The chemical structure of lignin in normal and compression wood of spruce has been studied.
Thioacidolysis using both methanethiol and ethanethiol as nucleophiles was used for degradation.
The results showed that these nucleophiles work equally well in thioacidolysis, but that
methanethiol gives a somewhat higher yield than ethanethiol. In spruce compression wood, several
of the monomeric, dimeric and trimeric products were structurally elucidated using gas chromatography-mass spectrometry analysis of the desulphurated thioacidolysis products. Some of the
oligomeric structures contained a p-hydroxyphenyl unit, but most of the material seemed to be
present in non-condensed β-O-4 structures. A catechol structure was found and the results also indicated
the presence of β-5 stilbene structures in native spruce lignin.
Thioacidolysis, periodate oxidation and size exclusion chromatography (SEC) were used for the analysis of spruce lignin samples. The standard thioacidolysis method was modified by including a pre-swelling of the lignocellulosic sample before the reaction, and this gave a higher yield of monomer products. Middle lamella and compression wood lignin was found to contain a larger amount of oligomers after thioacidolysis than normal wood lignin. Analysis by thioacidolysis of a thermomechanical pulp (TMP) revealed that approximately 10% of the β-O-4 structures in the lignin were cleaved due to the mechanical grinding effect. From kraft pulp, the mixture of thioacidolysis products contains a major fraction having a considerably larger molecular mass than the products from spruce wood and TMP. The structure of this material is still unknown.
Bauer-McNett fractions of an aspen chemi-thermomechanical pulp showed a significant variation
with respect to lignin content and sugar composition, and two of the fractions were found to be
particularly rich in middle lamella and secondary wall material, respectively. The secondary wall
lignin of aspen was found to contain larger amounts of non-condensed β-O-4 aryl ether structures
than the middle lamella lignin and the difference was attributed to a larger amount of syringyl
structures as revealed by thioacidolysis. Size exclusion chromatography of the thioacidolysis products
from birch and spruce wood showed a larger part of lignin oligomers being present in the degraded
spruce lignin. Moreover, birch lignin had a lower phenolic content than both aspen and
spruce lignin. Thioacidolysis followed by Raney-nickel desulphuration was used for the analysis of
hardwood lignin trimers.
Electrospray ionisation Fourier transform ion cyclotron resonance mass spectrometry (MS) and
size exclusion chromatography (SEC) was utilised for analysis of milled wood lignin (MWL) and
lignin model compounds degraded by thioacidolysis. MS and SEC showed that thioacidolysis was
an efficient method for degradation of the β-O-4 bond in lignin. Moreover, the molecular weight of
thioacidolysed pinoresinol and MWL were analysed using matrix assisted laser desorption ionisation
time-of-flight mass spectrometry. Tandem MS was successfully applied for the structural determination
of thioacidolysed pinoresinol with and without acetylation of the product. The MS results
of thioacidolysis on MWL resulted in the identification of a tetrameric lignin structure
containing a 5-O-4, 5-5, β-1 linkage pattern.