API-mass spectrometry of polyketides. I. A study on the fragmentation of triketide lactones

The fragmentation of delta-lactones, particularly triketide lactones, has been studied to provide information on the behavior of polyketides under atmospheric pressure ionization mass spectrometry (API-MS). The principal fragmentation patterns of triketide lactones are characterized by two sequential dehydrations followed by loss of CO to give hydrocarbon fragments. A particular goal of this study was an understanding of the origins of the two water molecules from the dehydrations. 18O- and 2H-isotope labeling experiments with delta-valerolactone suggest a mechanism for lactone fragmentation in which ionization by proton transfer is followed by rapid equilibration of ring-opened and ring-closed forms, which results in exchange of the ionizing proton into the hydrocarbon framework of the compound and randomization of the oxygens of the lactone. Subsequent fragmentation primarily involves sequential loss of water and CO. Similar experiments with the more complex triketide lactones show that their mass spectra share common features with that of delta-valerolactone, together with an additional water loss from the 3-hydroxyl group.