posted on 2024-02-27, 19:17authored byBiyang Zhu, Cong Ma, Lijun You
Increasing
hydrogen peroxide (H2O2)-based
systems have been developed to degrade various polysaccharides due
to the presence of highly reactive free radicals, but published degradation
mechanisms are still limited. Therefore, this study aimed to clarify
the degradation mechanism of six typical glucosidic bonds from different
disaccharides in an ultraviolet (UV)/H2O2 system.
The results showed that the H2O2 concentration,
disaccharide concentration, and radiation intensity were important
factors affecting pseudo-first-order kinetic constants. Hydroxyl radical,
superoxide radical, and UV alone contributed 58.37, 18.52, and 19.17%
to degradation, respectively. The apparent degradation rates ranked
in the order of cellobiose ≈ lactose > trehalose ≈
isomaltose
> turanose > sucrose ≈ maltose. The reaction pathways
were
then deduced after identifying their degradation products. According
to quantum chemical calculations, the cleavage of α-glycosidic
bonds was more kinetically unfavorable than that of β-glycosidic
bonds. Additionally, the order of apparent degradation rates depended
on the energy barriers for the formation of disaccharide-based alkoxyl
radicals. Moreover, energy barriers for homolytic scissions of glucosidic
C1–O or C7–O sites of these alkoxyl
radicals ranked in the sequence: α-(1 → 2) ≈ α-(1
→ 3) < α-(1 → 4) < β-(1 → 4)
< α-(1 → 6) < α-(1 → 1) glucosidic
bonds. This study helps to explain the mechanisms of carbohydrate
degradation by free radicals.