Dmitri V. Zagorevskii

A mass spectrometry study of tirapazamine and its metabolites: insights into the mechanism of metabolic transformations and the characterization of reaction intermediates

Tandem mass spectrometry methods were used to study the sites of protonation and for identification of 3-amino-1,2,4-benzotriazine 1,4-dioxide (1, tirapazamine), and its metabolites (3-amino-1,2,4-benzotriazine 1-oxide (3), 3-amino-1,2,4-benzotriazine 4-oxide (4), 3-amino-1,2,4-benzotriazine (5), and a related isomer 3-amino-1,2,4-benzotriazine 2-oxide (6). Fragmentation pathways of 3 and 5 indicated the 4-N-atom as the most likely site of protonation. Among the N-oxides studied, the 4-oxide (4) showed the highest degree of protonation at the oxygen atom. The differences in collision-induced dissociation of isomeric protonated 1-, 2- and 4-oxides allowed for their identification by LC/MS/MS. Gas phase and liquid phase protonation of tirapazamine occurred exclusively at the oxygen in the 4-position. A loss of OH radical from these ions (2+) resulted in ionized 3. Neutralization-reionization mass spectrometry (NR MS) experiments demonstrated the stability of the neutral analogue of protonated tirapazamine in the gas phase in the μs time-frame. A significant portion of the neutral tirapazamine radicals (2) dissociated by loss of hydroxyl radical during the NR MS event, which indicates that previously proposed mechanisms for redox-activated DNA damage are reasonable. The activation energy for loss of hydroxyl radical from activated tirapazamine (2) was estimated to be ∼14 kcal mol−1. Stable neutral analogues of [3+H]+ and [5+H]+ ions were also generated in the course of NR MS experiments. Structures of these radicals were assigned to the molecules having an extra hydrogen atom at one of the ring N-atoms. Quantum chemical calculations of protonated 1, 3, 4 and 5 and the corresponding neutrals were performed to assist in the interpretation of experimental results and to help identify their structures.

Metastable ion study of substituted cyclopentadienylmanganese cations in the gas phase

The behavior of some substituted cyclopentadienylmanganese ions has been studied by tandem mass spectrometry. This metastable ion study showed that only C5H5Mn+ and (C5H4CN)Mn+ ions retain their nido-cluster structure (1), which is characterized by a simple metal-ligand bond cleavage. Other substituted ions, RXC5H4Mn+, rearrange to a different extent, depending on the nature of the substituent. The first rearrangement step is R radical migration to the central metal atom, leading to RMnC5H4X+-type ions (2). These ions decompose by elimination of X (for X=CO) or with formation of RMnX+, but further rearrangements can also occur. These are the reverse migration of R from the metal atom to the π-ligand (for R=H, Ph) and cyclopentadienyl ring expansion (for X=CH2). Collisional activation mass spectra contained an Mn+ ion peak, which can indicate the existence of stable type 1 structures for most cyclopentadienylmanganese ions. Carboxyl and hydroxymethyl derivatives exist, presumably as ions of type 2. The neutralization-reionization mass spectra of RXC5H4Mn+ ions are also discussed.

The generation of low-valence tin derivatives, RSn(I), in the gas-phase by neutralization-reionization mass spectrometry

Neutralization-reionization mass spectrometry (NRMS) was applied to the generation and characterization of low-valence Sn(I) derivatives. The observation of recovery signals in the NR mass spectra of RSn+ ions (R=H, Cl, Br, CH3, C2H, C6H5) demonstrated that their neutral counterparts are stable species in the gas-phase with a lifetime of at least 5 μs. According to quantum chemical calculations, a favorable Franck–Condon factor may contribute to the stability of RSn neutrals generated in the NR event. The experimental results for tin acetylide and phenyltin are the first examples of the generation of these previously unknown molecular species.