Dr. Joe Banoub - November 17
A critique on the structural analysis of lignins, sequencing of lignin oligomers and lignomics
Lignins in the cell walls (CWs) of vascular plants are intimately mixed with the carbohydrate components and forma complex glycolignin network. In recent years, it was shown that the chemical structure of the various lignin biomolecules depends on the botanical origin and chemical composition of the vegetal fibres. It has also been proposed that the several different chemical, enzymatic and mechanical extraction methods are accountable for the major structural divergence that occurs after extraction and isolation. As a result, it appears that the only logic step to determine the natural structure of lignin is to isolate it from the vegetal matrix, without causing any structural change. Therefore, preparing pure samples of unchanged lignin is not an easy endeavour with the consequence that structural determination of lignin is perhaps more challenging than with other biopolymers.
A new nomenclature concerning the chemical state of lignins will be implemented, namely, virgin released lignins (VRLs) and processed modified lignins (PML). VRLs are obtained by liberation of lignins through degradation of vegetable matter by either chemical hydrolysis and/or enzymatic hydrolysis. PMLs are produced by subjecting the VRL to a series of further chemical transformations and purifications that are likely to alter their original chemical structures.
This presentation will discuss several MS application using soft ionization methods with a special emphasis on electrospray ionization, atmospheric pressure photoionization and matrix-assisted laser desorption/ionization MS and tandem MS (MS/MS) for the elucidation of the chemical structure of VRLs and PMLs lignins.
It will be proposed that native lignin polymers, present in the lignocellulosic biomass, are not made Lignin is a complex aromatic heteropolymer that forms a matrix with hemicelluloses as has been frequently reported. Instead, we propose that the lignins are composed of vast series of linear related oligomers, having different lengths that are covalently linked in a criss-cross pattern to cellulose and hemicellulose fibres forming the network of vegetal matter. Consequently, structural elucidation of VRLs, which presumably have not been purified and processed by any other type of additional chemical treatment and purification, may reflect the structure of the native lignin.
Finally, we will debunk the case that lignins have been reported to have very high molecular weights, as these might not exist! Since, such polymers have never been identified by the mild ionizing techniques used in modern MS.