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), CPbased on coniferyl alcohol and p-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 M 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 M 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 p-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.

Seeing biomass recalcitrance through fluorescence

“…Their fluorescence spectra (Fig. 3 ) indicates maximum λ EX /λ EM of 372 nm/439 nm and 382 nm/450 nm, respectively, in agreement with the values previously presented 36 . Patterns of native lignocellulosic samples (Fig.…”

“…In-depth characterization of model CA dilignols suggested that fluorophores are not significantly electronically coupled to one another and can be seen as isolated fluorophores 27 . But even model lignin polymers fluorescence differs significantly depending on their composition 36 . Recently, it has been proposed that lignin fluorescence may originate from its aggregation due in particular to clustering of the carbonyl groups 37 .…”

Fluorescence lifetime imaging of lignin autofluorescence in normal and compression wood

“…Compression wood lignin has greater p ‐hydroxyphenyl unit content localised to the outer secondary wall (Terashima & Fukushima, ). There are indications that in lignin, polymers containing a higher amount of p ‐hydroxyphenyl units, chain structures consisting of alternating C = C/C‐C bonds are formed (Donaldson et al ., ; Djikanovic et al ., ). Such structures may be responsible for shorter lifetimes, higher fluorescence intensity and red shift of the long‐wavelength emission maxima in compression wood compared to normal wood.…”

Maize Internode Autofluorescence at the Macroscopic Scale: Image Representation and Principal Component Analysis of a Series of Large Multispectral Images

“…It has the advantage of requiring little tissue preparation, and there are imaging devices adapted to observe samples on a macroscopic scale [2,34]. In plant cell walls, lignin and hydroxycinnamic acids account for most of the autofluorescence and can be differentiated by their autofluorescence profiles [33][34][35][36][37][38][39][40][41]. Hydroxycinnamic acids emit blue fluorescence under UV excitation at approximately 350 nm [37,38], while lignin excited using UV and visible light emits blue, green and red fluorescence [35,[39][40][41].…”

“…In plant cell walls, lignin and hydroxycinnamic acids account for most of the autofluorescence and can be differentiated by their autofluorescence profiles [33][34][35][36][37][38][39][40][41]. Hydroxycinnamic acids emit blue fluorescence under UV excitation at approximately 350 nm [37,38], while lignin excited using UV and visible light emits blue, green and red fluorescence [35,[39][40][41]. The relative proportions of phenolic compounds and their environment (pH, presence of quenching molecules, etc.)…”