On the structure of lignin in annual plants and its application in modern biotechnology
Recently, an increasing number of scientific papers have been published exploring various aspects of the biological activity of humic acids isolated from soil, peat, and various caustobioliths. Today, these studies serve as the fundamental basis for the production of modern, highly effective organomineral fertilizers, such as those produced by Agro.Bio.
It is generally accepted that lignin or its individual structural units is one of the most important components of the humic acid molecule. Lignin monomers have been shown to actively participate in plant cellular metabolism, and some authors believe that lignin components perform critical regulatory functions. Therefore, studying the structure of lignin is of great practical and scientific interest. While the structure of lignin in perennial plants (especially conifers) has been extensively studied, very little data exists on the lignin of deciduous trees and annual agricultural crops.
The lack of knowledge about lignin from annual plants means that this complex and useful polymer is not always used to its full potential. The aim of this study is to investigate the structure of lignin from non-food plant materials, specifically agricultural waste: buckwheat hulls and millet hulls.
Extraction and Fractionation of Lignin
Lignins were isolated from millet films and buckwheat hulls using dioxane and an alcohol-benzene mixture. A 1% hydrogen chloride (HCl) gas mixture was used as a catalyst. Based on their water solubility, dioxane lignins and alcohol-benzene lignins were separated into the following fragments:
- Fragment B: Low molecular weight part of lignin, soluble in water.
- Fragment B: A higher molecular weight part of the lignin macromolecule, soluble in alcohol-benzene, but insoluble in water.
- Fragment A: The most compacted part of the lignin macromolecule, not extracted from the cellolignin composition by the alcohol-benzene mixture.
Fragment B of lignin from millet films and buckwheat hulls was separated into individual components using chromatography. Valuable organic compounds were found among the lignin degradation products: vanillin, syringaldehyde, and para-hydroxybenzaldehyde.
CHEMICAL COMPOSITION OF FRAGMENT B
The average molecular weight portion of the lignin macromolecule (fragment B) was analyzed for its elemental composition and functional group content. The resulting data are presented in the table below.
Table 1. Content of functional groups and elemental composition of fragment B of alcohol-benzene lignin
| Raw materials | Fragment yield, % | WITH, % | H, % | -ОСН₃, % | OH total, % | OH phenol, % | C=O, % |
|---|---|---|---|---|---|---|---|
| Fragment B of lignin from millet films | 11:40 | 62.96 | 7.25 | 16.99 | 14.69 | 8.11 | 2.86 |
| Fragment B of buckwheat husk lignin | 7.00 | 63.57 | 6.94 | 16.81 | 11,12 | 8.47 | 2.15 |
(Note: Values are given as a percentage of the dry matter of the fragment).
Based on these data, semi-empirical formulas were calculated for the B fragments of alcohol-benzene lignins per phenylpropane structural unit C 6 -C 3 . The semi-empirical formula has the following form:
- For millet films:
C 9 H 9.31 O 0.81 (OCH 3 ) 1.08 (OH) 1.81 (C=O) 0.20 - For buckwheat hulls:
C 9 H 10 O 1.08 (OCH 3 ) 1.06 (OH) 1.33 (C=O) 0.14
As we can see, the formulas differ in their oxygen, hydroxyl, and carbonyl group content. For each phenylpropane unit in the B fragments of the lignins studied, there is, on average, one methoxyl group. The millet husk lignin fragment has almost two hydroxyl groups per unit, while the buckwheat husk lignin has approximately one and a half. Carbonyl groups are distributed more sparsly: one per five units in the millet husks and one per seven in the buckwheat husks.
SPECTROSCOPIC ANALYSIS (UV AND IR SPECTRA)
The ultraviolet and infrared spectra of alcohol-benzene lignin molecules were studied. It has been historically proven that absorption bands at λmax 2800–2850 Å for coniferous woods and λmax 2740–2760 Å for hardwoods are aromatic in nature (esterified residues of pyrocatechol and pyrogallol).
In our experiments, the spectra of dioxane lignins and alcohol-benzene lignins were recorded using an SF-4 spectrophotometer. All studied lignin preparations from annual plants were characterized by clearly defined bands in the 283–287 nm range and less distinct K-bands in the 309–314 nm range. The ultraviolet spectrum of fragment B of dioxane lignin from buckwheat husks has an additional maximum at 275 nm.
Pyrogallol dimethyl ethers (which include the syringaldehyde group) shift absorption toward shorter wavelengths, corresponding to the 275 nm band. Absorption in the 284–286 nm region is due to the presence of the guaiacyl group, and in the 310–315 nm region, to the presence of the carbonyl system.
CONCLUSIONS AND PROSPECTS FOR IMPLEMENTATION
Research data confirm the presence of guaiacyl and syringol groups, as well as a carbonyl conjugated with an aromatic ring, in agricultural waste lignins. While deciduous wood lignins consist of derivatives of coniferyl and sinapic alcohols (1:1), annual plant lignins additionally contain derivatives of p-oxycoumaric alcohol.
Consequently, millet films, buckwheat hulls and other crop residues from agricultural enterprises are not just waste, but a valuable reserve for the production of humic acids and physiologically active substances.
Agro.Bio Expert Comment: Today, this deep scientific approach is being put into practice in Agro.Bio's technologies. Using advanced methods for processing organic raw materials rich in lignin and humic acids, the company produces environmentally friendly organomineral growth stimulants (such as Potassium Humate and Mind Agro.Bio). These products activate plant metabolism at the cellular level, helping farmers achieve maximum yields and crop resilience.