[Determination of the derivatization reactivity between α/ß-dicarbonyl compounds and standard citrullinated peptides based on matrix-assisted laser desorption ionization-time-of-flight mass spectrometry].

Protein citrullination is an irreversible post-translational modification process regulated by peptidylarginine deiminases (PADs) in the presence of Ca2+. This process is closely related to the occurrence and development of autoimmune diseases, cancers, neurological disorders, cardiovascular and cerebrovascular diseases, and other major diseases. The analysis of protein citrullination by biomass spectrometry confronts great challenges owing to its low abundance, lack of affinity tags, small mass-to-charge ratio change, and susceptibility to isotopic and deamidation interferences. The methods commonly used to study the protein citrullination mainly involve the chemical derivatization of the urea group of the guanine side chain of the peptide to increase the mass-to-charge ratio difference of the citrullinated peptide. Affinity-enriched labels are then introduced to effectively improve the sensitivity and accuracy of protein citrullination by mass spectrometry. 2,3-Butanedione or phenylglyoxal compounds are often used as derivatization reagents to increase the mass-to-charge ratio difference of the citrullinated peptide, and the resulting derivatives have been observed to contain α-dicarbonyl structures. To date, however, no relevant studies on the reactivity of dicarbonyl compounds with citrullinated peptides have been reported. In this study, we determined whether six α-dicarbonyl and two ß-dicarbonyl compounds undergo derivatization reactions with standard citrullinated peptides using matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS). Among the α-dicarbonyl compounds, 2,3-butanedione and glyoxal reacted efficiently with several standard citrullinated peptides, but yielded a series of by-products. Phenylglyoxal, methylglyoxal, 1,2-cyclohexanedione, and 1,10-phenanthroline-5,6-dione also derivated efficiently with standard citrullinated peptides, generating a single derivative. Thus, a new derivatization method that could yield a single derivative was identified. Among the ß-dicarbonyl compounds, 1,3-cyclohexanedione and 2,4-pentanedione successfully reacted with the standard citrullinated peptides, and generated a single derivative. However, their reaction efficiency was very low, indicating that the ß-dicarbonyl compounds are unsuitable for the chemical derivatization of citrullinated peptides. The above results indicate that the α-dicarbonyl structure is necessary for realizing the efficient and specific chemical derivatization of citrullinated peptides. Moreover, the side chains of the α-dicarbonyl structure determine the structure of the derivatives, derivatization efficiency, and generation (or otherwise) of by-products. Therefore, the specific enrichment and precise identification of citrullinated peptides can be achieved by synthesizing α-dicarbonyl structured compounds containing affinity tags. The proposed method enables the identification of citrullinated proteins and their modified sites by MS, thereby providing a better understanding of the distribution of citrullinated proteins in different tissues. The findings will be beneficial for studies on the mechanism of action of citrullinated proteins in a variety of diseases.

Screening and identification of peptidyl arginine deiminase 4 inhibitors from herbal plants extracts and purified natural products by a trypsin assisted sensitive immunoassay based on streptavidin magnetic beads.

Peptidyl arginine deiminase 4 (PAD4) plays a critical role in many autoimmune diseases including rheumatoid arthritis. Herein, a trypsin assisted highly immunoassay method was established to determine PAD4 activity and screen potent inhibitors from herbal plants extracts and purified natural products. The method was applied to determine endogenous PAD4 activity in both cell and tissue lysates, as well as the inhibitory effects of 20 herbal plants and 50 purified natural products. The Cinnamomi ramulus extract showed strongest inhibitory potency with IC50 value lower than 5 µg/mL. Meanwhile, pyrroloquinoline quinone (PQQ), widely used as a dietary supplement, was discovered as a promising PAD4 inhibitor with an IC50 value lower than 4 µM. The inhibition kinetic analysis, drug affinity response target stability (DARTS) and molecular docking were performed to confirm the interaction between PQQ and PAD4. This method has great potential for researchers to monitor activities and discover potential inhibitors of PAD4.

Atomically precise photothermal nanomachines.

Interfacing molecular machines to inorganic nanoparticles can, in principle, lead to hybrid nanomachines with extended functions. Here we demonstrate a ligand engineering approach to develop atomically precise hybrid nanomachines by interfacing gold nanoclusters with tetraphenylethylene molecular rotors. When gold nanoclusters are irradiated with near-infrared light, the rotation of surface-decorated tetraphenylethylene moieties actively dissipates the absorbed energy to sustain the photothermal nanomachine with an intact structure and steady efficiency. Solid-state nuclear magnetic resonance and femtosecond transient absorption spectroscopy reveal that the photogenerated hot electrons are rapidly cooled down within picoseconds via electron-phonon coupling in the nanomachine. We find that the nanomachine remains structurally and functionally intact in mammalian cells and in vivo. A single dose of near-infrared irradiation can effectively ablate tumours without recurrence in tumour-bearing mice, which shows promise in the development of nanomachine-based theranostics.

Low-Temperature Oxidation Reaction Processes of Cyclopentanone Unraveled by In Situ Mass Spectrometry and Theoretical Study.

Although cyclopentanone (CPO) is a promising bio-derived fuel, thermodynamic data of its low-temperature oxidation under high-pressure conditions are lacking. In this work, the low-temperature oxidation mechanism of CPO is investigated in a flow reactor in the temperature range of 500-800 K and at a total pressure of 3 atm by a molecular beam sampling vacuum ultraviolet photoionization time-of-flight mass spectrometer. The electronic structure and pressure-dependent kinetic calculations are carried out at the UCCSD(T)-F12a/aug-cc-pVDZ//B3LYP/6-31+G(d,p) level to explore the combustion mechanism of CPO. Experimental and theoretical observations showed that the dominant product channel in the reaction of CPO radicals with O2 is HO2 elimination, yielding 2-cyclopentenone. The hydroperoxyalkyl radical (•QOOH) generated by 1,5-H-shifting is easily reacted with second O2 and forms ketohydroperoxide (KHP) intermediates. Unfortunately, the third O2 addition products are not detected. In addition, the decomposition pathways of KHP during the low-temperature oxidation of CPO are further assessed, and the unimolecular dissociation pathways of CPO radicals are confirmed. The results of this study can be used for future research on the kinetic combustion mechanisms of CPO under high pressure.

[Distribution Characteristics of Soil Phosphorus Forms and Phosphatase Activity at Different Altitudes in the Soil of Water-Level-Fluctuation Zone in Pengxi River, Three Gorges Reservoir].

Phosphatases play important roles in converting organic phosphorus into inorganic phosphorus in soil. However, studies from this perspective on the water-level-fluctuation zone (WLFZ) of the Three Gorges Reservoir are limited. In this study, phosphatase activity and the forms of phosphorus were analyzed. Soil samples were collected in the river basin of the Penxi River in the WLFZ during a drying period. The correlation between phosphatase activity and phosphorus forms and the impacts of phosphatase activity on the phosphorus forms were analyzed. The results showed that the contents of H2O-Pi, NaHCO3-Pi, and NaOH-Pi in the soils of the WLFZ were higher than those in the soils by the river. In addition, a higher altitude resulted in higher contents of bio-enzymatically hydrolysable phosphorus and NaOH-Po. Furthermore, redundancy analysis (RDA) showed that the contents of organic matter and amorphous Fe and Mn were the main factors affecting soil organic phosphorus forms. The average activities of acid phosphomonoesterase (ACP), alkaline phosphomonoesterase (ALP), phosphodiesterase (PDE) (all in p-NP), and phytase (PAE) (in P) in the soils of the WLFZ were 1.40, 2.60, 0.44, and 11.43 µmol·(g·h)-1, respectively. Moreover, the activities of different phosphatases increased with altitude. Soil plant biomass and microbial biomass were important reasons for the difference in spatial distribution of phosphatase activity in the soil of the WLFZ. Phosphatase activities were significantly positively correlated with the contents of organic phosphorus forms but negatively correlated with the content of bioavailable phosphorus. A higher soil phosphatase activity and a lower content of bioavailable phosphorus were usually detected in soil samples taken at a higher altitude. In the early stage of flooding, phosphatase converted organic phosphorus into inorganic phosphorus at a relatively high rate, and the risk of phosphorus release to the overlying water body was also high. This study contributed to a comprehensive understanding of the geochemical cycle of soil phosphorus in the soil of the WLFZ.

Insights into a Low-Rank Naomaohu Coal Structural Information by Multistage Fractions Coupled with LIAD-VUVPI-TOFMS.

In-depth insights into the chemical composition and structural information of coal are an effective way to improve the efficiency of coal utilization. Laser-induced acoustic desorption coupling with vacuum ultraviolet photoionization time-of-flight mass spectrometry (LIAD-VUVPI-TOFMS) was applied to structural characterization of cyclohexane extracts of low-rank Naomaohu coal. The characterization of four types (12 model compounds) of mixed coal model compounds (three compounds per category)-saturated hydrocarbons, substitute aromatic hydrocarbons, aromatic hydrocarbons, and aromatic heteroatom rings-demonstrated that the approach can provide intact molecular weight information. The cyclohexanone extract (E CYC) was obtained by microwave-assisted extraction and separated into four group components (F1-4) by column chromatography to achieve component classification and simplify analysis. The molecular weight and structure were obtained by LIAD-VUVPI-TOFMS and synchronous fluorescence spectroscopy, combined with microwave-assisted extraction and column chromatography to separate product characteristics. Chemical components of a total of 248 species were observed, of which 46 are derived from aliphatic hydrocarbons embedded in the coal skeleton structure, 132 species are derived from aromatic hydrocarbons embedded in the coal skeleton structure, 61 are derived from possible coal skeleton units (compounds have obvious stacking and bonding effects), and 9 could not be determined (aromatic hydrocarbons or a possible coal skeleton structure unit).

Developing the Low-Temperature Oxidation Mechanism of Cyclopentane: An Experimental and Theoretical Study.

Invited for the cover of this issue are Zichao Tang and co-workers at Xiamen University, Yangtze University and the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. The image depicts molecular beam sampling vacuum ultraviolet photoionization time-of-flight mass spectrometer (MB-VUV-PI-TOFMS) as an advanced analytical tool for detecting highly oxygen intermediates in the low temperature oxidation of fuels. Read the full text of the article at 10.1002/chem.202103546.

Developing the Low-Temperature Oxidation Mechanism of Cyclopentane: An Experimental and Theoretical Study.

At present, the reactivity of cyclic alkanes is estimated by comparison with acyclic hydrocarbons. Due to the difference in the structure of cycloalkanes and acycloalkanes, the thermodynamic data obtained by analogy are not applicable. In this study, a molecular beam sampling vacuum ultraviolet photoionization time-of-flight mass spectrometer (MB-VUV-PI-TOFMS) was applied to study the low-temperature oxidation of cyclopentane (CPT) at a total pressure range from 1-3 atm and low-temperature range between 500 and 800 K. Low-temperature reaction products including cyclic olefins, cyclic ethers, and highly oxygenated intermediates (e. g., ketohydroperoxide KHP, keto-dihydroperoxide KDHP, olefinic hydroperoxides OHP and ketone structure products) were observed. Further investigation of the oxidation of CPT - electronic structure calculations - were carried out at the UCCSD(T)-F12a/aug-cc-pVDZ//B3LYP/6-31+ G(d,p) level to explore the reactivity of O2 molecules adding sequentially to cyclopentyl radicals. Experimental and theoretical observations showed that the dominant product channel in the reaction of CPT radicals with O2 is HO2 elimination yielding cyclopentene. The pathways of second and third O2 addition - the dissociation of hydroperoxide - were further confirmed. The results of this study will develop the low-temperature oxidation mechanism of CPT, which can be used for future research on accurately simulating the combustion process of CPT.

Deciphering the Superatomic Behavior of Group V Metal Monoxides.

The valence orbitals of Group V metal monoxides exhibit atomic-like properties which mimic that of coinage metal element atoms. The electronic structures of MO-1/0 (M = V, Nb, and Ta) have been determined by negative ion photoelectron velocity map imaging. Electron affinities and vibrational frequencies for the ground state and excited states of MO (M = V, Nb, and Ta) molecules have been identified as well as photoelectron angular distributions. On the basis of the equivalent-electron principle, MO- (M = V, Nb, and Ta) molecules bear valence electron configurations similar to those of coinage metal elemental atoms, despite having more complicated electronic states for molecules, and concomitant mimicry of magnetic superatom. Generally, other than low-spin states of coinage metal atoms, Group V metal monoxides demonstrate a high-spin state except for TaO, possessing the potential applications to inexpensive superatoms in industrial catalysis.

Development of a miniature time-of-flight mass spectrometer coupled with an improved substrate-enhanced laser-induced acoustic desorption source (SE-LIAD/TOF-MS).

A novel, compact and sensitive SE-LIAD/TOF-MS has been described. It facilitates fast sample preparation, and a full mass spectrum is acquired efficiently and sensitively. More importantly, it features the detection of non-acidic and non-basic or non-polar species, which is not suitable for determination by ESI and MALDI techniques. In this technique, standard samples, carbazole and melamine, are prepared on a Ti foil with a quartz plate attached to the backside of the Ti foil to perform a laser-induced acoustic desorption experiment (SE-LIAD) coupled to TOF-MS for analysis. Enhanced signals are observed with about 5.6 to 13.8 times higher than that obtained in the standard LIAD method, dependent on different ionization techniques. Compared to the EI spectra, the PI spectra for both species show intact and sharp molecular peaks. The limits of detection (LOD) of melamine were evaluated experimentally in the range from ∼2-6 pg (EI/MS mode) to ∼0.3-0.5 ng (VUV-SPI/MS mode). Thus, the method in this study exhibits rapid qualitative and quantitative analysis with good sensitivity, being free of the complex matrix influences.

Dissection of bicapped octahedral copper hydride cluster to form two chiral tetrahedral copper hydride cluster series exhibiting auto deracemization and photoluminescence.

Three series of copper hydride clusters [Cu8H6L6]2+ (1), [Cu4HX2L4]+ where X- = Cl- (2a), Br- (2b), I- (2c), N3- (2d) and SCN- (2e), and [Cu4HX3L3] where X- = Br- (3b) and I- (3c) (L = 2-(diphenylphosphino)pyridine, dppy) were synthesized and characterized by single-crystal X-Ray crystallography and standard spectroscopic techniques. The metal core of 1, Cu8, can be described as a bicapped octahedron, while those of 2 and 3 series adopt tetrahedral structures. The hydride positions were deduced from difference electron density maps and corroborated by NMR and DFT calculations. For 1, there are two µ4-H-, one each in the two tetrahedral cavities of the two capping atoms and four µ3-H- on the six triangular faces around the waist of the octahedron. For [Cu4HX2L4]+ and [Cu4HX3L3] series, the single µ4-H- resides in the center of the Cu4 tetrahedron. It was found that these three series of copper clusters are intimately connected and can convert from one to another under specific reaction conditions. Their transformation pathways were investigated in detail. Spontaneous resolution to form optically pure enantiomeric single crystals was observed for [Cu4H(SCN)2L4]+ (2e) and [Cu4HBr3L3] (3b). Photoluminescence was observed for [Cu4HX2L4]+, as well as [Cu4HX3L3] with strong emissions from green to yellow regions.

Probing the geometric and electronic structures of the lanthanide oxide HoOn-1/0 (n = 1-3) clusters.

The complex 4f and 5d orbits of lanthanide oxide clusters increases the complexity and difficulty in both theoretical and experimental research. Combining the photoelectron imaging spectroscopy and ab initio calculations, the structural and electronic properties of HoO- were studied. The adiabatic detachment energy (ADE) and vertical detachment energy (VDE) of HoO- have been measured to be 1.31(3) eV and 1.42(2) eV, respectively. To determine the vibrational structure and observed spectral bands in the photoelectron spectrum, Franck-Condon simulation of the ground-state transition for HoO- has been performed. The fundamental frequency of ground-state HoO is estimated to be 893 ± 73 cm-1. Density functional method (DFT) was used to study the neutral and anionic clusters of HoOn-1/0 (n = 1-3), and the most stable cluster structures were obtained. Based on the DFT calculations, the theoretical ADEs and VDEs of anionic HoOn- (n = 1-3) clusters were obtained and the photoelectron spectra (PES) of HoOn- (n = 1-3) clusters were simulated, which might stimulate further experimental investigations on the Ho oxide clusters. In addition, the corresponding molecular orbitals (MOs) were also discussed to reveal the interaction between Ho and O atoms. This study can help us to understand the chemical bonding in Ho-containing molecules and will provide some light in their surface chemistry and photochemistry investigation.

Aromaticity Criterion Is Not the Only Factor to Decide the Ring Stability of Boron Oxide Families: c-M2O2-/0 Clusters (M = B, Al, Ga, and In).

Generally, compared to conjugated chain molecules, aromaticity provides additional stability for the cyclic, planar, and conjugated molecules. Thus, the concept of aromaticity was undeniably utilized to explain the unique stability for extensive cyclic molecules (notably for benzene, recently reported boron rings, and all-metal multiply aromatic Al42- salts) to guide chemical syntheses. However, can aromaticity alone describe the stability for all of those cyclic and planar clusters or molecules? In this regard, we observed the four-membered prototypical rings: c-M2O2-/0 clusters (M = B, Al, Ga, and In) possessing unique rhombic (four-center, four-electron) π and σ o-bonds, which are considered to have 3-fold aromaticity. Moreover, we not only elucidated the key role of ring strain energy (RSE) to determine the stability of these rings but also unexpectedly revealed that the electrostatic interaction (ionicity) plays a fundamental role in the stability of Al2O2-/0 clusters through systematically experimental and theoretical investigations into the isolated M2O2-/0 clusters (M = B, Al, Ga, and In). Detailed geometries, molecular orbital, and chemical bonding nature were analyzed to unravel those influences. This work provides a clue in which RSE and the electrostatic effect should be carefully taken into account for the stability of diverse cyclic clusters or molecules compared to the expected stability factor from aromaticity.

Unsaturated binuclear homoleptic nickel carbonyl anions Ni2(CO)n- (n = 4-6) featuring double three-center two-electron Ni-C-Ni bonds.

The homoleptic homodinuclear nickel carbonyl anions Ni2(CO)n- (n = 4-6) are mass-selected in the gas phase and examined with anion photoelectron velocity-map imaging spectroscopy combined with density functional calculations. The doubly carbonyl-bridged structures are found to be favorable for Ni2(CO)n- (n = 4-6). The nature of Ni-Ni bonding in these complexes is analysed with the aid of a range of state-of-the-art quantum chemistry methods. Despite the absence of direct multiple Ni-Ni bonds, the two nickel atoms in Ni2(CO)n- (n = 4-6) complexes are joined by two bridging carbonyl ligands via the sharing three-center two-electron Ni-C-Ni bond in turn to achieve the (16,16), (16,18), and eventually the favored (18,18) configurations.

Multicenter electron-sharing σ-bonding in the AgFe(CO)4- complex.

The heterodinuclear silver tetracarbonyl-iron anion was generated in the gas phase and studied by joint photoelectron velocity map imaging spectroscopy and quantum chemical calculations. The AgFe(CO)4- anion is characterized to be an 18-electron complex with the silver atom covalently bonded to the anionic tetracarbonyl-iron, an isolobal analogue of the methyl radical. The bonding analyses using a range of state-of-the-art quantum chemistry methods revealed a peculiar decentralized bonding situation, where the silver atom is covalently bonded to both the iron center and the vicinal carbon atoms in the form of an electron-sharing σ bond.

Thermodynamics and Kinetics of Gas-Phase CO Oxidation on the Scandium Monoxide Carbonyl Complexes.

The CO chemisorption onto the ScO+ cation was investigated using infrared photodissociation spectroscopy combined with density functional theory calculations. The spectra were recorded in the CO stretching vibrational region for the OSc(CO)n+ (n = 4-6) complex series. Comparisons of the experimental spectra with the simulated ones have established the geometries and present strong evidence that all of the CO ligands are chemisorbed, which could not be readily oxidized by scandium monoxide core into CO2. Complementary calculations demonstrate that, regardless of the thermodynamic feasibility, the CO oxidation on the scandium monoxide carbonyl complexes is kinetically unfavorable due to the significant barriers involved in the CO oxidation process. Nevertheless, the consecutive CO adsorption has a positive influence on the Sc-O bond activation.

The photoelectron-imaging spectroscopic study and chemical bonding analysis of VO2 -, NbO2 - and TaO2.

The transition-metal di-oxides, namely VO2 -, NbO2 - and TaO2 - have been studied using photoelectron velocity map imaging (PE-VMI) in combination with theoretical calculations. The adiabatic electron affinities of VO2 -, NbO2 - and TaO2 - are confirmed to be 2.029(8), 1.901(10) and 2.415(8) eV, respectively. By combining Franck-Condon (FC) simulation with theoretical calculations, the vibrational feature related to Nb-O and Ta-O stretching modes for the ground state has been unveiled. The photoelectron angular distribution (PAD) for VO2 -, NbO2 - and TaO2 - is correlated to the photo-detachment of the highest occupied molecular orbitals (HOMOs), which primarily gets involved in s- and d-orbitals of the V, Nb and Ta atoms. A variety of theoretical calculations have been used to analyze the chemical bonding features of VO2 -1/0, NbO2 -1/0 and TaO2 -1/0, which show that the strong M-O (M = V, Nb and Ta) bond is mainly characterized as ionicity.

Does gold behaves as hydrogen? A joint theoretical and experimental study.

It has been established that the noble-metal-H analogue has been found in a large number of noble-metal-ligand clusters in view of geometric and electronic structures. Here, we demonstrated a different view of noble-metal-H analogue between noble-metal and hydrogen in M(SCH3)2 - (M = Cu, Ag, Au and H) systems. Although H(SCH3)2 - is a typical ion-hydrogen bonding cluster dramatically different from the chemical bonding clusters of M(SCH3)2 - (M = Cu, Ag and Au), the comparison of the two typical bonding patterns has not yet been fully investigated. Through a series of chemical bonding analyses, it is indicated that the evolution has been exhibited from typical ionic bonding in Cu(SCH3)2 - to a significant covalent bonding nature in Au(SCH3)2 - and hydrogen bonding dominating in H(SCH3)2 -. The comparison of M(SCH3)2 - (M = Cu, Ag and Au) with H(SCH3)2 - illustrates the differences in bonding between noble metals and hydrogen, which are mainly related to their diverse atomic orbitals participating in chemical bonding.

Improving the Photostability of Ultrasmall Au Clusters via a Combined Strategy of Surface Engineering and Interfacial Modification.

Photostability is a critical issue for evaluating the use of photocatalysts to realize large-scale implementation of solar energy conversion. Recently emerged ultrasmall gold (Au) clusters with distinctive physicochemical properties have been regarded as visible-light photosensitizers for photoredox catalysis, whereas the poor stability under visible-light exposure greatly restricts their photocatalytic applications. Herein, we provide a proof-of-concept study on enhancing the photostability of ultrasmall Au clusters via a combined strategy of surface engineering and interfacial modification. The photostability of Au clusters on the surface of TiO2 nanosheets with less hydroxyl group can be improved to some extent as compared to that on TiO2 nanoparticles with abundant hydroxyl groups under continuous visible-light irradiation (λ > 420 nm). Moreover, the subsequent modification of branched polyethylenimine (BPEI) between TiO2 nanosheets and Au clusters further improves their photostability upon light illumination. Consequently, the as-constructed TiO2 nanosheet-BPEI-Au cluster composites exhibit stable visible-light activity toward Cr(VI) photoreduction. It is hoped that the joint strategy via surface engineering and interfacial modification provides a facile guideline for stabilizing ultrasmall Au clusters toward targeting applications in the photoredox catalysis process.

Spectroscopic identification of the low-lying electronic states of S2 molecule.

As is well-known, the S2 molecule is a ubiquitous intermediate in the combustion, atmosphere, and interstellar space. The six low-lying bound states of S2 have been characterized via photoelectron velocity map imaging and a high-level multi-reference configuration interaction method with the Davidson correction. Spectroscopic constants have been extracted by fitting the potential energy curves extrapolated to the complete basis set limit with a series of Dunning's correlation-consistent basis sets: aug-cc-pV(Q, 5)Z. The calculated spectroscopic parameters well reproduce the experimental results in this work. On the basis of the theoretical calculations, Franck-Condon simulations are performed to assign six adjacent electronic states, especially for three higher overlapping electronic states (c1Σu -, A'3Δu, and A3Σu +). The dissociation energy De of the S2 - is evaluated to be 4.111 (4) eV in this work, in agreement with the theoretical prediction (4.056 eV).