Inherited Structure Properties of Larch Arabinogalactan Affected via the TEMPO/NaBr/NaOCl Oxidative System

Abstract

Arabinogalactan (AG), extracted from larch wood, is a β-1,3-galactan backbone and β-1,6-galactan side chains with attached α-1-arabinofuranosyl and β-1-arabinopyranosyl residues. Although the structural characteristics of arabinogalactan II type have already been studied, its functionalization using 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) oxidation remains a promising avenue. In this study, the oxidation of AG, a neutral polysaccharide, was carried out using the TEMPO/NaBr/NaOCl system, resulting in polyuronides with improved functional properties. The oxidation of AG was controlled by analyzing portions of the reaction mixture using spectrophotometric and titration methods. To determine the effect of the TEMPO/NaBr/NaOCl system, air-dried samples of native and oxidized AG were studied by Fourier-transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy, as well as by gel permeation chromatography. Compounds that model free (1,1-diphenyl-2-picrylhydrazyl (DPPH)) and hydroxyl radicals (iron(II) sulfate, hydrogen peroxide, and salicylic acid) were used to study the antioxidant properties. It was found that, in oxidized forms of AG, the content of carboxyl groups increases by 0.61 mmol compared to native AG. The transformation of oxidized AG into the H⁺ form using a strong acid cation exchanger leads to an increase in the number of active carboxyl groups to 0.76 mmol. Using FTIR spectroscopy, characteristic absorption bands (1742, 1639, and 1403 cm^-1) were established, indicating the occurrence of oxidative processes with a subsequent reduction in the carboxyl group. The functionality of AG was also confirmed by gel permeation chromatography (GPC), which is reflected in an increase in molecular weights (up to 15,700 g/mol). A study of the antioxidant properties of the oxidized and protonated forms of AG show that the obtained antioxidant activity (AOA) values are generally characteristic of polyuronic acids. Therefore, the TEMPO oxidation of AG and other neutral polysaccharides can be considered a promising approach for obtaining compounds with the necessary controlled characteristics.

Keywords: 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO); arabinogalactan; flocculation; larch; oxidation.

Figure 1

Structure of arabinogalactan units.

Figure 2

Scheme of TEMPO/NaBr/NaOCl-mediated oxidation of larch wood arabinogalactan.

Figure 3

Gel permeation chromatograms of native and purified AG-T after dialysis.

Figure 4

Calibration curve of D-galacturonic acid solutions.

Figure 5

Dynamics of 0.1 M NaOH consumption and uronic acid content.

Figure 6

Potentiometric titration curves of arabinogalactan samples: (1) native AG; (2) AG-T; (3) AG-TH.

Figure 7

Dissolving the native arabinogalactan (a) 100 mg/mL; (b) 50 mg/mL; (c) 33 mg/mL.

Figure 8

Dissolving the arabinogalactan samples up to a 100 mg/mL concentration (a) AG-T; (b) AG-TH.

Figure 9

Dissolving the arabinogalactan samples up to a 33 mg/mL concentration: (a) native AG; (b) AG-T; (c) AG-TH.

Figure 10

IR spectra of arabinogalactan samples’ absorbance units: (1) native AG; (2) AG-T; (3) AG-TH.

Figure 11

¹H (a) and ¹³C (b) NMR spectra of native and oxidized AG recorded in D₂O.

Figure 12

Molecular weight distribution of arabinogalactan samples.

Figure 13

TGA/DTG thermal degradation profiles of native and oxidized AG samples.

Figure 14

Activity of arabinogalactan samples scavenging (a) DPPH and (b) hydroxyl radicals.

Figure 15

Impact of AG-TH volume on flocculation rate and flocculation activity in the presence of CaCl₂ and FeCl₃.