Enzymatic Oxidation of Carbohydrate Byproducts for Use in Formation of Chitosan Hydrogels

Chitosan hydrogels are used in diverse applications ranging from pharmaceuticals and biomedical materials to food and agriculture. This study introduces a biology-inspired approach to create fully bio-based hydrogels by combining chitosan with bio-based di/polycarbonyl crosslinkers produced through the enzymatic oxidation of carbohydrates. Two such crosslinkers, Ox-XOS and Ox-Lac, were synthesized by oxidizing carbohydrates: Ox-XOS was produced by oxidizing xylooligosaccharides (XOS) with pyranose dehydrogenase from Agaricus bisporus (AbPDH1), and Ox-Lac was produced by oxidizing lactose (Lac) with galactose oxidase from Fusarium graminearum (FgrGalOx). The efficacy of enzymatic oxidation of lactose and XOS was analyzed using liquid chromatography and mass spectroscopy, showing high degrees of oxidation, and carbonyl groups were confirmed using ATR-FTIR and ¹H NMR. Compared with unmodified XOS, Ox-XOS showed a lower reaction temperature towards hexamethylenediamine by differential scanning calorimetry and demonstrated stronger gel formation ability with polyallylamine and chitosan. Rheological measurements showed <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>O</mml:mi> <mml:mo>(</mml:mo> <mml:mn>10</mml:mn> <mml:mo>)</mml:mo></mml:mrow> <mml:annotation>$O(10)$</mml:annotation></mml:semantics> </mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>O</mml:mi> <mml:mo>(</mml:mo> <mml:msup><mml:mn>10</mml:mn> <mml:mrow><mml:mn>3</mml:mn></mml:mrow> </mml:msup> <mml:mo>)</mml:mo></mml:mrow> <mml:annotation>$O(10^{3})$</mml:annotation></mml:semantics> </mml:math> increases in the storage moduli ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>G</mml:mi> <mml:mo>'</mml:mo></mml:mrow> <mml:annotation>$G&#x00026;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;prime;$</mml:annotation></mml:semantics> </mml:math> ) of chitosan hydrogels formed with Ox-Lac and Ox-XOS compared with unmodified lactose and XOS, indicating considerable increases in the hydrogels' resistance to deformation. These findings demonstrate the potential of enzymatically oxidized carbohydrates as crosslinkers to enhance chitosan hydrogels with potential utility in both high-value and large-volume sectors.