Abstract
Understanding the chemical structure of lignin in the plant phloem contributes to the systematics of lignins of various biological origins, as well as the development of plant biomass valorization. In this study, the structure of the lignin from birch phloem has been characterized using the combination of three analytical techniques, including 2D NMR, Py-GC/MS, and APPI-Orbitrap-HRMS. Due to the specifics of the phloem chemical composition, two lignin preparations were analyzed: a sample obtained as dioxane lignin (DL) by the Pepper’s method and DL obtained after preliminary alkaline hydrolysis of the phloem. The obtained results demonstrated that birch phloem lignin possesses a guaiacyl–syringyl (G-S) nature with a unit ratio of (S/G) 0.7–0.9 and a higher degree of condensation compared to xylem lignin. It was indicated that its macromolecules are constructed from β-aryl ethers followed by phenylcoumaran and resinol structures as well as terminal groups in the form of cinnamic aldehyde and dihydroconiferyl alcohol. The presence of fatty acids and flavonoids removed during alkaline treatment was established. Tandem mass spectrometry made it possible to demonstrate that the polyphenolic components are impurities and are not incorporated into the structure of lignin macromolecules. An important component of phloem lignin is lignin–carbohydrate complexes incorporating xylopyranose moieties.
Highlights
With the depletion of fossil fuel reserves, lignin is becoming increasingly important as the potential renewable source of aromatic substances, being the second most abundant plant biopolymer in nature
It can be seen that phloem lignin, regardless of whether it has undergone alkaline hydrolysis, demonstrates similar ratios of S and G type units (S/G) predominating in its structure
The most important distinguishing feature of phloem lignin is the predominance of guaiacyl structures over syringyl ones (S/G = 0.7–0.9) providing a higher degree of condensation due to the formation of C-C bonds involving the 5th position in the phenylpropane aromatic nucleus
Summary
With the depletion of fossil fuel reserves, lignin is becoming increasingly important as the potential renewable source of aromatic substances, being the second most abundant (after cellulose) plant biopolymer in nature. Studies on the chemical structure of lignin preparations isolated from various plants are ongoing, significantly expanding our knowledge about the biopolymer and its transformations in different processes. The overwhelming majority of studies of woody plant lignins available in the literature were performed on preparations isolated from xylem. Inner bark (phloem) lignins are of the greatest interest since phloem, unlike xylem, is a living tissue, the study of which allows for a new knowledge to be gained about the processes of lignin biosynthesis and the ontogenesis of anatomical elements of plant biomass. The works on obtaining the reliable data on the chemical composition and structure of the phloem lignin is only starting
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