Solvent effects in tandem mass spectrometry: mechanistic studies indicating how a change in solvent conditions and pH can dramatically alter CID spectra.

Dramatically different CID (collision-induced dissociation) spectra are obtained when the complex [Zn(dien-glucose)](+) is electrosprayed from acidic and basic solutions. To understand this peculiar phenomenon, an in-depth mechanistic study was performed on one of the product ions that is present when the initial complex is diluted in basic solution but absent when the complex is diluted with acidic solution. On the basis of the results of this study, the differences in the CID spectra can be rationalized by the fact that the complex electrosprayed from basic solution was kinetically trapped, with the deprotonation site distal from the metal center. Under acidic conditions, the deprotonation site is at a hydroxyl group coordinated to the metal ion. A variety of experiments support this hypothesis. The studies herein underscore the importance of using identical solvent conditions when comparing sets of CID spectra. The data also highlight a very interesting phenomenon involving deprotonation of a hydroxyl group, which was several atoms away from the Zn(II) metal center.