Theoretical Study on the Formation and Decomposition Mechanisms of Coelenterazine Dioxetanone.

Bioluminescence has been drawing broad attention due to its high signal-to-noise ratio and high bioluminescence quantum yields, which has been widely applied in the fields of biomedicine, bioanalysis, and so on. Among numerous bioluminescent substrates, coelenterazine is famous for its wide distribution. However, the oxygenation reaction mechanism of coelenterazine is far from being completely understood. In this paper, the formation and decomposition mechanisms of coelenterazine dioxetanone were investigated via density functional theory (DFT) and time-dependent (TD) DFT approaches. The results showed that the oxygenation reaction first occurred along the triplet-state potential energy surface (PES), after the intersystem crossing (ISC), second jumped to the diradical-state PES, and ultimately formed coelenterazine dioxetanone. For the decomposition mechanism of dioxetanone, the computational results showed that the chemiexcitation of neutral dioxetanone was more efficient than that of other dioxetanone species. Moreover, the diradical properties and the degree of ionic character are modified by the counter ions.

An ultrastable La-MOF for catalytic hydrogen transfer of furfural: in situ activation of the surface.

The poor stability of metal-organic frameworks (MOFs) severely limits their catalytic application. The in situ activation of stable MOF catalysts not only simplifies the catalytic process, but also reduces energy consumption. Therefore, it is meaningful to explore the in situ activation of the MOF surface in the actual reaction process. In this paper, a novel rare-earth MOF La2(QS)3(DMF)3 (LaQS) was synthesized, which exhibited ultra-high stability not only in organic solvents but also in aqueous solutions. When LaQS was used as a catalyst for the catalytic hydrogen transfer (CHT) of furfural (FF) to furfuryl alcohol (FOL), the FF conversion and FOL selectivity reached 97.8% and 92.1%, respectively. Meanwhile, the high stability of LaQS ensures an enhanced catalytic cycling performance. The excellent catalytic performance is mainly attributed to the acid-base synergistic catalysis of LaQS. More importantly, it has been confirmed by control experiments and DFT calculation that the in situ activation in catalytic reactions leads to the formation of acidic sites in LaQS, together with the uncoordinated oxygen atoms of sulfonic acid groups in LaQS as Lewis bases, which can synergistically activate FF and isopropanol. Finally, the mechanism of in situ activation-caused acid-base synergistic catalysis of FF is speculated. This work provides meaningful enlightenment for the study of the catalytic reaction path of stable MOFs.

Ultrahigh Metal Content Carbon-Based Catalyst for Efficient Hydrogenation of Furfural: The Regulatory Effect of Glycerol.

The development of high-content non-noble metal nanocatalysts is important for multiphase catalysis applications. However, it is a challenge to solve the agglomeration in the preparation of high-content metal catalysts. In this paper, a carbon-based catalyst (Co@CN-G-600) with 71.28 wt % cobalt metal content was prepared using a new strategy of gas-phase carbon coating assisted by glycerol. The core of this strategy is to maintain the spacing of metallic cobalt by continuous replenishment of dissociated ligands during pyrolysis over gas-phase glycerol. This approach is also applicable to other non-noble metals. When Co@CN-G-600 was further used as a catalyst for the selective hydrogenation of furfural (FF) to prepare furfuryl alcohol (FOL), the yield of FOL was >99.9% under mild conditions of 80 °C, compared to only 8.23% catalytic yield at up to 130 °C for Co@CN-600 without glycerol. The excellent catalytic performance mainly lies in the fact that the introduction of glycerol modulates the size effect, electronic effect, and acidic site intensity of the high-content Co catalyst, which promotes the activation of FF and hydrogen. Meanwhile, the optimized specific surface area and pore structure by glycerol improve the accessibility of high-density active sites and promote more efficient mass transfer. In addition, the introduction of glycerol produced a graphitic carbon layer encapsulation structure relative to Co@CN-600, which substantially improved the cycling stability of the catalyst. This study resolves the paradox of high content and high dispersion of non-noble metal catalysts in the synthesis process and provides a general pathway and example for the preparation of stable high-content metal catalysts.

Mechanistic Insight into the Chemiluminescent Decomposition of Cypridina Dioxetanone and the Chemiluminescent, Fluorescent Properties of the Light Emitter of Cypridina Bioluminescence.

Cypridina bioluminescence has been increasingly used in bioimaging, bioanalysis, and biomedicine, due to high quantum yield and high signal-to-noise ratio. However, there is still no consensus regarding different aspects of the chemiluminescent mechanism of this system, which impairs the development of new applications. Herein, we have used a theoretical DFT and TD-DFT approach to (i) determine the identity of the dioxetanone species responsible for efficient chemiexcitation and (ii) identify the bioluminescent emitter and determine if light-emission occurs from the fluorescent or chemiluminescent state. Our results demonstrate that upon oxygenation of the imidazopyrazinone scaffold, a dioxetanone with a neutral amide group and a cationic guanidinopropyl group is formed. This species is efficiently chemiexcited (with no obvious charge transfer step) to the corresponding oxyluciferin with a neutral amide and cationic guanidinopropyl groups. After the "dark" chemiluminescent state, this oxyluciferin species is converted into a bright blue-emitting fluorescent state.

Bottom-Up Construction of Active Sites in a Cu-N4-C Catalyst for Highly Efficient Oxygen Reduction Reaction.

Bottom-up construction of efficient active sites in transition metal-nitrogen-carbon (M-N-C) catalysts for oxygen reduction reaction (ORR) from single molecular building blocks remains one of the most difficult challenges. Herein, we report a bottom-up approach to produce a highly active Cu-N4-C catalyst with well-defined Cu-N4 coordination sites derived from a small molecular copper complex containing Cu-N4 moieties. The Cu-N4 moieties were found to be covalently integrated into graphene sheets to create the Cu-N4 active sites for ORR. Furthermore, the activity was boosted by tuning the structure of active sites. We find that the high ORR activity of the Cu-N4-C catalyst is related to the Cu-N4 center linked to edges of the graphene sheets, where the electronic structure of the Cu center has the right symmetry for the degenerate π* orbital of the O2 molecule. These findings point out the direction for the synthesis of the M-N-C catalysts at the molecular level.

Facile Synthesis, Characterization of Poly-2-mercapto-1,3,4-thiadiazole Nanoparticles for Rapid Removal of Mercury and Silver Ions from Aqueous Solutions.

Industrial pollution by heavy metal ions such as Hg2+ and Ag⁺ is a universal problem owing to the toxicity of heavy metals. In this study, a novel nano-adsorbent, i.e., poly-2-mercapto-1,3,4-thiadiazole (PTT), was synthesized and used to selectively adsorb mercury and silver ions from aqueous solutions. PTT nanoparticles were synthesized via chemical oxidative dehydrogenation polymerization under mild conditions. Oxidant species, medium, monomer concentration, oxidant/monomer molar ratio, and polymerization temperature were optimized to obtain optimum yields. The molecular structure and morphology of the nanoparticles were analyzed by ultraviolet-visible (UV-Vis), Fourier transform infrared (FT-IR), matrix-assisted laser desorption/ionization/time-of-flight (MALDI/TOF) mass and X-ray photoelectron (XPS) spectroscopies, wide-angle X-ray diffraction (WAXD), theoretical calculations and transmission electron microscopy (TEM), respectively. It was found that the polymerization of 2-mercapto-1,3,4-thiodiazole occurs through head-to-tail coupling between the S(2) and C(5) positions. The PTT nanoparticles having a peculiar synergic combination of four kinds of active groups, S⁻, ⁻SH, N⁻N, and =N⁻ with a small particle size of 30⁻200 nm exhibit ultrarapid initial adsorption rates of 1500 mg(Hg)·g-1·h-1 and 5364 mg(Ag)·g-1·h-1 and high adsorption capacities of up to 186.9 mg(Hg)·g-1 and 193.1 mg(Ag)·g-1, becoming ultrafast chelate nanosorbents with high adsorption capacities. Kinetic study indicates that the adsorption of Hg2+ and Ag⁺ follows the pseudo-second-order model, suggesting a chemical adsorption as the rate-limiting step during the adsorption process. The Hg2+ and Ag⁺-loaded PTT nanoparticles could be effectively regenerated with 0.1 mol·L-1 EDTA or 1 mol·L-1 HNO3 without significantly losing their adsorption capacities even after five adsorption⁻desorption cycles. With these impressive properties, PTT nanoparticles are very promising materials in the fields of water-treatment and precious metals recovery.

Theoretically obtained insight into the mechanism and dioxetanone species responsible for the singlet chemiexcitation of Coelenterazine.

Coelenterazine is a widespread bioluminescent substrate for a diverse set of marine species. Moreover, its imidazopyrazinone core is present in eight phyla of bioluminescent organisms. Given their very attractive intrinsic properties, these bioluminescent systems have been used in bioimaging, photodynamic therapy of cancer, as gene reporter and in sensing applications, among others. While it is known that bioluminescence results from the thermolysis of high-energy dioxetanones, the mechanism and dioxetanone species responsible for the singlet chemiexcitation of Coelenterazine are not fully understood. The theoretical characterization of the reactions of model Coelenterazine dioxetanones showed that efficient chemiexcitation is caused by a neutral dioxetanone with limited electron and charge transfer, by accessing a region of the PES where ground and excited states are nearly-degenerated. This finding was supported by calculation of equilibrium constants, which showed that only neutral dioxetanone is present in conditions associated with bioluminescence. Moreover, while cationic amino-acids easily protonate amide dioxetanone, anionic ones cannot deprotonate the neutral species. These results indicate that, contrary to existent theories, efficient chemiexcitation can occur with significant electron and/or charge transfer. In fact, these processes can be prejudicial to chemiexcitation, as anionic dioxetanones showed a less efficient chemiexcitation despite the occurrence of significant electron and charge transfer.

Highly Productive Synthesis, Characterization, and Fluorescence and Heavy Metal Ion Adsorption Properties of Poly(2,5-dimercapto-1,3,4-thiadiazole) Nanosheets.

Poly(2,5-dimercapto-1,3,4-thiadiazole) (PBT) nanosheets were synthesized by chemical oxidative synthesis under mild conditions. The media, oxidant species, monomer concentrations, oxidant/monomer molar ratio, and temperature were optimized to achieve higher yields and better performance. The molecular structure, morphology, and properties of the nanosheets were analyzed by Fourier transform infrared (FT-IR), ultraviolet-visible (UV-Vis), and fluorescence spectroscopies, wide-angle X-ray diffraction (WAXD), matrix-assisted laser desorption/ionization/time-of-flight (MALDI-TOF) mass spectrometry, X-ray photoelectron spectroscopy (XPS), scanning electronic microscopy (SEM), transmission electron microscopy (TEM), and simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC). It was found that the polymerization of 2,5-dimercapto-1,3,4-thiadiazole occurs via dehydrogenation coupling between two mercapto groups to form the ⁻S⁻S⁻ bond. PBTs show the highest polymerization yield of up to 98.47% and form uniform nanosheets with a thickness of 89~367 nm. poly(2,5-dimercapto-1,3,4-thiadiazole) polymers (PBTs) exhibit good chemical resistance, high thermostability, interesting blue-light emitting fluorescence, and wonderful heavy metal ion adsorption properties. Particularly, the PBT nanosheets having a unique synergic combination of three kinds of active ⁻S⁻, ⁻SH, and =N⁻ groups with a moderate specific area of 15.85 m² g-1 exhibit an ultra-rapid initial adsorption rate of 10,653 mg g-1 h-1 and an ultrahigh adsorption capacity of up to 680.01 mg g-1 for mercury ion, becoming ultrafast chelate nanosorbents with a high adsorption capacity. With these impressive properties, PBT nanosheets are very promising materials in the fields of water treatment, sensors, and electrodes.

A Computational Investigation of the Equilibrium Constants for the Fluorescent and Chemiluminescent States of Coelenteramide.

In spite of recent advances in understanding the mechanism of coelenterate bioluminescence, there is no consensus about which coelenteramide specie and/or state are the light emitter. In this study, a systematic investigation of the geometries and spectra of all possible light emitters has been performed at the TD ωB97XD/6-31+G(d) level of theory, including various fluorescent and chemiluminescent states in vacuum, in a hydrophobic environment and in aqueous solution. To deduce the most probable form of the fluorescent and chemiluminescent coelenteramide emitter, the equilibrium constants for the fluorescent and chemiluminescent states connecting the various species have been calculated. ωB97XD gives a qualitatively good description of fluorescent and chemiluminescent structures. Coelenteramide is formed in a "dark" chemiluminescent state and must evolve to a bright fluorescent state. Moreover, the photoacidity of the phenol group is significantly higher in the fluorescent state than in the chemiluminescent state, which allows the formation of phenolate coelenteramide and clarifies its role as the bioluminescent emitter.

Catechol degradation on hematite/silica-gas interface as affected by gas composition and the formation of environmentally persistent free radicals.

Environmentally persistent free radicals (EPFRs) formed on a solid particle surface have received increasing attention because of their toxic effects. However, organic chemical fate regulated by EPFRs has rarely been investigated, and this information may provide the missing link in understanding their environmental behavior. Previous studies have suggested that the reduction of transition metals is involved in EPFRs formation. We thus hypothesize that an oxidative environment may inhibit EPFRs formation in particle-gas interface, which will consequently release free radicals and accelerate organic chemical degradation. Our result indicates that a 1% hematite coating on a silica surface inhibited catechol degradation in N2, especially at low catechol loadings on solid particles (SCT). However, under an O2 environment, catechol degradation decreased when SCT was <1 µg/mg but increased when SCT was >1 µg/mg. Stable organic free radicals were observed in the N2 system with g factors in the 2.0035-2.0050 range, suggesting the dominance of oxygen-centered free radicals. The introduction of O2 into the catechol degradation system substantially decreased the free radical signals and decreased the Fe(II) content. These results were observed in both dark and light irradiation systems, indicating the ubiquitous presence of EPFRs in regulating the fate of organic chemicals.

The effect of micro-environment on luminescence of aequorin: the role of amino acids and explicit water molecules on spectroscopic properties of coelenteramide.

Despite the fact that the luminescence reaction mechanism of aequorin has been intensively investigated, details in luminescence such as the effect of important amino acids residues and explicit water molecules on spectroscopic properties of coelenteramide remain unclear. In this work, the effect of amino acids residues His16, Tyr82, Trp86, Phe113, Trp129, Tyr132, explicit water molecules Wat505 and Wat405 on the spectral properties of CLM(-) has been studied by CAM-B3LYP, TD M06L and TD CAM-B3LYP methods in hydrophobic environment and aqueous solution. In hydrophobic environment, the amino acids or water molecules have no significant effect on the absorption. Tyr82 and Trp86 move close to CLM(-) changes the hydrogen bond network, and thus, the spectral properties is significantly affected by the hydrogen bonds between His16H(+)+Tyr82+Trp86 and CLM(-). Tyr82, Trp86 hydrogen bonding to CLM(-) upshifts the excited energy and helps emission spectra shift to blue region. Therefore, it is concluded that His16H(+)+Tyr82+Trp86 modify the emission spectra. The molecular electrostatic potential indicated that the greater electron density is located at the oxygen atom of 6-p-hydroxyphenyl group of CLM(-), and it facilitates the formation of hydrogen bond with His16H(+)+Tyr82+Trp86. It is a critical condition for the modification of emission spectra. It is expected to help to understand the interactions between emitter and amino acids in the micro environment.

Poly(1-amino-5-chloroanthraquinone): highly selective and ultrasensitive fluorescent chemosensor for ferric ion.

Poly(1-amino-5-chloroanthraquinone) (PACA) was firstly synthesized by a chemically oxidative interfacial polymerization. The PACA has been developed as a fluorescent sensor for the determination of Fe(III) in semi-aqueous solution at pH 7.0. The sensor exhibited remarkably high sensitivity toward Fe(3+) since the fluorescence of the polymer could be significantly quenched even though trace Fe(3+) was added. The sensor showed a linear fluorescence emission response over a wide concentration range from 1.0 × 10(-10) to 1.0 × 10(-4) M, with an ultra-low detection limit of 2.0 × 10(-11) M. The quenching of the fluorescence was found to be static one due to the formation of non-fluorescent complex in the ground state.

Diaqua-bis-(4,4'-bipyridine-κN)bis-(2,4,5-trifluoro-3-hy-droxy-benzoato-κO)manganese(II).

In the title compound, [Mn(C(7)H(2)F(3)O(3))(2)(C(10)H(8)N(2))(2)(H(2)O)(2)], the Mn(II) ion, situated on a centre of inversion, has a distorted octa-hedral coordination geometry and is coordinated by two N atoms from two 4,4'-bipyridine ligands, two O atoms from two 2,4,5-trifluoro-3-hy-droxy-benzoate ligands and two water mol-ecules. Inter-molecular O-H⋯N hydrogen bonds link the mol-ecules into a chain along the a axis. Inter-actions between neighboring chains occur through O-H⋯O hydrogen bonds, which link the chains into a two-dimensional supra-molecular network parallel to the ac plane. In addition, O-H⋯O hydrogen bonds between the water mol-ecules and carboxyl-ate groups also exist in the the crystal structure.

Theoretical investigation on the origin of yellow-green firefly bioluminescence by time-dependent density functional theory.

The question whether the emitter of yellow-green firefly bioluminescence is the enol or keto-constrained form of oxyluciferin (OxyLH(2)) still has no definitive answer from experiment or theory. In this study, Arg220, His247, adenosine monophosphate (AMP), Water324, Phe249, Gly343, and Ser349, which make the dominant contributions to color tuning of the fluorescence, are selected to simulate the luciferase (Luc) environment and thus elucidate the origin of firefly bioluminescence. Their respective and compositive effects on OxyLH(2) are considered and the electronic absorption and emission spectra are investigated with B3LYP, B3PW91, and PBE1KCIS methods. Comparing the respective effects in the gas and aqueous phases revealed that the emission transition is prohibited in the gas phase but allowed in the aqueous phase. For the compositive effects, the optimized geometry shows that OxyLH(2) exists in the keto(-1) form when Arg220, His247, AMP, Water324, Phe249, Gly343, and Ser349 are all included in the model. Furthermore, the emission maximum wavelength of keto(-1)+Arg+His+AMP+H(2)O+Phe+Gly+Ser is close to the experimental value (560 nm). We conclude that the keto(-1) form of OxyLH(2) is a possible emitter which can produce yellow-green bioluminescence because of the compositive effects of Arg220, His247, AMP, Water324, Phe249, Gly343, and Ser349 in the luciferase environment. Moreover, AMP may be involved in enolization of the keto(-1) form of OxyLH(2). Water324 is indispensable with respect to the environmental factors around luciferin (LH(2)).

A time-dependent density functional theory investigation on the origin of red chemiluminescence.

Is the resonance-based anionic keto form of oxyluciferin the chemical origin of multicolor bioluminescence? Can it modulate green into red luminescence? There is as yet no definitive answer from experiment or theory. The resonance-based anionic keto forms of oxyluciferin have been proposed as a cause of multicolor bioluminescence in the firefly. We model the possible structures by adding sodium or ammonium cations and investigating the ground- and excited-state geometries as well as the electronic absorption and emission spectra. A role for the resonance structures is obvious in the gas phase. The absorption and emission spectra of the two structures are quite different--one in the blue and another in the red. The differences in the spectra of the models are small in aqueous solution, with all the absorption and emission spectra in the yellow-green region. The resonance-based anionic keto form of oxyluciferin may be one origin of the red-shifted luminescence but is not the exclusive explanation for the variation from green (approximately 530 nm) to red (approximately 635 nm). We study the geometries, absorption, and emission spectra of the possible protonated compounds of keto(-1) in the excited states. A new emitter keto(-1)'-H is considered.