Acidic proteins and nucleic acids such as RNA are most readily ionized in
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deprotonated protein or RNA ions can be used as precursors in top-down mass
spectrometry. Because the performance of the dissociation method used critically
depends on precursor ion negative net charge, it is important that the extent of
charging in ESI can be manipulated efficiently. We show here that (M –
nH)n– ion net charge of
proteins and RNA can be controlled efficiently by the addition of organic bases
to the electrosprayed solution. Our study also highlights the fact that ion
formation in ESI in negative mode is only poorly understood.
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