Reductive Degradation of O -Acyl Oximes under Fast-Atom Bombardment Conditions

A degradation phenomenon of O-acyl oximes under fast-atom bombardment (FAB) conditions is described from the point of view of reactivity with the liquid matrix and irradiation time with the xenon neutral beam. Mass spectra of O-acyl oximes were compared with those of the free oxime and hydroxylamine as model compounds, by using two different matrices, m-nitrobenzyl alcohol and DTT/TG12 (a 1:2 mixture of dithiothreitol and thioglycerol). A characteristic fragment ion was observed in the FAB mass spectra of O-acyl oximes and a free oxime only when DTT/TG12 was used. Collision-activated dissociation (CAD) was used to search for the origin and to confirm the structure of the characteristic fragment ion. The CAD spectra of the molecular-related ions, [M – H]⁺,M^+• and [M + H]⁺, demonstrated that this characteristic fragment ion was not derived from any of these precursor ions. The CAD spectra of the characteristic fragment ion, which originated from O-acyl oximes, were very similar to those from free oximes and these results indicated that the fragment ion had the same structure. It was concluded that the degradation, under FAB conditions, was caused by a reductive reaction of the oxime moiety (–O–N=C) in O-acyl oximes regardless of its acyl moiety and that the degradation was initiated by the thiol group(s) in the matrix. The abundance of the ion originating from reductive degradation increased with increasing irradiation time with the xenon neutral beam.