Successfully derivatized unmodified lysine residues with a glycine moiety as evidenced by the control reactions and the tandem MS spectra. The Gly tagged peptide has the same retention time and charge state as the GlyGly tagged peptide. The ionization efficiencies of the two tagged peptides are different but a linear relationship was determined which will allow for quantitation calculations as long as a correction factor is included. The Gly tag is sufficiently analogous to the GlyGly tag with respect to mass spectrometry for quantitation purposes providing the opportunity to use this technique to determine how much a protein of interest is ubiquitinated.
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