There is need for
well-defined lignin macromolecules for research
related to their use in biomaterial and biochemical applications.
Lignin biorefining efforts are therefore under investigation to meet
these needs. The detailed knowledge of the molecular structure of
the native lignin and of the biorefinery lignins is essential for
understanding the extraction mechanisms as well as chemical properties
of the molecules. The objective of this work was to study the reactivity
of lignin during a cyclic organosolv extraction process adopting physical
protection strategies. As references, synthetic lignins obtained by
mimicking the chemistry of lignin polymerization were used. State-of-the-art
nuclear magnetic resonance (NMR) analysis, a powerful tool for the
elucidation of lignin inter-unit linkages and functionalities, is
complemented with matrix-assisted laser desorption/ionization-time-of-flight-mass
spectrometry (MALDI-TOF MS), to gain insights into linkage sequences
and structural populations. The study unraveled interesting fundamental
aspects on lignin polymerization processes, such as identifications
of molecular populations with high degrees of structural homogeneity
and the emergence of branching points in lignin structure. Furthermore,
a previously proposed intramolecular condensation reaction is substantiated
and new insights into the selectivity of this reaction are introduced
and supported by density functional theory (DFT) calculations, where
the important role of intramolecular π–π stacking
is emphasized. The combined NMR and MALDI-TOF MS analytical approach,
together with computational modeling, is important for deeper fundamental
lignin studies and will be further exploited.