Fast-atom bombardment mass spectrometry and low energy collision-induced tandem mass spectrometry of tauroconjugated bile acid anions

Abstract

Fragmentation of negative ions produced by fast-atom bombardment (FAB) from 14 tauroconjugated bile acids and some of their deuterated analogs has been studied by mass spectrometry and by collision-induced dissociation (CED) tandem mass spectrometry at low energy.Low energy collision-induced dissociation of the deprotonated molecules [M - H](-) of these tauroconjugated bile acids leads to both charge-driven and charge-remote fragmentations (CRF). The former yields neutral loss from the side chain with charge migration during the fragmentation process. These fragments dominate the CID spectra, but are absent from the FAB spectra. Their relative abundances are dependent on the number and the positions of the hydroxyl groups in the steroid nucleus and thus permit distinction among some positional isomers.The CRF fragments correspond to cleavages in the side chain up to fragmentations across the steroid rings with charge retention on the sulfonate group. These CRF fragments, which also are useful for structural identification, are less intense in CID than in FAB spectra. It appears that these charge-remote fragments are favored by unsaturation in the steroid rings, either as keto groups or as endocyclic double bonds. Tandem mass spectrometry combined with the use of deuterated analogs demonstrates that the structures of the survivor pseudomolecular ions and of the CRF fragments are not rearranged.

Nano-electrospray tandem mass spectrometry for the analysis of neurosteroid sulphates†

“…21 Charge remote fragmentation reactions appear to be of most importance at high collision energies and studies performed at low collision energies (`100 eV) on instruments such as triple quadrupoles give product ion spectra containing less prominent CRF ions. 18,22 In recent years electrospray 23 (ES) and matrix-assisted laser desorption/ionisation 24 (MALDI) have replaced FAB as the most popular methods of desorption/ionisation of polar biological molecules. Although FAB is an excellent desorption/ionisation method for steroid sulphates, it suffers as a result of the presence of ubiquitous matrix-associated chemical noise, making low level analysis of steroid sulphates difficult if not impossible.…”

Tandem mass spectrometry in the study of fatty acids, bile acids, and steroids

“…Because FAB was initially incorporated on magnetic‐sector instruments, most of the early CID studies were performed at high collision‐energy. However, Libert et al (1991); Stroobant et al (1995); and Lemonde, Johnson, & Clayton (1999) have shown that informative CID spectra of bile acids can also be obtained at low collision‐energy (Table 2). Libert et al (1991) found that taurine‐conjugated Δ 4 ‐unsaturated bile acids give particularly informative low‐energy CID spectra; this result is in contrast to the spectra of glycine‐conjugated and unconjugated bile acids, which mostly show fragment ions generated by small neutral losses (Eckers et al, 1990, Eckers, East, & Haskins, 1991).…”

“…Such molecules are suitable for ionization by negative‐ion FAB, and give abundant [M‐H] − ions and few fragment ions. To gain further structural information, MS/MS experiments have been performed mostly at high‐collision energies on magnetic‐sector instruments (Tomer, Crow, & Gross, 1983; Jensen, Tomer, & Gross, 1985; Kingston et al, 1985; Liehr, Kingston, & Beynon, 1985; Tomer & Gross, 1988; Adams, 1990; Gross, 1992; Gross & Hu, 1994; Cheng & Gross, 2000), although studies have also been performed at low‐collision energies on triple‐quadrupole instruments (Libert et al, 1991; Sala et al, 1991; Stroobant et al, 1995). The major disadvantage of the FAB ionization method is the accompanying presence of matrix‐associated chemical noise.…”

Applications and mechanisms of charge-remote fragmentation

“…The [M À H] À and [M Na] ions of steroid derivatives (bile salts and steroid conjugates) undergo charge-remote fragmentation, providing structural information of the ring systems, including positions of unsaturation, hydroxy, or keto groups (Grif®ths, Zhang & Sjo Èvall, 1993Marschall et al, 1994;Rossi, Johnson & Yost, 1994;Ikeda et al, 1995). Stroobant et al (1995) to occur. The charge-driven chemistry, occurring along the side chain, results in the losses of parts of the chain as neutrals, making it possible to elucidate the structure of the chain.…”