MALDI Application: Distinguishing Lysine and Glutamine in a Peptide
Lysine and glutamine are not easily distinguished by the most common approaches to peptide sequencing which involve mass spectrometers with low to moderate resolving power and low-energy collision-induced dissociation (CID). Lysine (C6H14N2O2 with a mass of 146.1055 u) and glutamine (C5H10N2O3 with a mass of 146.0691) differ by only 0.036 u. In this study, we demonstrate the measurement of a mixture of Substance P and a synthesized peptide (3-Gln ) with glutamine substituted for lysine in the Susbstance P sequence. Because the mass difference between Substance P and 3-Gln is 0.036 u, a resolving power of greater than 37,000 is required to separate each peptide. Additionally, we show that the TOF-TOF mode can be used to distinguish lysine and glutamine in these peptides by comparing the peak area ratio between a ions and d ions in the high-energy CID mass spectra.
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