Do thermally-labile 1,4-benzodiazepines rearrange under electrospray and particle bombardment?

Nordiazepam (N), diazepam (D), lorazepam (L), oxazepam (O) and temazepam (T) are 1,4-benzodiazepines. L, O and T are substituted in position 3 of the seven-membered ring by a hydroxyl group and are known to be thermally unstable. N and D are unsubstituted in position 3 and are expected to be thermally stable. We have studied the stability of all these molecules under electrospray conditions and under particle bombardment (MeV ions and UV photons). The fragmentations induced by low energy collision-induced dissociation and high energy collision-activated dissociation of molecules protonated by electrospray were compared with the spontaneous fragmentations of these molecules ionized by particle bombardment. The fragmentation mechanisms were determined using labeled compounds and by means of ab initio calculations using 1,4-diazepine and 3-hydroxy-1,4-diazepine as models. The fragmentation is dramatically dependent upon the substitution in position 3 and upon the internal energy of protonated molecules. At low collision energies, the non-hydroxylated benzodiazepines eliminate CO by opening of the diazepine ring whereas 3-hydroxy-1,4-benzodiazepines eliminate water after ring contraction. At high collision energies, all protonated benzodiazepines eliminate a hydrogen atom by simple bond cleavage. Molecular orbital calculations give arguments in favor of an isomerization in the gas phase of the protonated 3-hydroxybenzodiazepines and of a partial thermal decomposition of 1,4-benzodiazepines occurring before protonation under particle bombardment.