Protonation Of Nitrogen Research Articles

Exploring the factors influencing the ketoenamine-enolimine tautomeric equilibrium of pyridoxal 5′-phosphate in branched-chain aminotransferases

Pyridoxal 5′-phosphate (PLP), the active form of vitamin B6, is a critical coenzyme for various enzymes. It generates a Schiff base with the substrate and exhibits ketoenamine and enolimine tautomeric forms due to intramolecular proton transfer. This study aims to ascertain the predominant tautomeric form of the PLP Schiff base in MtIlvE and analyze its influencing factors. Molecular dynamics simulations indicate that the ketoenamine tautomer has higher binding free energy than the enolimine tautomer. Density functional theory calculations suggest that, despite their ability to interconvert at a relatively low energy barrier, the ketoenamine tautomer is thermodynamically more stable. Factors affecting the keto-enol tautomeric equilibrium were investigated by constructing various QM-cluster models. Our results demonstrate that both the protonation of the pyridine nitrogen and the presence of Tyr209, which stabilizes the O3 anion, shift the tautomeric equilibrium toward the ketoenamine configuration. These findings provide a theoretical basis for investigating enzyme catalytic mechanisms.

Pinning Down Small Populations of Photoinduced Intermediates Using Transient Absorption Spectroscopy and Time-Dependent Density Functional Theory Difference Spectra to Provide Mechanistic Insight into Controlling Pyridine Azo Dynamics with Protons.

In this work, the impact of protonation on the photoisomerization (trans → cis) and reversion (cis → trans) of three pyridine-based azo dyes (PyrN) is investigated by using a combination of transient absorption spectroscopy and time-dependent density functional theory computed difference spectra. The photophysical behaviors of the PyrN dyes are altered by the addition of one or two protons. Protonation of basic pyridine nitrogens results in an ultrafast accelerated reversion mechanism after photoisomerization, while protonation of azo bond nitrogens restricts cis isomer formation entirely. Computed difference spectra provide spectral signatures that are critical for the assignment of low-population long-lived states, providing direct evidence of the accelerated reversion mechanism. Thus, the addition of organic acids can selectively control the photophysics of azo dyes for a wide range of applications, including materials design and pharmaceuticals.

Physically-Based Descriptors of the Nitrogen Reduction Reaction on Transition Metal Nitrides

Due to the high energy intensity and consequent carbon footprint of the Haber-Bosch process for ammonia production, researchers have been intensely studying the electrochemical nitrogen reduction reaction (NRR) as a means to directly produce ammonia using clean energy with lower emissions of carbon by-products. One exciting class of materials for NRR catalysis are transition metal nitrides (TMNs), as they have been shown to have good stability, flexible electronic structures, and decent electrochemical activity [1]. Previous studies using density functional theory (DFT) have explored the energetics of known NRR pathways (associative, dissociative, and Mars-van Krevelen) on the mononitrides of all naturally occurring d-block metals in rock salt and zincblende structures [2]. The most favorable NRR mechanism (determined to be the pathway that contains the smallest maximum potential change across all steps of the reaction) on these materials was shown to be the Mars-van Krevelen mechanism. Uncovering electronic, structural, and dynamic descriptors that correlate well with the energetics of this favorable NRR mechanism on TMNs will be useful in mitigating the need to run full DFT calculations when expanding the search space of potential catalyst candidates, such as ternary transition metal nitrides (TTMNs) and perovskite transition metal oxynitrides (TMONs). Previous literature has shown that the proton adsorption energy is a good descriptor for catalytic NRR activity on TMNs [3], oxygen vacancy content [4] and the oxygen p-band center of bulk perovskites [5] are good descriptors for catalytic oxygen evolution reaction performance, and the metal d-band center of pure transition metal catalysts is a good descriptor for reactivity and adsorption energies [6]. Inspired by these results, this work uses DFT on a set of TMNs to calculate a variety of potential physics-based descriptors, including: metal d and nitrogen p-band centers, hybridization between nitrogen p and metal d orbitals, nitrogen vacancy content, surface Bader charges, and phonon band centers. Since the energetics of the NRR can be modulated by the properties of either the atoms in the bulk or in the top surface layers, we calculate all of our descriptors from atoms in the TMN bulk form as well as from surface and subsurface atoms in the TMN slab form.Our results reveal several correlations between the descriptors and material attributes (such as adsorption energies and vacancy formation energies), which in turn govern the energetics of the NRR on the TMNs. We find that the N2 adsorption energy is negatively correlated with the metal d-band center, the N adsorption energy is positively correlated with the nitrogen p-band center, and both the proton adsorption energy and nitrogen vacancy formation energy are negatively correlated with nitrogen p and metal d orbital hybridization. Interestingly, we also find that for some attributes, the subsurface layer atoms dictate the attribute values and not the surface layer atoms. This suggests that heterogeneous catalysis reactivity may be modulated by atomic properties not purely from the bulk or purely from the surface, but rather from multiple subsurface layers as well.Overall, the insights gained from our descriptor analysis can be used to constrain the search and design space of future potential NRR catalysts, such as TTMNs and TMONs. In doing so, the descriptors can be used in future works in either a high-throughput screening or machine learning framework to allow for the inverse material design of optimal NRR catalysts. These optimal catalysts can in turn lead to the emergence of electrochemical ammonia synthesis in industrial production.References Qiao Z, Johnson D and Djire A. Cell Rep Phys Sci 2021; 2.Abghoui Y and Skúlason E. Catal Today 2017; 286: 69-77.Abghoui Y, Garden AL, Hlynsson VF, Björgvinsdóttir S, Ólafsdóttir H and Skúlason E. Physical Chemistry Chemical Physics 2015; 17: 4909-4918.Marelli E, Gazquez J, Poghosyan E, Müller E, Gawryluk DJ, Pomjakushina E, Sheptyakov D, Piamonteze C, Aegerter D, Schmidt TJ, Medarde M and Fabbri E. Angewandte Chemie - International Edition 2021; 60: 14609-14619.Jacobs R, Hwang J, Shao-Horn Y and Morgan D. Chemistry of Materials 2019; 31: 785-797.Nørskov JK, Abild-Pedersen F, Studt F and Bligaard T. Proc Natl Acad Sci U S A 2011; 108: 937-943.

Novel Insights into the Catalytic Mechanism of Collagenolysis by Zn(II)-Dependent Matrix Metalloproteinase-1.

Collagen hydrolysis, catalyzed by Zn(II)-dependent matrix metalloproteinases (MMPs), is a critical physiological process. Despite previous computational investigations into the catalytic mechanisms of MMP-mediated collagenolysis, a significant knowledge gap in understanding remains regarding the influence of conformational sampling and entropic contributions at physiological temperature on enzymatic collagenolysis. In our comprehensive multilevel computational study, employing quantum mechanics/molecular mechanics (QM/MM) metadynamics (MetD) simulations, we aimed to bridge this gap and provide valuable insights into the catalytic mechanism of MMP-1. Specifically, we compared the full enzyme-substrate complex in solution, clusters in solution, and gas-phase to elucidate insights into MMP-1-catalyzed collagenolysis. Our findings reveal significant differences in the catalytic mechanism when considering thermal effects and the dynamic evolution of the system, contrasting with conventional static potential energy surface QM/MM reaction path studies. Notably, we observed a significant stabilization of the critical tetrahedral intermediate, attributed to contributions from conformational flexibility and entropy. Moreover, we found that protonation of the scissile bond nitrogen occurs via proton transfer from a Zn(II)-coordinated hydroxide rather than from a solvent water molecule. Following C-N bond cleavage, the C-terminus remains coordinated to the catalytic Zn(II), while the N-terminus forms a hydrogen bond with a solvent water molecule. Subsequently, the release of the C-terminus is facilitated by the coordination of a water molecule. Our study underscores the pivotal role of protein conformational dynamics at physiological temperature in stabilizing the transition state of the rate-limiting step and key intermediates, compared to the corresponding reaction in solution. These fundamental insights into the mechanism of collagen degradation provide valuable guidance for the development of MMP-1-specific inhibitors.

Supramolecular synthons in hydrates and solvates of lamotrigine: a tool for cocrystal design.

The molecule of anti-epileptic drug lamotrigine [LAM; 3,5-diamino-6-(2,3-dichlorophenyl)-1,2,4-triazine] is capable of the formation of multicomponent solids. Such an enhanced tendency is related to the diverse functionalities of the LAM chemical groups able to form hydrogen bonds. Two robust synthons are recognized in the supramolecular structure of LAM itself formed via N-H...N hydrogen bond: homosynthon, so-called aminopyridine dimer or synthon 1 [R22(8)] and larger homosynthon 2 [R32(8)]. The synthetic procedures for a new hydrate and 11 solvates of LAM (in the series: with acetone, ethanol: two polymorphs: form I and form II, 2-propanol, n-butanol, tert-butanol, n-pentanol, benzonitrile, acetonitrile, DMSO and dioxane) were performed. The comparative solid state structural analysis of a new hydrate and 11 solvates of LAM has been undertaken in order to establish robustness of the supramolecular synthons 1 and 2 found in the crystal structure of LAM itself as well as LAM susceptibility to build methodical solid state supramolecular architecture in the given competitive surrounding of potential hydrogen bonds. The aminopyridine dimer homosynthon 1 [R22(8)] has been switched from para-para (P-P) topology to ortho-ortho (O-O) topology in all crystal structures, except in LAM:n-pentanol:water solvate where it remains P-P. Homosynthon 2 [R32(8)] of the LAM crystal structure imitates in the LAM solvates as a heterosynthon by replacing the triazine nitrogen proton acceptor atoms of LAM with the proton acceptors of solvates molecules.

Study of mesomorphism behavior and protonation-induced emission of symmetrical benzalazines

Rod-shaped and polycatenar symmetric benzalazines with protonation-induced emission were synthesized, characterized, and studied for their thermal and optical behavior. Benzalazines lacking hydroxy groups at the ortho position did not show luminescence in the solid state or solution. In the presence of HCl (either in solution or vapor form) these compounds became good emitters. The protonation of azine nitrogen that may hinder the NN bond rotation by intermolecular interactions toward aggregates provided an explanation for the emission characteristics observed. The protonation-induced emission was thermally reversible. A benzalazine derived from 4-alkoxysalicylaldehyde with AIE property was prepared. For this compound with ortho-OH groups, the optical properties were not affected by the presence of acidic media. It exhibited a stable SmC phase characterized by POM, DSC, and XRD. The luminescence capability of the molecular arrangement within the smectic phase was investigated. The intense yellow-green emission in the crystals decreased significantly at the phase transition due to the non-radiative decay that occurs with increasing temperature, and loss of intermolecular H-bonding which prevents molecular rotations in the crystal.

A non-extraction sequential injection method for determination of loratadine using formation of its ion-association complex with bromocresol purple in acetonitrile

The formation of an ion-association complex (IA) between sulfonephthalein dye and basic nitrogen-containing compound in an organic solvent medium has been for the first time used to develop an automated SIA method. In highly polar aprotic solvents, the tautomeric equilibrium for such dyes is strongly shifted towards the colorless lactonic form. The addition of a basic nitrogen-containing substance leads to the formation of IA with a highly colored quinonoid form, which is accompanied by an increase in the absorbance of the dye band at approximately 400 nm. Protonation of pyridine nitrogen in loratadine, structure and binding places of IA were shown using quantum-chemical calculations. The very simple, direct and non-extraction spectrophotometric SIA method with high throughput of 43 h−1 was developed based on the formation of IA between loratadine and bromocresol purple in the medium of acetonitrile used both as solvent and carrier. The calibration graph was linear in the concentration range from 1.0 to 20 mg L−1 with correlation coefficient of 0.9992. The developed method was successfully applied to the analysis of pharmaceutical formulations.

CuI-amidobis(phosphine) catalyzed C(sp3)-C(sp3) direct homo- and hetero-coupling of unactivated alkanes via C(sp3)-H activation.

Herein, we present a CuI-dimer, [CuI{Ph2PC6H4C(O)NC6H4PPh2-o}]2, which catalyzed direct C(sp3)-H homocoupling of benzyl and cycloalkane derivatives with excellent yields and regio-selectivity. The method is very simple and tolerates various functionalities. Synergistic metal-ligand cooperativity was observed in Cu-N bond cleavage and protonation of nitrogen, and facilitates a bifunctional pathway, minimising the free energy corrugation for catalytic intermediates.

Structural chemistry of penta- and hexanitrato thorium(iv) complexes isolated using N–H donors

Fifteen Th(iv)–nitrate compounds, consisting of [Th(NO3)5(H2O)2]1− or [Th(NO3)6]2− units, were isolated from aqueous solution using a series of N–H heterocycles.

Unravelling the role of hydrogen peroxide in pH-dependent ORR performance of Mn-N-C catalysts

Although the Mn-N-C catalyst has low toxicity and high durability, its intense pH-dependent ORR activity is perplexing and prevents its commercial use in PEMFCs. Herein, we have developed a novel understanding of the role of hydrogen peroxide in the pH-dependent activity and stability of Mn-N-C catalysts. Instead of nitrogen protonation, the strong pH effects of ORR activity are caused by the much faster kinetics of the H2O2 reduction reaction in alkaline medium than in acid, due to the lower dissociation energy of the HO-OH bond in alkaline environment. Regarding durability, the deactivation induced by H2O2 demonstrates a strong pH dependence, primarily due to the differing intensities of reactive oxygen species at different pH values. Correspondingly, isopropanol effectively reduces Fenton reagents and increases the acid resistance of Mn-N-C sites. The present research provides a mechanistic understanding of pH effects and paves the way for the creation of more efficient catalysts.

Flower-shaped covalent organic framework synthesis and its anticancer drug delivery application

Fabrication of covalent organic frameworks (COFs) at room temperature is necessary due to their easy preparation, practical scale-up, and low cost to overcome the limitations of solvothermal and high-temperature synthesis. Herein, we fabricated flower-shaped COFs (FSCOFs) that were synthesized in various ratios of ortho-xylene and butanol mixture solvents. The FSCOFs exhibited excellent crystallinity with a pore size of approximately 10 nm, which was confirmed by PXRD and BET measurements. To utilize the FSCOFs as a drug carrier, doxorubicin (DOX) was loaded, followed by encapsulation of hyaluronic acid to increase the colloidal stability of FSCOFs in a physiological environment (FSCOFs-DOX-HA). The encapsulation efficiency and the drug loading capacity of FSCOFs-DOX-HA were 21.23 % and 29.82 %, respectively. Moreover, 92 % and 59 % of DOX is released in acidic environments and elevated GSH environments, proving the sensitivity of the FSCOFs as carriers. The excellent drug release mechanism of FSCOFs-HA-DOX is demonstrated by FSCOFs imine nitrogen protonation under acidic environment confirmed by its zeta potential changes under various physiological environments. The designed FSCOFs-DOX-HA was demonstrated to be a pH-sensitive drug carrier to deliver anticancer drugs into cancer cells. The MTT and drug internalization data demonstrated sustained DOX release from FSCOFs-DOX-HA. Finally, the real-time fluorescence demonstrates the effectiveness of the FSCOFs-DOX-HA upon cellular uptake and release of the anticancer drug. Thus, the FSCOFs system reported here opens a new path in COF research for drug carrier systems.

The preparation of a bismuth-based complex and the influence of double protonation

Bismuth as a relatively non-toxic and inexpensive metal has a wide range of applications. There are numerous compounds based on bismuth that exhibit excellent semiconductive, photochromic, intensely luminous, and other properties. In this work, we synthesized a new bismuth-based complex (C10H8N2BiCl4, named complex 1) by a hydrothermal method. The crystal structure of complex 1 is resolved using single-crystal X-ray diffraction. When replacing water by methanol as solvent, the color of the obtained crystals (denoted as complex 2) changes from colorless to pink. The analysis of IR and XPS suggests that partial double protonation of the pyridine nitrogen occurs in complex 2. Because of partial double protonation, complex 2 displays visible light absorption and stronger fluorescence intensity. Further investigation found that complex 1 could change its color from colorless to pink in the presence of formic acid and acetic acid, suggesting that complex 1 can be used as formic acid (HCOOH) and acetic acid (CH3COOH) sensors. This work provides new insights into the exploration of bismuth-based complexes and provides a new way to broaden light absorption and enhance luminescence.

Design Principles for Transition Metal Nitride Stability and Ammonia Generation in Acid

Transition metal nitrides have shown great promise for reducing or eliminating the use of expensive precious-metal-based catalysts (e.g., Pt) in proton exchange membrane fuel cells and electrolyzers, but the use of these nitrides in such technologies is severely hindered by their dissolution at acidic pHs [1-2]. More importantly, the decomposition of transition metal nitrides in acid generates ammonia from the protonation of lattice nitrogen, giving rise to many false positives in previous reports of nitride catalysts for electrochemical nitrogen reduction to ammonia. For example, while nitrides such as VN [3], NbN [4], and Mo2N [5] have been computationally predicted to catalyze the reduction of nitrogen to ammonia, the experimentally observed ammonia has been attributed to the decomposition of nitrides in acid [6-8]. To tackle this challenge, motivated by our previous work [9-12] on the design principles of transition-metal-oxide-based catalysts, we established the stability descriptors of transition metal nitrides in acid. Such stability descriptors not only offer a fundamental understanding of nitride dissolution but also provide new guiding principles to optimize the intrinsic stability of nitrides for diverse acidic applications.In this work [13], combining ab initio calculations with synchrotron X-ray spectroscopies, we identified electronic-structure-based design principles governing the extent and kinetics of nitride dissolution and ammonia production in acid. We found that lowering the nitrogen 2p band center of transition metal nitrides with respect to the Fermi level leads to weakened metal-nitrogen bonds, increased labile metallic character, and a reduced barrier for the protonation of lattice nitrogen to produce ammonia, correlating with faster dissolution kinetics of nitrides in acid. Moreover, increasing the solubility of dissolved metal ions in acid was found to be critical in preventing surface oxide passivation to ensure the complete conversion from transition metal nitrides to ammonia. Based on these observations, a new mechanistic picture was formulated, where the initial protonation step of lattice nitrogen is critical to trigger nitride dissolution, and this proposed reaction scheme was supported by the pH-dependent dissolution kinetics of nitrides in acid.These design principles and mechanistic insights for producing ammonia and dissolving metal cations from the decomposition of nitrides in acid are essential for a variety of clean energy applications. For instance, such design principles can be leveraged to boost the stability of nitride catalysts for proton-exchange membrane fuel cells and electrochemical ammonia synthesis, where the dissolution of nitrides in acid has hindered their functions. Moreover, these descriptors for nitride dissolution and ammonia formation in acid provide emerging opportunities for designing novel nitride chemistries for distributed, on-demand ammonia generation. As nitrides represent an exciting, yet markedly unexplored chemical space, this work provides a blueprint to design multinary nitrides in such a vast materials space for various acidic applications, including electrocatalysis and beyond.

Extended Light Absorption and Improved Charge Separation by Protonation of the Organic Ligand in a Bismuth-Based Metal-Organic Framework.

In this work, a new method to extend the light absorption and improve the photocatalytic activity of metal-organic frameworks (MOFs) with nitrogen-containing ligand is reported, namely, the protonation of nitrogen. Specifically, a protonated Bi-based MOF synthesized by a hydrothermal method (Bi-MMTAA-H, MMTAA=2-mercapto-4-methyl-5-thiazoleacetic acid) displays a wider visible light absorption than Bi-MMTAA-R with the same single-crystal structure, but synthesized by a reflux method. The redshifted light absorption was confirmed to be caused by the protonation of nitrogen in the thiazolyl ring in MMTAA. Moreover, this protonation also facilitates the charge separation and transfer and improves the photocatalytic activity of selective oxidation of α-terpinene to p-cymene. Our results provide a new idea for nitrogen-containing Bi-based MOFs to extend the light absorption and improve the photocatalytic performance.

Unraveling the Reaction Mechanism of Russell's Viper Venom Factor X Activator: A Paradigm for the Reactivity of Zinc Metalloproteinases?

Snake venom metalloproteinases (SVMPs) are important drug targets against snakebite envenoming, the neglected tropical disease with the highest mortality worldwide. Here, we focus on Russell's viper (Daboia russelii), one of the "big four" snakes of the Indian subcontinent that, together, are responsible for ca. 50,000 fatalities annually. The "Russell's viper venom factor X activator" (RVV-X), a highly toxic metalloproteinase, activates the blood coagulation factor X (FX), leading to the prey's abnormal blood clotting and death. Given its tremendous public health impact, the WHO recognized an urgent need to develop efficient, heat-stable, and affordable-for-all small-molecule inhibitors, for which a deep understanding of the mechanisms of action of snake's principal toxins is fundamental. In this study, we determine the catalytic mechanism of RVV-X by using a density functional theory/molecular mechanics (DFT:MM) methodology to calculate its free energy profile. The results showed that the catalytic process takes place via two steps. The first step involves a nucleophilic attack by an in situ generated hydroxide ion on the substrate carbonyl, yielding an activation barrier of 17.7 kcal·mol-1, while the second step corresponds to protonation of the peptide nitrogen and peptide bond cleavage with an energy barrier of 23.1 kcal·mol-1. Our study shows a unique role played by Zn2+ in catalysis by lowering the pKa of the Zn2+-bound water molecule, enough to permit the swift formation of the hydroxide nucleophile through barrierless deprotonation by the formally much less basic Glu140. Without the Zn2+ cofactor, this step would be rate-limiting.

Revealing the structure and mechanisms of action of a synthetic opioid with model biological membranes at the air-water interface

Synthetic opioids such as piperazine derivative called MT-45 interact with opioid receptors in a manner similar to morphine leading to euphoria, a sense of relaxation and pain relief and are commonly used as substituents of natural opioids. In this study we show the changes in the surface properties of nasal mucosa and intestinal epithelial model cell membranes formed at the air – water interface using Langmuir technique upon the exposure to MT-45. Both membranes constitute the first barrier to absorb this substance into the human body. The presence of the piperazine derivative affects the organization of both DPPC and ternary DMPC:DMPE:DMPS monolayers treated as simple models of nasal mucosa and intestinal cell membranes, respectively. This novel psychoactive substance (NPS) leads to the fluidization of the model layers, which may indicate their increased permeability. MT-45 has a greater influence on the ternary monolayers characteristic of the intestinal epithelial cells than nasal mucosa. It might be attributed to the increased attractive interactions between the components of the ternary layer, which in turn increase the interactions with a synthetic opioid. Additionally, the crystal structures of MT-45 determined by single-crystal and powder X-ray diffraction methods allowed us to both provide useful data for facilitating the identification of synthetic opioids as well as to attribute the effect of MT-45 to the ionic interactions between protonated nitrogen atoms and negatively charged parts of the polar heads of the lipids.

Activating One/Two-Photon Excited Red Fluorescence on Carbon Dots: Emerging n→π Photon Transition Induced by Amino Protonation.

Due to the complicated nature of carbon dots (CDs), fluorescence mechanism of red fluorescent CDs is still unrevealed and features highly controversial. Reliable and effective strategies for manipulating the red fluorescence of CDs are urgently needed. Herein, CDs with one-photon excited (622 nm, QYs ≈ 17%) and two-photon (629 nm) excited red fluorescence are prepared by acidifying o-phenylenediamine-based reaction sediments. Systematic analysis reveals that the protonation of amino groups increases the particle surface potential, disperse the bulk sediments into nano-scale CDs. In the meanwhile, amino protonation of pyridinic nitrogen (-N=) structure inserts numerous n orbital energy levels between the π → π* transition, narrows the gap distance for photon transition, and induces red fluorescence emission on CDs. Present research reveals an effective pathway to activate CDs reaction sediments and trigger red emission, thus may open a new avenue for developing CDs with ideal optical properties and promising application prospects.

Synthesis, Structural and Behavioral Studies of Indole Derivatives D2AAK5, D2AAK6 and D2AAK7 as Serotonin 5-HT1A and 5-HT2A Receptor Ligands.

Serotonin receptors are involved in a number of physiological functions and regulate aggression, anxiety, appetite, cognition, learning, memory, mood, nausea, sleep, and thermoregulation. Here we report synthesis and detailed structural and behavioral studies of three indole derivatives: D2AAK5, D2AAK6, and D2AAK7 as serotonin 5-HT1A and 5-HT2A receptor ligands. X-ray studies revealed that the D2AAK5 compound crystallizes in centrosymmetric triclinic space group with one molecule in the asymmetric unit. The main interaction between the ligands and the receptors is the salt bridge between the protonatable nitrogen atom of the ligands and the conserved Asp (3.32) of the receptors. The complexes were stable in the molecular dynamic simulations. MD revealed that the studied ligands are relatively stable in their binding sites, with the exception of D2AAK7 in the serotonin 5-HT1A receptor. D2AAK7 exerts anxiolytic activity in the EPM test, while D2AAK5 has a beneficial effect on the memory processes in the PA test.

Effect of buffers and pH in antenna sensitized Eu(III) luminescence

The photophysics of a europium(III) complex of 1,4,7,10-tetraazacycododecane-1,4,7-triacetic acid-10-(2-methylene)-1-azathioxanthone was investigated in three buffer systems and at three pH values. The buffers—phosphate buffered saline (PBS), 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), and universal buffer (UB)—had no effect on the europium luminescence, but a lower overall emission intensity was determined in HEPES. It was found that this was due to quenching of the 1-azathioxanthone first excited singlet state by HEPES. The effect of pH on the photophysics of the complex was found to be minimal, and protonation of the pyridine nitrogen was found to be irrelevant. Even so, pH was shown to change the intensity ratio between 1-azathioxanthone fluorescence and europium luminescence. It was concluded that the full photophysics of a potential molecular probe should be investigated to achieve the best possible results in any application.

Efficient additive-free formic acid dehydrogenation with a NNN-ruthenium complex.

A new ruthenium complex containing a pyridylidene amine-based NNN ligand was developed as a catalyst precursor for formic acid dehydrogenation, which, as a rare example, does not require basic additives to display high activity (TOF ∼10 000 h-1). Conveniently, the complex is air-stable, but sensitive to light. Mechanistic investigations using UV-vis and NMR spectroscopic monitoring correlated with gas evolution profiles indicate rapid and reversible protonation of the central nitrogen of the NNN ligand as key step of catalyst activation, followed by an associative step for formic acid dehydrogenation.