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Lignin is a promising renewable source of valuable organic compounds and environmentally benign materials. However, its involvement in economic circulation and the creation of new biorefining technologies require an understanding of its chemical composition and structure. This problem can be overcome by applying mass spectrometry analytical techniques in combination with advanced chemometric methods for mass spectra processing. The present study is aimed at the development of mass defect filtering to characterize the chemical composition of lignin at the molecular level. This study introduces a novel approach involving resolution-enhanced Kendrick mass defect (REKMD) analysis for the processing of atmospheric pressure photoionization Orbitrap mass spectra of lignin. The set of priority Kendrick fractional base units was predefined in model experiments and provided a substantially expanding available mass defect range for the informative visualization of lignin mass spectra. The developed REKMD analysis strategy allowed to obtain the most complete data on all the homologous series typical of lignin and thus facilitated the interpretation and assignment of elemental compositions and structural formulas to oligomers detected in extremely complex mass spectra, including tandem ones. For the first time, the minor modifications (sulfation) of lignin obtained in ionic liquid-based biorefining processes were revealed., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Ilya Pikovskoi reports financial support was provided by Russian Science Foundation. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Lignin is the second most abundant biopolymer in nature and a promising renewable feedstock for the production of aromatic compounds, composite materials, sorbents, etc. Being a complex mixture of oligomeric molecules with an irregular structure, natural lignin is an extremely difficult object to study. Its molecular level characterization requires advanced analytical techniques among which atmospheric pressure photoionization Orbitrap mass spectrometry holds a promising place. In the present study, Kendrick mass defect (KMD) analysis was proposed to facilitate the visualization and interpretation of Orbitrap mass spectra of the biopolymer on an example of Siberian pine dioxane lignin preparation. The use of the typical guaiacylpropane structure C 10 H 12 O 4 as a Kendrick base unit made it possible to effectively identify oligomer series with different polymerization degrees and structurally related compounds, as well as to reliably determine the elemental compositions and structures of oligomers with high molecular weights (> 1 kDa). For the first time, KMD analysis was applied to the interpretation of the complex tandem mass spectra of lignin oligomers, rapid discrimination of the product ion series, and the establishment of the main collision-induced dissociation pathways. It was demonstrated that especially promising was the use of KMD filtering in the study of broadband fragmentation tandem mass spectra, which allows for the structural characterization of all oligomers with a particular degree of polymerization., (© 2023. Springer-Verlag GmbH Germany, part of Springer Nature.)

Lignin is the second most abundant biopolymer in nature and is considered an important renewable source of aromatic compounds. One of the most promising analytical methods for molecular characterization of lignin is Orbitrap high-resolution mass spectrometry with atmospheric pressure photoionization (APPI), proved itself in the study of lignins of various origins and their depolymerization products. In this work, the photoionization of lignin using acetone, 1,4-dioxane, and THF as solvents for the biopolymer and APPI dopants providing the generation of protonated and deprotonated molecules of lignin oligomers has been studied. The ionization conditions were optimized on the basis of the dependences of the total ion current on temperature and the flow rate of the solution into the ion source. Lignin degradation processes under APPI conditions occur mainly with the cleavage of ether β-O-4 bonds between phenylpropane structural units, demethylation (negative ion mode), as well as the loss of water and formaldehyde (positive ion mode). Negative ion mode APPI provides a higher ionization efficiency in the region of high molecular weights, however, it is characterized by an increased fragmentation of β-O-4 ether bonds compared to APPI(+) leading to a partial depolymerization of lignin in the ion source. The combination of APPI with Orbitrap high-resolution mass spectrometry allows obtaining mass spectra of coniferous and deciduous wood lignins with resolved fine structure and containing signals of up to 3000 oligomers in the mass range of 300-1800 Da. This can be used for comprehensive characterization of lignins at molecular level and tracking changes in biopolymer chemical composition in various processes., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

The use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the study of lignin is still extremely limited due to its low ionization efficiency. We have developed an approach for obtaining high-intensity MALDI mass spectra of lignin, based on the use of ionic liquids as matrices. Thirty-two ionic liquids consisting of large nitrogen-containing cations and anions of aromatic acids, traditionally used as crystalline matrices, were tested. It was established that ionic liquids based on N,N-diisopropyl-N-ethylammonium, N-isopropyl-N-methyl-N-tert-butylammonium, 3-aminoquinolinium, pyridinium, and 1-methylimidazolium cations and anions of ferulic, α-cyanohydroxycinnamic, and 2,5-dihydroxybenzoic acids as MALDI matrices provided high efficiency of lignin desorption/ionization with generation of singly charged protonated molecules of its oligomers. The use of such matrices in combination with the MALDI quadrupole ion trap-time-of-flight technique allows high-intensity mass spectra of lignin to be obtained without interferences from the matrix in the molecular weight range up to 3 kDa, adequately reflecting the molecular mass characteristics of lignin preparations. Using ionic liquid matrices, MS 2 and MS 3 MALDI mass spectra of lignins for various precursor ions were first obtained, including in the region of large (> 2 kDa) molecular weights. Differences in tandem mass spectra of coniferous and deciduous lignins, reflecting the structural features of corresponding oligomers were demonstrated. Graphical abstract ᅟ.

Rationale: Mass spectrometry with atmospheric pressure ionization is the most promising method for studying the structure of natural lignin, which is the second most abundant biopolymer in nature. The goal of this study is to compare the efficiency and characteristics of different types of ionization techniques (ESI, APCI, and APPI) in the negative ion mode by the example of softwood lignin., Methods: As the subjects of the study, we selected a preparation of spruce dioxane lignin and several phenols, simulating the basic structural fragments of the lignin macromolecule. High-resolution mass spectra were recorded using an Orbitrap mass spectrometer. Acetone was used as a solvent for samples and a dopant in photoionization mode. The ionization conditions were optimized to achieve the maximum intensity of the mass spectra., Results: The formation of deprotonated lignin molecules is characteristic of all the studied types of ionization; partial fragmentation of the biopolymer occurs in all ionization modes. ESI in the presence of ammonia yields low-intensity signals, leads to a significant decrease in ionization efficiency with increasing molecular weight of lignin oligomers, gives high-intensity impurity peaks in the mass spectra, and demonstrates selectivity for more polar structures. The ionization efficiency increases sharply in the order of ESI < APCI < APPI. The two latter methods are characterized by similar mechanisms of ionization; they ensure detection of approximately 1900 spruce lignin oligomers in the range of molecular weights up to 1.8 kDa. The determination of the elemental composition of oligolignols enabled the four main groups of compounds to be distinguished., Conclusions: Photoionization using acetone as a dopant is distinguished by a significantly higher intensity of signals and the lowest sensitivity to contaminants present in the lignin preparation. This ionization method can be considered as preferred for studying the dioxane lignin preparations of woody plants. Copyright © 2016 John Wiley & Sons, Ltd., (Copyright © 2016 John Wiley & Sons, Ltd.)

Lignin is considered a promising renewable source of valuable chemical compounds and a feedstock for the production of various materials. Its suitability for certain directions of processing is determined by the chemical structure of its macromolecules. Its formation depends on botanical origin, isolation procedure and other factors. Due to the complexity of the chemical composition, revealing the structural differences between lignins of various origins is a challenging task and requires the use of the most informative methods for obtaining and processing data. In the present study, a combination of two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy and multivariate analysis of heteronuclear single quantum coherence (HSQC) spectra is proposed. Principal component analysis and hierarchical cluster analysis techniques demonstrated the possibility to effectively classify lignins at the level of belonging to classes and families of plants, and in some cases individual species, with an error rate for data classification of 2.3%. The reverse transformation of loading plots into the corresponding HSQC loading spectra allowed for structural information to be obtained about the latent components of lignins and their structural fragments (biomarkers) responsible for certain differences. As a result of the analysis of 34 coniferous, deciduous, and herbaceous lignins, 10 groups of key substructures were established. In addition to syringyl, guaiacyl, and p-hydroxyphenyl monomeric units, they include various terminal substructures: dihydroconiferyl alcohol, balanopholin, cinnamic acids, and tricin. It was shown that, in some cases, the substructures formed during the partial destruction of biopolymer macromolecules also have a significant effect on the classification of lignins of various origins. [ABSTRACT FROM AUTHOR]

Understanding the chemical structure of lignin in the plant phloem contributes to the systematics of lignins of various biological origins, as well as the development of plant biomass valorization. In this study, the structure of the lignin from birch phloem has been characterized using the combination of three analytical techniques, including 2D NMR, Py-GC/MS, and APPI-Orbitrap-HRMS. Due to the specifics of the phloem chemical composition, two lignin preparations were analyzed: a sample obtained as dioxane lignin (DL) by the Pepper's method and DL obtained after preliminary alkaline hydrolysis of the phloem. The obtained results demonstrated that birch phloem lignin possesses a guaiacyl–syringyl (G-S) nature with a unit ratio of (S/G) 0.7–0.9 and a higher degree of condensation compared to xylem lignin. It was indicated that its macromolecules are constructed from β-aryl ethers followed by phenylcoumaran and resinol structures as well as terminal groups in the form of cinnamic aldehyde and dihydroconiferyl alcohol. The presence of fatty acids and flavonoids removed during alkaline treatment was established. Tandem mass spectrometry made it possible to demonstrate that the polyphenolic components are impurities and are not incorporated into the structure of lignin macromolecules. An important component of phloem lignin is lignin–carbohydrate complexes incorporating xylopyranose moieties. [ABSTRACT FROM AUTHOR]

Ionic liquids (ILs) based on 1-butyl-3-methylimidazolium (bmim) cation have proved to be promising solvents for the fractionation of plant biomass with the production of cellulose and lignin. This study deals with the characterization of lignins isolated from coniferous (spruce) wood using [bmim]OAc and [bmim]MeSO4 ionic liquids and their binary mixtures with DMSO (80:20). Molecular weight distributions, functional composition, and structural features of IL lignins were studied by size-exclusion chromatography, NMR spectroscopy (1D and 2D) and atmospheric pressure photoionization high-resolution mass spectrometry. It was shown that the interaction of ILs with lignin leads to significant chemical changes in the biopolymer; a decrease in the degree of polymerization and in the content of free phenolic hydroxyl groups due to alkylation, the disappearance (in the case of [bmim]OAc) of carbonyl groups and a significant destruction of β-O-4 bonds. The chemical reactions between lignin and 1-butyl-3-methylidazolium cation with covalent binding of ionic liquids or products of their decomposition is evidenced by the presence of a large number of nitrogen-containing oligomers in IL lignins. [ABSTRACT FROM AUTHOR]