Structural Differences Between Lignin Model Polymers Synthesized from Various Monomers

Abstract

In a plant cell wall, lignin is synthesized from several monomeric precursors, combined in various ratios. The variation in monomer type and quantity enables multifunctional role of lignin in plants. Thus, it is important to know how different combinations of lignin monomers impact variability of bond types and local structural changes in the polymer. Lignin model polymers are a good model system for studies of relation between variations of the starting monomers and structural variations within the polymer. We synthesized lignin model polymers from three monomers, CF-based on coniferyl alcohol and ferulic acid in monomer proportions 5:1 and 10:1 (w/w), CP-based on coniferyl alcohol and -coumaric acid in proportion 10:1 (w/w) and CA-based on pure coniferyl alcohol. We studied structural modifications in the obtained polymers, by combining fluorescence microscopy and spectroscopy, FT-IR and Raman spectroscopy, in parallel with determination of polymers' molecular mass distribution. The differences in the low (w) region of the distribution curves of the 10:1 polymers in comparison with the CA polymer may be connected with the increased content of C=C bonds and decreased content of condensed structures, as observed in FT-IR spectra and indicated by the analysis of fluorescence spectra. The 5:1 CF polymer contains a different type of structure in comparison with the 10:1 CF polymers, reflected in its simpler (w) distribution, higher homogeneity of the fluorescence emitting structures and in the appearance of a new high-wavelength emission component. We propose that this component may originate from -conjugated chains, which are longer in this polymer. The results are a contribution to the understanding of the involvement of structural variations of lignin polymers in the cell wall structural plasticity.