Definitions: Peptide, modified peptide and precursor

Definitions: Peptide, modified peptide and precursor

In bottom‑up proteomics peptides are analyzed in a mass spectrometer, usually to infer information about the proteins they come from. When you analyze your mass spectrometry results in any of Biognosys' software, you get information for different elements at different levels, related to the detected peptides (Table 1).

  1. Peptide (stripped sequence): refers to the amino acid sequence regardless of charge state or modification of the analyzed peptide. Peptide level is mainly used for protein inference.
  2. Modified peptide: reports information about the posttranslational modified status of peptides. In our reports, this appears with the additional mass (if any) added to the amino acid residue in square brackets.
  3. Precursor: refers to the actual unique molecular unit being analyzed in the mass spectrometer. Precursor level contains information about the sequence, the modification status, and the charge state. Ultimately, fragment ions are produced from these precursors. 


Following this rationale, the number of precursors will always be equal or higher than the number of modified peptides, and the latter equal or higher than the number of stripped sequences (or peptides, Table 1).



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