Fragmentation Study of Limonoids from Turraae pubescens by Electrospray Ionization Quadrupole Time-of-flight Mass Spectrometry

Nine representative limonoids isolated from Turraae pubescens were investigated by electrospray ionization quadrupole time-of-flight tandem mass spectrometry in positive ion mode. Although the structures of these compounds are similar, the corresponding fragmentation patterns and mass spectrometry and tandem mass spectrometry (MS/MS) spectra are clearly different. For Turrapubin A-C, product ions can be detected in both low and high mass ranges. A McLafferty-type rearrangement is the only way for the cleavage of C9-C10. For 11-epi-toonacilin, Turrapubin E, Turraflorin A, 11-epi-23-hydroxytoonacilide, Turrapubin H and 11-epi-21-hydroxytoonacilide, the cleavage of C9-C10 goes through two different ways, including McLafferty-type rearrangement and homolytic cleavage. The relative abundances of product ions from McLafferty-type rearrangement for 11-epi-toonacilin, 11-epi-23-hydroxytoonacilide, and Turrapubin H are high, while those for Turrapubin E, Turraflorin A, and 11-epi-21-hydroxytoonacilide are low. A pair of epimers was distinguished unambiguously by MS/MS spectra. It was found that the substituent group at C-1, hydroxy group, O atom linked to C-14 and C-15, and the oxygenated furan ring were the important factors leading to the differences of their MS/MS spectra.