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

Abstract

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. 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. Six protonated lignin oligomer molecules were identified: [C19 H24 O8 + H]+ as H(8-O-4')G; [C50 H52 O19 + H]+ as H(8-O-4')H(8-O-4'')S(8-O-4''')S(8-O-4'''')G; [C58 H54 O18 +H]+ as H(8-O-4')H(8-O-4'')H(8-O-4''')G(8-O-4'''')S(8-O-4''''')G; [C58 H54 O19 +H]+ as H(8-O-4')H(8-O-4'')H(8-O-4''')S(8-O-4'''')S(8-O-4''''')G; [C61 H68 O25 + H]+ as H(8-O-4')G(8-O-4'')G(8-O-4''')S(8-O-4'''')S(8-O-4''''')G; and [C61 H68 O26 +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 [C39 H43 O15 +H]+• and [C40 H43 O16 +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. 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.