It was found based on results from alkaline nitrobenzene oxidation of natural and isolated dioxane lignins from several plants of the family Gramineae such as rice husks, rice straw, and corn and sorghum stems that the studied lignins consisted of three types of structural units with the predominance of guaiacyl structures.Grasses (Gramineae Juss.) are distributed in nature on all continents, represent a significant part of the biomass of many biocommunities, and are considered one of the most widespread and useful plants for man. The main components of the cell wall, e.g., cellulose, carbohydrates, proteins, extractable substances, of many grasses have by now been investigated. One of the main components of grasses is lignin, the content of which in grasses reaches 15-28% and which is not yet fully studied. It is also known that high-molecular-weight compounds of plant xylem, cellulose and lignin, exhibit valuable properties and can be used in various economic sectors.We reported previously on the isolation of dioxane lignins (DLAs) from rice husk and straw and from corn and sorghum stems. These were characterized by determining the chemical composition and calculating the semi-empirical formula [1,2].The goal of our work was to study the structure of natural lignins and DLAs from rice husk and straw and corn and sorghum stems using alkaline nitrobenzene oxidation (NBO).The yields of NBO products from the studied samples were different for both natural lignins and DLAs isolated from each plant (Fig. 1).HPLC was used to analyze total oxidation products of natural lignins from the grasses ( Table 1). The analysis showed that aromatic aldehydes, ketones, and phenolcarboxylic acids belonging to three types of structural units (p-coumaryl, guaiacyl, and syringyl) were formed by NBO of lignins from the samples.A comparison of the ratio of guaiacyl, syringyl, and p-coumaryl structural units in the alkaline NBO products of natural lignins and DLAs from the studied plants found that syringyl structures predominated in the oxidation products of ricestraw DLA. This may have indicated a low degree of condensation in this lignin (Tables 1 and 2). However, guaiacyl structures predominated in the oxidation products of natural lignin and rice-husk DLA. The observation of a rather significant quantity of p-hydroxybenzoic acid in the oxidation products of corn stems was a characteristic signature of this plant and reached 5-10%, in agreement with the literature on the content of esterified phenolcarboxylic acids in grasses [3].The ratio of syringaldehyde to vanillin (S/V) is given in many studies as an indicator of the degree of condensation of lignin preparations. For example, the ratio of syringyl and guaiacyl groups in mechanically milled lignin (MML) of wheatstraw (Triticum aestivum L.) internodes showed that the Maule color reaction [treatment of sections of plant material with KMnO 4 solution (1%) for 5 min and then HCl and NH 4 OH (a red color should appear for a positive reaction)] was negative for lignins with S/V < 2 [4...
Corn (Zea mais) and sorghum (Sorghum saccharatum) are annual herbaceous plants of the family Gramineae. The sugar content in sorghum stems reaches 12-18% [1]. Processing of these plants is complicated by the formation of multi-ton wastes of corn stems and cobs and sorghum stems. A comprehensive study of the compositions of these wastes is needed in order to find recycling pathways (Table 1). We used ground corn stems that were extracted with an alcohol:benzene mixture (1:2) [2] and the solid residue of sorghum stems after separation of sugars.The observed high Komarov lignin and cellulose contents in corn stems indicated that this sample was more woody. The comparatively high difficultly hydrolyzed polysaccharide (DHPS) content compared with that of readily hydrolyzed polysaccharide (RHPS) suggested that the carbohydrate complexes of corn and sorghum stems were DHPS, i.e., cellulose and hemicellulose.We performed basic hydrolysis of the studied plants by NaOH solution (8%) according to the literature [3]. A total of 19.2% of corn stems (suspended particles, 0.1 g; phenolic compounds, 0.43%) and 68.2% of sorghum stems (suspended particles, 1.6 g; phenolic compounds, 0.27%) underwent basic hydrolysis.Acidification of the basic hydrolysate by dilute HCl until the pH was 7 precipitated a dark-brown amorphous powder of suspended particles that were insoluble in water. It was found previously that a finely dispersed powder that precipitated was a fragment of the plant ligno-carbohydrate complex (LCC) [4]. Acid hydrolysis by H 2 SO 4 (2 N) of suspended particles and paper chromatography of the hydrolysates detected monosaccharides, mostly xylose and traces of arabinose and glucose. Furthermore, hydrolysates of suspended particles gave a positive reaction to phenol:H 2 SO 4 , thereby confirming that the sutdied samples contained lignin fragments. Therefore, the suspended particles were lignocarbohydrates, in other words, fragments of the LCC of the studied plants.An analysis of the basic components in the solid (remaining plant) showed that the lignin content decreased during basic hydrolysis. This was consistent with degradation of the macromolecule (Table 2).Extraction by CHCl 3 of the hydrolysates after removal of the LCC isolated fractions of phenolic compounds that were analyzed by HPLC (Table 3).The hydrolysis products contained aromatic aldehydes (p-hydroxybenzaldehyde, vanillin, syringaldehyde) and acids (vanillic, syringic, cinnamic, caffeic, veratric). This was characteristic of Gramineae plants. The phenolcarboxylic acids identified after basic hydrolysis may have been formed by hydrolysis of ether bonds of the lignin macromolecule. The dominant species in hydrolysates of both plants were p-coumaric, in contrast with those of rice husks and straw. The high content of p-hydroxybenzoic acid in the products from basic hydrolysis of natural lignin was characteristic for lignins of Gramineae plants.Basic hydrolysis of the studied plants showed that low-molecular-weight phenolic compounds could be produced. HPLC was...
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