Determination of Arylglycerol−β-Aryl Ether Linkages in Enzymatic Mild Acidolysis Lignins (EMAL): Comparison of DFRC/³¹P NMR with Thioacidolysis

Abstract

Enzymatic mild acidolysis lignins (EMAL) isolated from different species of softwood and Eucalyptus globulus were submitted to comparative analysis that included thioacidolysis, derivatization followed by reductive cleavage (DFRC), and DFRC followed by quantitative 31 P NMR (DFRC/ 31 P NMR). While gas chromatography (GC) was used to determine the monomer yields from both thioacidolysis and DFRC, 31 P NMR studies quantified the various phenolic hydroxy groups released by DFRC. The monomer yields from thioacidolysis and DFRC were substantially different, with thioacidolysis resulting in higher yields. In contrast, an excellent agreement was obtained in the total number of -aryl ether structures determined by thioacidolysis and DFRC/ 31 P NMR, indicating that the combination of DFRC with quantitative 31 P NMR overcomes, at least in part, the limitations presented by the DFRC method. Both thioacidolysis and DFRC/ 31 P NMR were further used to better understand the lignin isolation process from wood. The results show that mild rotary ball milling minimizes, but does not prevent, the degradation of -O-4 structures during the early stages of wood pulverization. The extent of such degradation was found to be higher for E. globulus than for a variety of softwoods examined. Furthermore, the structures of the EMALs isolated at yields ranging from 20% to 62% were very similar, indicating structural homogeneity in the lignin biopolymer within the secondary wall.Lignin is a complex and irregular natural polymer built up of different interunit linkages. 1 While the bulk of lignin in wood consists of nonphenolic -aryl ether units, other units, such as phenylcoumaran ( -5), resinol ( -), and dibenzodioxocins (5-5/ -O-4, R-O-4), are also present in lower amounts within the lignin macromolecule. 1,2 According to the current understanding, almost all lignin macromolecules in softwood and softwood pulps are covalently linked to polysaccharides, mainly hemicelluloses. 3,4 A primary problem in elucidating lignin structure has been the isolation of total lignin from wood in a chemically unaltered form. 5-9 Overall, two approaches have been used to isolate lignin from lignocellulosics: acidolysis methods 10,11 and extraction of lignin after ball milling. 5,6,12 Despite resulting in lignin preparations with high yields and purities, severe acid concentrations (usually 0.2 M) trigger some changes in lignin structure. 10,11 On the other hand, traditional ball-milling-based methods, such as the milled wood lignin (MWL) and cellulolytic enzyme lignin (CEL) protocols, offer less modified lignin than those obtained by severe acid treatments. However, the yields of such lignins are dependent on milling intensity. [7][8][9]13 Intensive milling protocols offered by vibratory-and orbital-milling devices provide higher lignin yields within relatively short milling intervals, although at the expense of the integrity of the lignin macromolecule and associated condensation and oxidation reactions. 8,9,13,14 Low-intensity milling minimizes structural c...

Analytical pyrolysis as a direct method to determine the lignin content in wood

“…As it is known that the lignin composition is different in tissuetypes [28] or cell wall types [19,44,45] analytical pyrolysis was shown to have the potential to discriminate them. The samples with unknown compression wood amount can now be tentatively assigned, and also the ones found around the average of PC 1 should maybe (re)assigned to the same tissue-type.…”

“…Methods such as thioacidolysis [18][19][20], derivatization followed by reductive cleavage (DFRC) [21,22], hydrogenolysis [23], acidolysis [24], ozonation [25], selective tosylation (T) of primary hydroxyl group, iodination (I), and zinc-metal treatment (Z) (TIZ) to cleave b-O-4 linkage [26] were used to characterize lignin structure/composition. DFRC was combined with 1 H-13 C HMQC NMR spectroscopy [27] or 31 P NMR [28,29] and compared to thioacidolysis [27,28] to elucidate lignin structure. The two of the most effective methods for elucidating lignin structure have proven to be thioacidolysis and DFRC, whereas the latter method provides lower monomer yields from b-aryl ether bonds [28].…”

“…DFRC was combined with 1 H-13 C HMQC NMR spectroscopy [27] or 31 P NMR [28,29] and compared to thioacidolysis [27,28] to elucidate lignin structure. The two of the most effective methods for elucidating lignin structure have proven to be thioacidolysis and DFRC, whereas the latter method provides lower monomer yields from b-aryl ether bonds [28]. All these methods were developed to investigate the lignin composition of lignocellulosic materials in native as well as processed state, e.g.…”

Aplicações de fibras lignocelulósicas na química de polímeros e em compósitos

“…122 Recentes progressos foram alcançados no isolamento de lignina de fibras vegetais com a acidólise enzimática em condições amenas EMAL (Enzymatic Mild Acidolysis Lignin), a qual produz lignina com características mais próximas às do material em sua forma nativa. [123][124][125] Zoia e colaboradores 79 aperfeiçoaram este método em relação ao rendimento e pureza da lignina, utilizando EMAL combinado com radiação de micro-ondas, em substituição à usual etapa de refluxo durante a acidólise.…”

Analytical Pyrolysis Characteristics of Enzymatic/Mild Acidolysis Lignin (EMAL)

“…Solid samples can be directly used in Py-GC/MS testing without pretreatment. The analysis of pyrolysis products not only assists in understanding the chemical structure of lignin, but more importantly, it helps to determine the distribution of lignin pyrolysis products under different thermochemical conditions (Guerra et al 2008;Lou et al 2010;Jiang et al 2017).…”