Structural Assignment of the Product Ion Generated from a Natural Ciguatoxin-3C Congener, 51-Hydroxyciguatoxin-3C, and Discovery of Distinguishable Signals in Congeners Bearing the 51-Hydroxy Group.

Ciguatoxins (CTXs) stand as the primary toxins causing ciguatera fish poisoning (CFP) and are essential compounds distinguished by their characteristic polycyclic ether structure. In a previous report, we identified the structures of product ions generated via homolytic fragmentation by assuming three charge sites in the mass spectrometry (MS)/MS spectrum of ciguatoxin-3C (CTX3C) using LC-MS. This study aims to elucidate the homolytic fragmentation of a ciguatoxin-3C congener. We assigned detailed structures of the product ions in the MS/MS spectrum of a naturally occurring ciguatoxin-3C congener, 51-hydroxyciguatoxin-3C (51-hydoxyCTX3C), employing liquid chromatography/quadrupole time-of-flight mass spectrometry with an atmospheric pressure chemical ionization (APCI) source. The introduction of a hydroxy substituent on C51 induced different fragmentation pathways, including a novel cleavage mechanism of the M ring involving the elimination of 51-OH and the formation of enol ether. Consequently, new cleavage patterns generated product ions at m/z 979 (C55H79O15), 439 (C24H39O7), 149 (C10H13O), 135 (C9H11O), and 115 (C6H11O2). Additionally, characteristic product ions were observed at m/z 509 (C28H45O8), 491 (C28H43O7), 481 (C26H41O8), 463 (C26H39O7), 439 (C24H39O7), 421 (C24H37O6), 171 (C9H15O3), 153 (C9H13O2), 141 (C8H13O2), and 123 (C8H11O).

Assignment of product ions produced from ciguatoxin-3C provides a deep insight into the fragmentation mechanism of polycyclic ether compounds.

RATIONALE: Marine polycyclic ethers have drawn attention owing to their unique chemical structures and involvement in food poisoning and fish killing. To study structural diversity, we performed a structural assignment of product ions produced from a representative ladder-shaped polycyclic ether, ciguatoxin-3C, and elucidated the mechanism of generation. METHODS: The product ions used for the structural assignment were produced from a precursor ion [M + H]+ using liquid chromatography/quadrupole time-of-flight mass spectrometry, by employing an atmospheric pressure chemical ionization source. RESULTS: Three charged sites were considered at both terminals of a molecule. Typical charge-remote fragmentation was produced at the respective charge sites, yielding a hybrid spectrum. C-C bonds bordering two ethers could cleave and trigger the fission of two other bonds. Prominent ions indicating the serial loss of water molecules resulted from the simultaneous deprivation of ethereal oxygen and hydrogen atoms. The resultant double bonds formed long chains of conjugated polyenes, which stabilized charge via resonance. CONCLUSIONS: Three alternative charge sites produce a hybrid spectrum. The simultaneous fission of three bonds was explained. For the first time, intense ions due to serial dehydration were explained by the elimination of ether oxygen atoms and the subsequent conjugation of double bonds. All product ions were considered by the structural features of polycyclic ether that facilitates the formation of conjugated polyenes.