A Fragmentation Study of Six C21 Steroidal Aglycones by Electrospray Ionization Ion-Trap Time-of-Flight Tandem Mass Spectrometry

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The fragmentation patterns of six C21 steroidal aglycones, metaplexigenin (1), caudatin (2), qingyangshengenin (3), penupogenin (4), 20-cinnamoylsarcostin (5), and gagamine (6), were analyzed by high-resolution electrospray ionization ion-trap time-of-flight tandem mass spectrometry (HR-ESI-IT-TOF-MS(n)). The [M-H]+ ions of steroids 1-3 that contain a carbonyl functional group at C-20 (Type I) and [M+H]+ ions of steroids 5-6 that possess a hydroxyl group at C-20 (Type II) were readily observed in MS analyses. The fragmentation pathways and diagnostic fragment ions for these six steroidal aglycones were proposed on the basis of their MS(n) analyses. The common fragmentation pathways for type I steroidal aglycones include the neutral loss of the ester group at C-12 and the hydroxyl moieties on the steroid skeleton, as well as the cleavage of ring D. Their diagnostic fragment ions were identified as m/z 361(B), 343 (C), 325 (D), 307 (F), 283 (G), 259 (E), and 243 (H). The fragmentation behavior of penupogenin (4) in type II was similar to those of type I, with m/z 363 (B'), 345 (C'), 327 (D'), 309 (F'), 283 (G), and 243 (H) as its diagnostic fragment ions. The ester group at C-20 was difficult to cleave in the MS(n) analyses of 20-cinnamoylsarcostin (5) and gagamine (6) so that the loss of this ester group was slower than that at C-12 and hydroxyl groups; the key ions at m/z 329 (I), 311 (J), 293 (K), and 275 (L) were characteristic for 5 and 6. The base ion peaks were derived from the loss of the substituent group at either C-12 or C-17 for both type I and type II steroidal aglycones.