A combined theoretical and experimental study of mechanisms of fragmentation active for PHB oligomers in negative-ion mode multistage mass spectrometry

First-principles studies of the potential energy profiles for dissociation of poly[(R, S)-3-hydroxybutanoic acid] [PHBn-H]− anions by proton rearrangement and direct fragmentation, as well as subsequent statistical RRKM analysis, indicate that fragmentation should proceed mainly through successive loss of a single 86 Da neutral unit (propene + CO2). This theoretically predicted fragmentation mechanism is inconsistent with that proposed in the literature based on experiments, which suggests that proton rearrangements are the only active fragmentation channels in CID of [PHBn-H]−. We have combined experimental ESI-MS/MS fragmentation studies of [PHBn-H]− with first-principles explorations of the potential energy surfaces, and molecular dynamics simulations of CID and thermal fragmentation events to understand this discordance. Both proton rearrangement and direct fragmentation channels are considered. The results reveal that the fragmentations observed at low collision energy favor the direct fragmentation channel and are more closely modeled as a thermal fragmentation process. By contrast, fragmentations observed at higher collision energy favor the indirect fragmentation channels and are more closely modeled as a fast CID process.

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► Identification of an alternative “direct” fragmentation channel of PHB.
► According to the RRKM theory this should be dominant fragmentation process.
► MS spectra consistent are with the dominance of indirect fragmentation processes.
► Thermal and CID MD simulations confirm the presence of the direct pathway.