Charge migration fragmentation in the negative ion mode of cyclopentenone and cyclopentanone intermediates in the biosynthesis of jasmonates

Abstract

Rationale Jasmonates are formed from 12‐oxo‐10,15(Z)‐phytodienoic acid (12‐OPDA) in plants and also from 12‐oxo‐10‐phytoenoic acid (12‐OPEA) in fungi. Collision‐induced dissociation (CID) of [M‐H]− generates characteristic product anions at m/z 165 [C11H17O]−. Our goal was to investigate the structure and mode of formation of this anion by CID of 12‐OPDA, 12‐OPEA, and 12‐oxophytonoic acid (12‐OPA). Methods We investigated the CID of the [M‐H]−, [M‐H‐CO2]−, and [M‐H‐H2O]− anions using electrospray ionization and MS/MS analysis of 12‐OPDA, 12‐OPEA, and 12‐OPA, and compared the results with the data obtained with the corresponding compounds labeled with 2H at C‐6 and C‐7 and with structural and side chain analogs. Results CID of [6,6,7,7‐2H4]12‐OPEA and [6,6‐2H2]12‐OPDA ([M‐H]− and [M‐H‐CO2]−) showed that one or two 2H atoms were transferred to anions at m/z 165 as judged by the signal intensities of m/z 165 + 1 or 165 + 2, respectively. CID of [6,6‐2H2]‐ and [6,6,7,7‐2H4]‐12‐OPA ([M‐H]− and [M‐H‐CO2]−) yielded the loss of H2 from the cyclopentanone and displayed the transfer of one 2H atom in analogy to 12‐OPEA. In contrast, CID of [6,6,7,7‐2H4]12‐OPEA and [6,6,7,7‐2H4]12‐OPA [M‐H‐H2O]− demonstrated the transfer of two 2H atoms (m/z 165 + 2). All spectra obtained by CID of [6,6,7,7‐2H4]12‐OPDA and [6,6,7,7‐2H4]12‐oxo‐9(13),15(Z)‐phytodienoic acid showed that one or two additional 2H atoms could be transferred to this anion at m/z 167 of [6,6‐2H2]12‐OPDA due to isotope scrambling. Conclusions CID of 12‐OPDA and 12‐OPEA generates cyclopentanone enolate anions at m/z 165 by charge‐driven hydride transfer as a common mechanism and by bond cleavage between C‐7 and C‐8 of the carboxyl side chains with either gain or loss of a hydrogen atom.

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“…It showed three main fragments in negative ionization mode, which were assigned to loss of water [M-H 2 O-H] - 273.186, loss of the carboxy group [M-CO 2 -H] - 247.207, as well as the analytical OPDA fragment of m/z 165.125 for [M-C 7 H 10 O 2 -H] - ( Figure 3B ). These fragments were in agreement with an earlier report ( Oliw and Hamberg, 2019 ). In a similar way the identity of dnOPDA was confirmed ( Figure 3B ).…”

“…The fragmentation pattern of these two compounds showed a remarkable similarity to -OPDA and dnOPDA, including the three fragments m/z 273.184, m/z 247.207 and m/z 165.129 of 12-OPDA, and m/z 219.175, m/z 191.144, m/z 165.13 and m/z 59.014 of dnOPDA, respectively (Figure 3B). In accordance with a report of Oliw and Hamberg, these two compounds were therefore assigned as iso-OPDA (12-oxo-9-(15Z)-phytodienoic acid) and iso-dnOPDA, respectively (Oliw and Hamberg, 2019).…”

“…), with Mass Hunter Qualitative Analysis software as analysis tool. For structure elucidation, the fragmentation patterns of compounds were compared with MS/MS spectra deposited in the Metlin ( Guijas et al., 2018 ) database, known from the literature ( Oliw and Hamberg, 2019 ) or by interpretation of fragmentation rules.…”