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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...

547.99.992The functional composition and structural porosity of hydrolyzed lignin from cotton-seed husks was altered by oxidation with hydrogen peroxide.