How Extraction and Purification Affect MALDI-TOF MS Characterization of Mangrove Condensed Tannins, An Ecologically Important Secondary Metabolites in Coastal Wetland Ecosystem

Mangrove plants are rich in tannins, especially condensed tannins (CTs), which play an important role in biogeochemistry in coastal wetland ecosystem due to their functions of binding nutrients and heavy metal chelation. This study aims to obtain authentic chemical structures of mangrove CTs by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Four organic solvents (n-hexane, ether, chloroform, and EtOAc (ethyl acetate)) were used for extraction tests and three purification methods (Method 1, Sephadex LH-20 absorbed tannins, and nontannins fraction were washed by 50% methanol (mp) solution; Method 2, Sephadex LH-20 absorbed tannins and nontannins fraction were washed by 100% ethanol (ep), and released in 70% acetone solution; and Method 3, Yb³⁺ selectivity precipitated tannins) were conducted to investigate their influences on the characterization of CTs from two mangrove species, Bruguiera gymnorhiza and Kandelia obovata. The results showed that (1) EtOAc was used as an extraction solvent, leading to unauthentic structural properties of CTs; (2) the distribution patterns of the polymers in mangrove CTs purified with 50% methanol elicited the least different trends with those of CTs in the two mangrove crude extracts, and the lower oligomers (dimmer-hexamer) and higher polymers were lost during purification of CTs by 100% ep. Therefore, based on the toxicity and price of solvents, the crude CTs from mangrove plants can be extracted with n-hexane or ether to remove lipid and pigment impurities and then purified with 50% methanol, which is a complete set of methods to obtain completely authentic structural information of mangrove CTs. This study can offer more accurate structural information of mangrove CTs and new insights for the conservation of mangrove living environments for follow-up research.