(−)ESI/CAD MSⁿ Procedure for Sequencing Lignin Oligomers Based on a Study of Synthetic Model Compounds with β-O-4 and 5-5 Linkages

Abstract

Seven synthesized G-lignin oligomer model compounds (ranging in size from dimers to an octamer) with 5-5 and/or β-O-4 linkages, and three synthesized S-lignin model compounds (a dimer, trimer, and tetramer) with β-O-4 linkages, were evaporated and deprotonated using negative-ion mode ESI in a linear quadrupole ion trap/Fourier transform ion cyclotron resonance mass spectrometer. The collision-activated dissociation (CAD) fragmentation patterns (obtained in MS and MS experiments, respectively) for the negative ions were studied to develop a procedure for sequencing unknown lignin oligomers. On the basis of the observed fragmentation patterns, the measured elemental compositions of the most abundant fragment ions, and quantum chemical calculations, the most important reaction pathways and likely mechanisms were delineated. Many of these reactions occur via charge-remote fragmentation mechanisms. Deprotonated compounds with only β-O-4 linkages, or both 5-5 and β-O-4 linkages, showed major 1,2-eliminations of neutral compounds containing one, two, or three aromatic rings. The most likely mechanisms for these reactions are charge-remote Maccoll and retro-ene eliminations resulting in the cleavage of a β-O-4 linkage. Facile losses of HO and CHO were also observed for all deprotonated model compounds, which involve a previously published charge-driven mechanism. Characteristic "ion groups" and "key ions" were identified that, when combined with their CAD products (MS experiments), can be used to sequence unknown oligomers.

Identification of lignin oligomers in Kraft lignin using ultra-high-performance liquid chromatography/high-resolution multiple-stage tandem mass spectrometry (UHPLC/HRMSn)

“…However, with techniques like Py-GC/MS and NMR, it is difficult to identify and characterise single lignin oligomers (LOs) in complex Kraft lignin samples. In recent years, several methods based on atmospheric pressure ionisation (API) in combination with tandem mass spectrometry (MS/MS) or multiple-stage tandem mass spectrometry (MS n ) have been established for the investigation of LOs in different types of lignin samples [ 8 , 13 ].…”

Lignin from bark as a resource for aromatics production by hydrothermal liquefaction

“…Recent studies like the work of Sheng et al () apply a procedure which is using negative ion‐mode electrospray ionization (ESI) with NaOH as a dopant and collision‐activated dissociation (CAD) fragmentation pattern. With this, they could provide new information about synthesized oligomer model compounds from coniferyl alcohol with β‐O‐4 and/or 5–5 bonds, which could help to analyze and better interpret real biomass samples.…”

“…Also, in this case several aspects must be still understood and considered. As an example, the synthesized oligomers always have a coniferyl alcohol end group; this could influence the fragmentation patterns of the deprotonated molecules (Sheng et al, ). However, this method shows potential for analysis of oligomers, but it is also highly complex and expensive.…”

“…However, this method shows potential for analysis of oligomers, but it is also highly complex and expensive. Several main requirements must be fulfilled (e.g., no fragmentations, only ions containing an entire intact molecule; Sheng et al, ) to get clear results. In addition, only straight chains were investigated, side chains could complicate the interpretation of the results.…”

Characterization of ionized lignin model compounds with α‐O‐4 linkages by positive‐ and negative‐ion mode electrospray ionization tandem mass spectrometry based on collision‐activated dissociation

“…The most common lignin linkages include β‐O‐4 (most common), 5‐5, β‐5, β‐β, β‐1, 4‐O‐5 and α‐O‐4 linkages (Figure 1). Our group has developed an ionization method for lignin degradation products containing β‐O‐4 and/or 5‐5 linkages 13–16 . The method is based on negative‐ion mode electrospray ionization (ESI) with sodium hydroxide dopant.…”