Structural elucidation of the wheat straw lignin polymer by atmospheric pressure chemical ionization tandem mass spectrometry and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry

The structure of the cell wall of the lignocellulose fibers which constitute the skeletal substance of all terrestrial plants can be schematically viewed as bundles of cellulose microfibrils ordered in parallel in a matrix of amorphous hemicelluloses and lignins.1,2 Obtaining cellulose fibers from vegetable matter using traditional industrial paper procedures consists of degrading large amounts of lignins and hemicelluloses and making them soluble in aqueous media.3 Lignin, one of the most abundant naturally occurring materials, was identified as a soluble residue of acidic hydrolysis of wood more than 150 years ago.4 It is only in the last 50 years that the polymeric structure of lignin based on phenylpropenyl alcohol monomers (also called lignols) has been proposed. Knowledge about the lignin structure is still fragmentary. As shown in Fig. 1, it has been established that the monomeric lignol precursor in conifers is predominantly coniferyl alcohol (Fig. 1(A)), whereas in deciduous trees it is sinapyl alcohol (Fig. 1(B)) and in grasses and herblike dicotyledons it is p-cumaryl alcohol (Fig. 1(C)).5 In the last two decades, the biological heterogeneity of its molecular structure has been reported and it is well accepted that the chemical structure of the various lignin polymers depends on the botanical origin and chemical composition of the vegetable polymeric fibers.6 It has been proposed that the native lignin polymer is produced through random polymerization processes. Although Freudenberg and Neish have postulated that the lignols polymerize to yield native lignin macromolecules by a radical mechanism, the exact biosynthetic pathway has not yet been definitely established.7 The structure of the heterogenous lignin macromolecule remains unsolved and is a daunting task, especially as the extraction protocol appears to modify and degrade the lignin polymer. Thus, the initial step in lignin analysis will be to isolate lignin from the