Ion Ion Reactions
Ion Ion Reactions
Proteins are involved in nearly every aspect of cellular function. In fact, the characterization of proteins has become such a significant part of modern biology, it has inspired a new discipline: Proteomics – the classification of the protein complement expressed by the genome of an organism. Technology development has, and continues, to drive rapid evolution in this field. We use linear quadrupole ion trap mass spectrometers to perform ion/ion reactions – reactions of small-molecule anions with peptide/protein cations in the gas phase – for protein characterization. In general, these reactions can be classified in three categories:
(1) reactions that remove charge from the peptide (proton transfer),
(2) reactions that transfer an electron to the peptide (electron transfer dissociation), and
(3) reactions that result in the formation of a complex (anion attachment).
We study the first two of these reactions and use them - alone or in sequence - to identify and characterize proteins on a global-scale (proteomics).
Sequencing peptides
The electron transfer reaction results in the attachment of an electron to the protonated peptide. The odd-electron peptide then undergoes very rapid (femtosec) rearrangement with subsequent dissociation of the N – C bond. The process occurs efficiently and randomly across the entire sequence of peptides and whole proteins. The aim is to produce a collection of peptide fragment ions that differ in mass by a single amino acid, allowing one to read the amino acid sequence of the peptide. And, by repeating this sequence in a rapid, automated fashion (3- 4 analyses/sec), we can characterize several hundred peptides as they elute from a nanoflow HPLC separation column (~ 2 hour gradient). Most proteomics strategies use enzymes to cleave the analyte proteins into small peptides – peptides that are then sequenced with mass spectrometers. As an alternate method, we are pursuing the use of our technology to directly interrogate intact proteins in the gas-phase. By performing our experiment in the context of the whole protein, we can begin to elucidate important biological events such as global patterns of modification and protein alternative splicing events.
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