Structural Estimation of High-Molecular-Weight and Ultra-hydrophilic Trichloroacetic Acid Precursors Originated from Raphidophytes

Raphidophytes, algae that grow in lakes and ponds, can proliferate in drinking water sources and increase the level of trichloroacetic acid (TCAA) formation during chlorination. Raphidophytes lack cell walls, making them highly susceptible to disruption by minor physical effects, which in turn necessitates the removal of dissolved TCAA precursors during water treatment. TCAA precursors are presumably nonprotein biopolymers with ultrahydrophilic properties that pose challenges for their removal through conventional treatment. In this study, we estimated the substructures of TCAA precursors from lab-cultured raphidophytes (<i>Gonyostomum semen</i>) and wild raphidophytes (<i>Gonyostomum</i> spp.), using nuclear magnetic resonance (NMR), infrared (IR) spectroscopy, and high-resolution mass spectrometry (HRMS). ¹H and ¹³C NMR and IR spectra revealed the presence of phenolic substructures within the TCAA precursors. Additionally, the presence of glycan substructures is confirmed. Predicted molecular formulas obtained by HRMS supported the existence of glycans. Further evaluation of the TCAA formation potentials of the model substances indicated that the glycan substructures did not contribute to TCAA formation; instead, TCAA was derived solely from the phenolic functional groups. Additionally, the phenolic substructure may be substituted with electron-donating functionalities, which enhances TCAA formation. This study pioneers the elucidation of the detailed structures of these novel and unique TCAA precursors.