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: [C19H24O8 + H]+ as H(8-O-4')G; [C50H52O19 + H]+ as H(8-O-4')H(8-O-4'')S(8-O-4''')S(8-O-4'''')G; [C58H54O18+ H]+ as H(8-O-4')H(8-O-4'')H(8-O-4''')G(8-O-4'''')S(8-O-4''''')G; [C58H54O19 + H]+ as H(8-O-4')H(8-O-4'')H(8-O-4''')S(8-O-4'''')S(8-O-4''''')G; [C61H68O25 + H]+ as H(8-O-4')G(8-O-4'')G(8-O-4''')S(8-O-4'''')S(8-O-4''''')G; and [C61H68O26 + 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 [C39H43O15 + H]+• and [C40H43O16 + 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.