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 [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 H-2 at C-6 and C-7 and with structural and side chain analogs. Results CID of [6,6,7,7-H-2(4)]12-OPEA and [6,6-H-2(2)]12-OPDA ([M-H](-) and [M-H-CO2](-)) showed that one or two H-2 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-H-2(2)]- and [6,6,7,7-H-2(4)]-12-OPA ([M-H](-) and [M-H-CO2](-)) yielded the loss of H-2 from the cyclopentanone and displayed the transfer of one H-2 atom in analogy to 12-OPEA. In contrast, CID of [6,6,7,7-H-2(4)]12-OPEA and [6,6,7,7-H-2(4)]12-OPA [M-H-H2O](-) demonstrated the transfer of two H-2 atoms (m/z 165 + 2). All spectra obtained by CID of [6,6,7,7-H-2(4)]12-OPDA and [6,6,7,7-H-2(4)]12-oxo-9(13),15(Z)-phytodienoic acid showed that one or two additional H-2 atoms could be transferred to this anion at m/z 167 of [6,6-H-2(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.