Mass spectrometry studies of peptides cationized by trivalent metal ions

The field of proteomics is dedicated to understanding how a protein’s structure and function relates to human health and disease. Peptide sequencing by mass spectrometry is important to the proteomics movement. Unfortunately, sequencing of many peptides and proteins, such as those with residues containing acidic and neutral side chains, can be difficult. Acidic side chains undergo facile deprotonation that make analysis challenging and can hinder formation of positive mode ions. New methods of sample preparation and dissociation techniques are needed to increase sequence information. This dissertation includes an extensive study of the effects on electron transfer dissociation (ETD) mass spectrometry of biological and model acidic non-phosphorylated and phosphorylated peptides adducted to trivalent lanthanide metal cations. Mass spectra contained herein provide abundant information about the primary structure of peptides. The ETD process requires multiply positively charged ions that can be difficult to obtain with acidic peptides. This work demonstrates that addition of trivalent lanthanide metal cations allows highly acidic peptides to be analyzed by ETD by forming multiply positively charged precursor ions by electrospray ionization (ESI). Using trivalent lanthanide cations as ionizing agents yields extensive sequence information for highly acidic peptides including definitive identification of phosphorylation sites. Peptides forming [M + Met + H]4+ and [M + Met]3+ generate full sequence coverage in many cases, but [M + Pr – H]2+ generates less sequence coverage. (Met is the trivalent metal cation.) The spectra contain primarily a mix of non-metallated and metal adducted c- and z- ions. All metallated product ions incorporate at least two acidic sites or a highly acidic phosphoresidue, which strongly suggests that the trivalent metal cation coordinates with residues that contain highly acidic side chains. All trivalent lanthanide cations are suitable for sequencing highly acidic peptides except europium and radioactive promethium. ETD spectra contain high signal-to-noise ratios making identification of product ions straightforward. Sequence coverage generally improves with increasing peptide chain length. Trivalent chromium enhances protonation of neutral peptides, which is important to ETD analysis. ESI conditions, particularly drying and nebulizing gas pressures are critical to formation of [M + 2H]2+ by neutral peptides.