Eco-friendly ozonation of alginate: Physicochemical characterization and degradation mechanism exploration through mass spectrometry

The ozone degradation has been proven to be an effective degradation method for alginate, while the degradation mechanism remained to be unconfirmed. In this study, two high-molecular-weight alginates with different mannuronic/guluronic (G/M) ratios, HM and HG (G/M 0.49 vs 1.40), were depolymerized using established ozonation technology platform. Notably, HM can be degraded faster than HG especially within initial 30 min, indicating that the β-1, 4-mannuronic bonds are more susceptible to be ozonated than α-1, 4-guluronic bonds. However, HM/HG degraded to LMWA in 2 h and reached a plateau. Therefore, we employed mass spectrometry (MS) to profile the degraded products of LMWA polymannuronate (PM) and polyguluronate (PG) in more intense conditions. The results indicated that the oxidation process continued until all reducing ends were converted to carboxyl groups. The o-diol could directly oxidize to o-dialdehyde. This study provides a MS based elucidation of the mechanism by which alginate cleaves to oligosaccharides through ozonation.