioluminescence light emitter, Cypridina oxyluciferin. It was found that the introduction of electron-donating (methoxy and dimethylamino) groups onto the 2-phenyl moiety of the TPy derivatives, as well as the phenyl and 4-(dimethylamino)phenyl groups at C2 and C6, respectively, increases the fluorescence yield and appearance of solvatochromic character. The mechanism of increasing the fluorescence yield depending on the substituents is discussed. These findings provide useful information on designing new TPy fluorophores.

Depth detection limit of a fluorescent object in tissue-like medium with background emission in continuous-wave measurements: a phantom study.

Significance: Although the depth detection limit of fluorescence objects in tissue has been studied, reports with a model including noise statistics for designing the optimum measurement configuration are missing. We demonstrate a variance analysis of the depth detection limit toward clinical applications such as noninvasively assessing the risk of aspiration. Aim: It is essential to analyze how the depth detection limit of the fluorescence object in a strong scattering medium depends on the measurement configuration to optimize the configuration. We aim to evaluate the depth detection limit from theoretical analysis and phantom experiments and discuss the source-detector distance that maximizes this limit. Approach: Experiments for detecting a fluorescent object in a biological tissue-mimicking phantom of ground beef with background emission were conducted using continuous wave fluorescence measurements with a point source-detector scheme. The results were analyzed using a model based on the photon diffusion equations. Then, variance analysis of the signal fluctuation was introduced. Results: The model explained the measured fluorescence intensities and their fluctuations well. The variance analysis showed that the depth detection limit in the presence of ambient light increased with the decrease in the source-detector distance, and the optimum distance was in the range of 10 to 15 mm. The depth detection limit was found to be ∼ 30 mm with this optimum distance for the phantom. Conclusions: The presented analysis provides a guide for the optimum design of the measurement configuration for detecting fluorescence objects in clinical applications.

Spectroscopic studies of the color modulation mechanism of firefly (beetle) bioluminescence with amino-analogs of luciferin and oxyluciferin.

Spectroscopic properties of amino-analogs of luciferin and oxyluciferin were investigated to confirm the color modulation mechanism of firefly (beetle) bioluminescence. Fluorescence solvatochromic character of aminooxyluciferin analogs indicates that the bioluminescence of aminoluciferin is useful for evaluating the polarity of a luciferase active site.

Bioluminescence characteristics of the fruiting body of Mycena chlorophos.

Bioluminescent fungi are widely distributed on land and most belong to the class Basidomycetes. Light of about 530 nm wavelength maximum is emitted continuously. The molecular basis for the light-emitting process remains unclear. We investigated the characteristics of the bioluminescence using cultivated fruiting bodies of M. chlorophos. Only fresh fruiting bodies exhibited long-lasting light emission; rapid decay of light emission was observed with frozen and freeze-dried samples. Freeze-dried samples can be stored at room temperature under dry conditions and may be useful for the isolation of luciferin. The light emission of the fresh fruiting bodies was maintained in various buffers at varying pH; it could be stopped with pH 4 acetate buffer and could be recovered at pH 6. The isolation of luciferin from the fresh fruiting bodies might be possible by the control of buffer pH. The effect of temperature on the light emission of fruiting bodies indicated that bioluminescence in M. chlorophos may involve enzymatic reaction(s). The solubilization of bioluminescent components from the fruiting bodies could not be achieved with various surfactants.

Spectroscopic studies of the light-color modulation mechanism of firefly (beetle) bioluminescence.

To reveal the light-color modulation mechanism of firefly (beetle) bioluminescence, we investigated the spectroscopic properties of the phenolate anion 1-O(-) generated from 5,5-dimethyloxyluciferin (1-OH) using various base/solvent combinations. Phenolate anion 1-O(-) is a model compound for the keto form of wild-type oxyluciferin phenolate anion (OL(-)), which is postulated to be the emitter of the bioluminescence. The fluorescence maxima of 1-O(-) were found to depend on the base/solvent combination used, and they varied in the range 541-640 nm, which covers the almost whole range of the bioluminescence emission maximum. In a polar solvent, where (1)(1-O(-))* and the countercation (the conjugate acid of a base) make a solvent-separated ion pair or a free ion couple, the emission maxima of 1-O(-) were found to be modulated by the solvent polarity. In a less polar solvent, where (1)(1-O(-))* and the countercation are formed as a contact ion pair, the strength of the covalent character of the O8'...H bond between (1)(1-O(-))* and the countercation is operative. The effect of the base/solvent combination on the emission properties of (1)(1-O(-))* was also verified using fluorescence lifetime measurements and density functional theory calculations on 1-O(-) and its ion-pair models. On the basis of these results, we propose the following light-color modulation mechanism: (1) the light emitter is the excited singlet state of OL(-) [(1)(OL(-))*], and (2) light emission from (1)(OL(-))* is modulated by the polarity of the active-site environment of a luciferase and the degree of covalent character of the O8'...H bond between (1)(OL(-))* and a protonated basic moiety in the active site. Mechanisms for variation of the bioluminescence colors and their applications are discussed.

A luciferin analogue generating near-infrared bioluminescence achieves highly sensitive deep-tissue imaging.

In preclinical cancer research, bioluminescence imaging with firefly luciferase and D-luciferin has become a standard to monitor biological processes both in vitro and in vivo. However, the emission maximum (λmax) of bioluminescence produced by D-luciferin is 562 nm where light is not highly penetrable in biological tissues. This emphasizes a need for developing a red-shifted bioluminescence imaging system to improve detection sensitivity of targets in deep tissue. Here we characterize the bioluminescent properties of the newly synthesized luciferin analogue, AkaLumine-HCl. The bioluminescence produced by AkaLumine-HCl in reactions with native firefly luciferase is in the near-infrared wavelength ranges (λmax=677 nm), and yields significantly increased target-detection sensitivity from deep tissues with maximal signals attained at very low concentrations, as compared with D-luciferin and emerging synthetic luciferin CycLuc1. These characteristics offer a more sensitive and accurate method for non-invasive bioluminescence imaging with native firefly luciferase in various animal models.

Multicolor Bioluminescence Obtained Using Firefly Luciferin.

Firefly bioluminescence is widely used in life science research as a useful analysis tool. For example, the adenosine-5`-triphosphate (ATP)-dependent enzymatic firefly bioluminescence reaction has long been utilized as a microbial monitoring tool. Rapid and sensitive firefly luciferin-luciferase combinations are used not only to measure cell viability but also for reporter-gene assays. Recently, bioluminescence was utilized as a noninvasive, real-time imaging tool for living subjects to monitor cells and biological events. However, the number of commercialized luciferase genes is limited and tissue-permeable near-infrared (NIR) region emitting light is required for in vivo imaging. In this review, recent studies describing synthetic luciferin analogues predicted to have red-shifted bioluminescence are summarized. Luciferase substrates emitting red, green, and blue light that were designed and developed in our laboratory are presented. The longest emission wavelength of the synthesized luciferin analogues was recorded at 675 nm, which is within the NIR region. This compound is now commercially available as "Aka Lumine®".

Fluorescence Properties of Diphenylthiazolo[4,5-b]pyrazines Tuned by Donor-Acceptor Substituent Effects.

Fluorescence properties of 2,6- and 2,5-diphenylthiazolo[4,5-b]pyrazine (TPy) derivatives having an electron-donating substituent (methoxy and dimethylamino) on the 6- and 5-phenyl groups were studied. It was found that 2,6-diphenyl derivatives fluoresce more efficiently than 2,5-diphenyl derivatives. Furthermore, a 2,6-diphenyl derivative having an additional cyano group on the 2-phenyl ring was an excellent fluorophore showing a wide solvatochromism with great fluorescence yields. Based on the obtained spectroscopic data and mechanistic explanations concerning the substituent effects on the fluorescence properties, useful information on designing new TPy fluorophores is provided.

Analysis of sugar epimers using mass spectrometry: N-acetyllactosamine-6,6'-disulfate and the 2'-epimer.

Galbeta1-4GlcNAc-6,6'-disulfate and 2'-epimer corresponding to Galbeta1- 4ManNAc-6,6'-disulfate were distinguished by mass spectrometry by utilizing fast atom bombardment (FAB), electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) methods. As for the steric information, negative-ion ESI mass spectrometry/mass spectrometry (MS/MS) provides the most extensive data, but FAB MS/MS also reveals detailed structural information of interest in our case, where MALDI MS is not yet fully equipped with post-source decay.

Anoikis induction and inhibition of peritoneal metastasis of pancreatic cancer cells by a nuclear factor-κB inhibitor, (-)-DHMEQ.

The transcription factor nuclear factor-κB (NF-κB) plays a crucial role in pancreatic cancer (PC) progression. NF-κB is also involved in resistance to anoikis, a special type of apoptosis induced when cells are detached from the extracellular matrix or other cells. Anoikis resistance is related to the metastatic abilities of tumor cells; however, little is known about anoikis induction as it relates to inhibition of PC metastasis by NF-κB inhibitors. Here we used a specific NF-κB inhibitor, (-)-dehydroxymethylepoxyquinomicin (DHMEQ), to investigate anoikis induction and peritoneal metastasis suppression following NF-κB inhibition. We transduced Gluc, a secretory form of luciferase, into a PC cell line, AsPC-1 (AsPC-1-Gluc), for our in vivo experiments. (-)-DHMEQ induced anoikis in AsPC-1-Gluc cells as measured by cell survival assays and flow cytometry. The DNA-binding activity of p65 was enhanced immediately after cell detachment from culture dishes in ELISA assays. Some antiapoptotic proteins such as cellular inhibitor of apoptotic protein-1 were consequently upregulated on Western blots. (-)-DHMEQ prevented this increase in p65 activity and the subsequent expressions of antiapoptotic molecules. In a murine xenograft model, anoikis-resistant PC cell lines tended to metastasize to the peritoneum more than anoikis-sensitive cells, suggesting a correlation between anoikis sensitivity and peritoneal metastasis. (-)-DHMEQ successfully inhibited peritoneal metastasis of AsPC-1-Gluc cells. We monitored metastasis inhibition by ex vivo chemiluminescent detection of Gluc secreted from tumor cells into murine plasma and by in vivo imaging. Our results suggest that (-)-DHMEQ inhibited peritoneal dissemination by preventing anoikis resistance of PC cells.

Distinction of sialyl anomers on ESI- and FAB-MS/MS: stereo-specific fragmentations.

An anomeric pair of the lysoglyceroganglioside 1-O-octadecyl-3-O-(N-acetyl)neuraminyl-sn-glycerol sodium salt was studied to see if sialic anomers were distinguishable by mass spectra. It was evident that, in the electrospray ionization and fast-atom bombardment product-ion spectra: (1) in the positive MS(2) product-ion spectrum, the beta- anomer showed an unexpected aglycone-side sodiated sodium alkoxide ion, which was absent for the alpha-anomer; (2) in both polarities the beta-anomer showed dehydration much more easily than the alpha-anomer; and (3) in the negative MS(2) product-ion spectrum, the beta-anomer also readily showed decarboxylation. Our hypothesis is that, although several easily interconvertible conformations may be allowed, the one having the large aglycone in the equatorial orientation affects the collision-induced dissociation fragmentations.

The reaction mechanism for the high quantum yield of Cypridina (Vargula) bioluminescence supported by the chemiluminescence of 6-aryl-2-methylimidazo[1,2-a]pyrazin-3(7H)-ones (Cypridina luciferin analogues).

To establish the reaction mechanism of the high-quantum-yield bioluminescence in Cypridina (Vargula), we investigated the chemiluminescence of 6-aryl-2-methylimidazo[1,2-a]pyrazin-3(7H)-ones (1H) as Cypridina luciferin analogues in DMSO-1,1,3,3-tetramethylguanidine and in diglyme-acetate buffer. We found that the chemiluminescence of 1H with an electron-donating aryl group, such as a 4-(dimethylamino)phenyl, 3-indolyl or 3-(1-methyl)indolyl group, gave a high quantum yield (Phi(CL)) in diglyme-acetate buffer. This indicates that the reaction mechanism producing this high Phi(CL) involves the chemiexcitation of a neutral dioxetanone intermediate possessing an electron-donating aryl group to the singlet excited state of neutral acetamidopyrazine (the light emitter). In addition, we investigated the fluorescence of acetamidopyrazines and performed DFT calculations for neutral dioxetanones and the transition states (TS) of the dioxetanone's decomposition. The results made it clear that the electron-donating aryl group gives the TS and the singlet-excited acetamidopyrazine (S(1)) a strong intramolecular charge transfer (ICT) character, and their similar ICT character leads to the ICT TS --> S(1) route in the charge transfer-induced luminescence (CTIL) mechanism for efficient chemiexcitation. The reaction mechanism of the chemiluminescence of 1H can explain the highly efficient chemiexcitation of Cypridina bioluminescence.

Firefly luciferase exhibits bimodal action depending on the luciferin chirality.

Firefly luciferase is able to convert L-luciferin into luciferyl-CoA even under ordinary aerobic luciferin-luciferase reaction conditions. The luciferase is able to recognize strictly the chirality of the luciferin structure, serving as the acyl-CoA synthetase for L-luciferin, whereas d-luciferin is used for the bioluminescence reaction. D-Luciferin inhibits the luciferyl-CoA synthetase activity of L-luciferin, whereas L-luciferin retards the bioluminescence reaction of D-luciferin, meaning that both enzyme activities are prevented by the enantiomer of its own substrate.

Essential 110Cys in active site of membrane-associated prostaglandin E synthase-2.

The amino acid sequence of membrane-associated prostaglandin (PG) E synthase-2 (mPGE synthase-2), which has a broad specificity in its thiol requirement for a catalytic activity, has the consensus region from 104Leu to 120Leu found in glutaredoxin and of thioredoxin. The sequence of Cys-x-x-Cys in the consensus region is the active site for thioredoxin and mPGE synthase-2 also has this amino acid sequence (110Cys-x-x-113Cys). The mutation from 110Cys to Ser or the double mutation from 110Cys and 113Cys to Ser caused loss of PGE synthase activity, whereas the single mutation from 113Cys to Ser did not affect the enzyme activity. These results indicate that 110Cys, but not 113Cys, is the essential amino acid in the active site of mPGE synthase-2. 110Cys is an important amino acid in PGE synthase activity and plays the critical role as Cys at the same position in redoxin. Moreover, we found that the reduced form of lipoic acid (dihydrolipoic acid) serves as one of the natural activators of mPGE synthase-2 in the cells.

Synthesis of prostaglandin F ethanolamide by prostaglandin F synthase and identification of Bimatoprost as a potent inhibitor of the enzyme: new enzyme assay method using LC/ESI/MS.

Prostaglandin (PG) D(2) ethanolamide (prostamide D(2)) was reduced to 9alpha,11beta-PGF(2) ethanolamide (9alpha,11beta-prostamide F(2)) by PGF synthase, which also catalyzes the reduction of PGH(2) and PGD(2) to PGF(2alpha) and 9alpha,11beta-PGF(2), respectively. These enzyme activities were measured by a new method, the liquid chromatographic-electrospray ionization-mass spectrometry (LC/ESI/MS) technique, which could simultaneously detect the substrate and all products. PGF(2alpha), 9alpha,11beta-PGF(2), PGD(2), PGH(2), 9alpha,11beta-prostamide F(2), and prostamide D(2) were separated on a TSKgel ODS 80Ts column, ionized by electrospray, and detected in the negative mode. Selected ion monitoring (SIM) of m/z 353 ([M-H](-)), 353 ([M-H](-)), 351 ([M-H](-)), 333 ([M-H-H(2)O](-)), 456 ([M+59](-)), and m/z 358 ([M-37](-)) was used for quantifying PGF(2alpha), 9alpha,11beta-PGF(2), PGD(2), PGH(2), 9alpha,11beta-prostamide F(2), and prostamide D(2), respectively. The detection limit for PGF(2alpha) and 9alpha,11beta-PGF(2) was 0.01pmol; that for PGH(2) and PGD(2), 0.1pmol; and that for prostamide D(2) and 9alpha,11beta-prostamide F(2), 0.5 and 0.03pmol, respectively. The LC/ESI/MS technique for measuring PGF synthase activity showed higher sensitivity than other methods. Using this method, we found that Bimatoprost, the ethyl amide analog of 17-phenyl-trinor PGF(2alpha) and an anti-glaucoma agent, inhibited all three reductase activities of PGF synthase when used at a low concentration. These results suggest that Bimatoprost also behaves as a potent PGF synthase inhibitor in addition to having prostamide-like activity.