The utility of charge-derivatized peptides has been a substantial development from this Facility, first in the context of FAB analyses, and more recently in the context of MALDI. It was unexpected that charge-localized peptides respond well in MALDI. The mechanism of ionization in this experiment is not well-understood, but is presumably similar to the mechanism through which other cations such as sodium ions are formed. As in the case of protonated peptides, charge-derivatized peptides form only a single peak in a MALDI mass spectrum using a linear time-of-flight mass spectrometer. However, their MS/MS spectra (Post-Source Decay mass spectra) are very different from those for protonated peptides. For many peptides, the charge-localized derivatives form predominantly a-type fragment ions. Sidechain losses are also observed for some residues, and the mechanism for their formation may be similar to those through which the a-type fragments are formed. The goal of this project is to understand the mechanisms through which fragment ions are formed from charge-derivatized spectra, such that sequence information can be extracted from the PSD mass spectra. Incorporation of continued MALDI advances, such as delayed extraction (DE) is also important for optimizing information obtained from these experiments.
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