Zichao Tang

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 β-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 β-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 β-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.

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.

Nitrogen-doped carbon materials demonstrate high performance as electrodes in fuel cells and higher oxygen reduction reactivity than traditional Pt-based electrodes. However, the formation process of nitrogen-doped carbon materials has long been a mystery. In this study, the formation mechanism of nitrogen-doped carbon materials from polyaniline (PANI) pyrolysis was studied by the combination of in situ pyrolysis vacuum ultraviolet photoionization time-of-flight mass spectrometry (Py-VUVPI-TOF MS) and substrate-enhanced, laser-induced acoustic desorption source time-of-flight mass spectrometry (SE-LIAD-TOF MS). The initial pyrolysis species, including free radicals and intermediates, were investigated via in situ Py-VUVPI-TOF MS during the temperature-programmed desorption process (within tens of microseconds). The pyrolysis residues were collected and further investigated via SE-LIAD-TOF MS, revealing the product information of the initial pyrolysis products. The results show that the edge doping of carbon materials depends on free radical reactions rather than the direct substitution of carbon atoms by nitrogen atoms. Meanwhile, pyrrole nitrogen and pyridine nitrogen are formed by the free radical cyclization reaction and the amino aromatization reaction at the initial stage of pyrolysis, while the formation of graphitic nitrogen depends on the further polymerization reaction of pyrrole nitrogen and pyridine nitrogen.

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).

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.

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.

In contrast to the normal LIAD method, the enhanced signals were obtained using substrate-enhanced LIAD source (SE-LIAD) being about 5-fold to 14-fold enhancement.

Ultrasmall gold (Au) clusters have been regarded as one of the prototypes materials for solar energy conversion due to their unique strong molecular-like light absorption properties. However, the light-induced aggregation of Au clusters into nanoparticles is one of the most important factors that restricts its application in photocatalysis. Although Au clusters aggregation has been widely demonstrated, the underlying mechanism for cluster fusion is still unclear due to the lack of experimental evidence. Herein, we report the direct observation of Au clusters on TiO₂ nanosheets aggregating when used as visible light photocatalysts for the reduction of nitroaromatics. Through in situ high-resolution transmission electron microscopy (TEM), the coexistence of two fusion mechanisms of Au clusters on TiO₂ under ultraviolet-visible (UV-Vis) light irradiation in air is identified, i.e., the migration and coalescence (MC) and Ostwald ripening (OR). Additionally, the correlation between the photostability of Au clusters and reaction atmospheres has been investigated, among which Au clusters have higher stability in an inert N₂ atmosphere or vacuum than the oxidizing atmospheres (i.e., air and O₂). These results indicate the inherent stability of Au cluster during photocatalysis, and instability comes from the consuming of ligand layer. This work not only discloses the underlying mechanism of Au cluster sintering but also provides guidelines for enhancing metal clusters-based photocatalysts stability.