Separation of lignosulfonate lignin into antibacterial lignin oligomer fractions

Abstract

Introduction: Antibiotic resistance is rising globally, increasing the demand on new antibacterial substances.
Background: Studies suggest that lignin exhibits antibacterial properties, however this property is unexploited commercially. The valorization of lignin is difficult due to its complex and variable structure.
Aims: This study aims therefore to fractionate lignosulfonate lignin (LSL) into chemically well-defined fractions with elevated antibacterial properties. The influence of quaternary ammonium groups with long alkyl chains on antibacterial activity is also investigated.
Methods: LSL was fractionated using a ternary solvent system consisting of pressurized carbon dioxide, ethanol, and water. The fractions were analyzed using nuclear magnetic resonance (NMR) and ion mobility high resolution mass spectrometry (IM-HRMS). The effect of solvent composition on extraction selectivity was investigated with a simplex-like design of experiment. Each NMR spectra was divided into 99 different intervals, and the peak area in each interval was correlated to solvent composition using the reduced cubic Scheffe model. The fractions were derivatized via the Mannich and Menshutkin reactions which introduce tertiary amines and alkyl groups, respectively. A variant of the Etest and the microdilution broth assay were used to screen antibacterial susceptibility and determine minimum inhibitory concentrations (MIC).
Results: The obtained extracts varied in composition regarding type of functional groups and oligomer length. 83 out of the 99 regression models were valid giving tools for correlating extraction selectivity in terms of functional groups to solvent composition. Analysis of the derivatized fractions suggest that the reactions were unsuccessful. Antibacterial susceptibility testing suggest that the antibacterial effect is elevated in fractions extracted with intermediate polar solvent compositions, yielding antibacterial fractions with MIC values of 2500 µg/mL.
Conclusion: Antibacterial LSL fractions were obtained using intermediate polar solvent compositions, although with low potency compared to commercial antibiotics. More research should be dedicated towards optimizing introduction of quaternary ammonium groups on LSL.

Popular Abstract

Interest in replacing fossil-based polymers is growing, due to fossil feedstocks being nonrenewable and having a negative impact on the environment. A more sustainable alternative is lignin which is an important component of plants and is therefore bio-based and renewable. Lignin is produced as a side-product in the paper and pulp industry, and has historically mostly been burned as a source of heat. An interesting property of lignin is its antibacterial activity, which has not been thoroughly investigated. However, due to the prevalence of antibiotic resistance, the need for new antibacterial substances has increased. One reason to why lignin has been an under-utilized resource is that it has complex molecular structure. It is composed of many different building blocks that can be linked to each other in different ways to form larger structures. This makes it difficult to produce useful lignin-based polymers, but some heterogeneity is necessary to make it difficult for the bacteria to develop resistance. The aim of this project was therefore to separate the lignin sample obtained from industry into fractions that have a less complicated chemical composition, and then test if the antibacterial activity is enhanced in the fractions. This was performed by fractionating the lignin sample using different combinations of three solvents: liquid carbon dioxide, ethanol, and water. The obtained fractions were analyzed using techniques that give information about their composition and mass. The fractions were also chemically modified to alter their structure with the aim to improve antibacterial activity. Both the chemically modified and non-chemically modified fractions were then applied on bacteria to observe if they will affect their growth. In total, seven different fractions were obtained with differing composition regarding molecular structure and number of lignin building blocks linked together. The results suggested that the chemical modification did not improve the antibacterial activity of the lignin. However, the tests on the bacteria showed that the fractions separated with intermediate polar solvent mixtures had enhanced antibacterial activity compared to the original lignin sample. These fractions differed from the other fractions mainly regarding number of lignin building blocks linked together. However, the antibacterial activity was not that high compared to commercial antibiotics. In conclusion, it is possible to fractionate lignin obtained from the industry into fractions with increased antibacterial activity, but more work is needed to establish lignin-based antibiotics or materials with antibacterial properties as a viable alternative.

• author: Bajramova, Azemina
• supervisor: Peter Spégel; Mynta Norberg
• organization: Department of Chemistry
• course: KEMR30 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: Antibacterial activity, Lignosulfonate lignin, Simplex, Supercritical fluid extraction, Quaternary ammonium groups, Analytical chemistry
• language: English
• id: 9121750
• date added to LUP: 2023-06-09 10:14:54
• date last changed: 2023-06-09 10:14:54