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

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 [C 11 H 17 O] − . 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‐CO 2 ] − , and [M‐H‐H 2 O] − 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 2 H at C‐6 and C‐7 and with structural and side chain analogs. Results CID of [6,6,7,7‐ 2 H 4 ]12‐OPEA and [6,6‐ 2 H 2 ]12‐OPDA ([M‐H] − and [M‐H‐CO 2 ] − ) showed that one or two 2 H 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‐ 2 H 2 ]‐ and [6,6,7,7‐ 2 H 4 ]‐12‐OPA ([M‐H] − and [M‐H‐CO 2 ] − ) yielded the loss of H 2 from the cyclopentanone and displayed the transfer of one 2 H atom in analogy to 12‐OPEA. In contrast, CID of [6,6,7,7‐ 2 H 4 ]12‐OPEA and [6,6,7,7‐ 2 H 4 ]12‐OPA [M‐H‐H 2 O] − demonstrated the transfer of two 2 H atoms ( m/z 165 + 2). All spectra obtained by CID of [6,6,7,7‐ 2 H 4 ]12‐OPDA and [6,6,7,7‐ 2 H 4 ]12‐oxo‐9(13),15( Z )‐phytodienoic acid showed that one or two additional 2 H atoms could be transferred to this anion at m/z 167 of [6,6‐ 2 H 2 ]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.