Top‐down lignomic matrix‐assisted laser desorption/ionization time‐of‐flight tandem mass spectrometry analysis of lignin oligomers extracted from date palm wood

Abstract

Rationale We report for the first time the top‐down lignomic analysis of the virgin released lignin (VRL) oligomers obtained from the Saudi date palm wood (SDPW), using a matrix‐assisted laser desorption/ionization time‐of‐flight/time‐of‐flight (MALDI‐TOF/TOF) instrument. In addition, we are proposing new collision‐induced dissociation tandem mass spectrometry (CID‐MS/MS) fragmentation routes for this series of unreported VRL oligomers. Methods We have used direct MALDI‐TOF‐MS analysis of the mixture of lignin oligomers without any chromatographic pre‐separation. High‐energy CID‐MS/MS analyses were used to confirm the precursor ion structures. Results Six protonated lignin oligomer molecules were identified: [C 19 H 24 O 8 + H] + as H(8‐O‐4')G; [C 50 H 52 O 19 + H] + as H(8‐O‐4')H(8‐O‐4'')S(8‐O‐4''')S(8‐O‐4'''')G; [C 58 H 54 O 18 + H] + as H(8‐O‐4')H(8‐O‐4'')H(8‐O‐4''')G(8‐O‐4'''')S(8‐O‐4''''')G; [C 58 H 54 O 19 + H] + as H(8‐O‐4')H(8‐O‐4'')H(8‐O‐4''')S(8‐O‐4'''')S(8‐O‐4''''')G; [C 61 H 68 O 25 + H] + as H(8‐O‐4')G(8‐O‐4'')G(8‐O‐4''')S(8‐O‐4'''')S(8‐O‐4''''')G; and [C 61 H 68 O 26 + H] + as C(8‐O‐4')G(8‐O‐4'')G(8‐O‐4''')S(8‐O‐4'''')S(8‐O‐4''''')G units (H = coniferyl, S = sinapyl, and G = p ‐coumaryl). Two distonic cations were identified as [C 39 H 43 O 15 + H] +• and [C 40 H 43 O 16 + H] +• deriving from two tetrameric lignin oligomers. The high‐energy MS/MS analyses allowed the confirmation of the proposed structures of this series of lignin oligomers. Conclusions To our knowledge, this is the first elucidation of the lignin structure of the Saudi seedling date palm wood that was accomplished using a top‐down lignomic strategy that has not previously been published. The complex high‐energy CID‐MS/MS fragmentations presented herein are novel and have never been described before.