Practice question

In peptide mass spectrometry, residue mass equals amino acid monoisotopic molecular mass minus mass of water eliminated during peptide bond formation, 18.015 Da. Phenylalanine molecular mass is 165.19 Da; subtracting water yields residue mass approximately 147.18 Da. This value is distinct from tyrosine residue 163 Da, arginine 156 Da and methionine 131 Da, allowing unambiguous assignment. Characteristic phenylalanine immonium ion at m/z 120 further confirms presence. Knowledge of residue masses enables database searching, de novo sequencing and accurate interpretation of b- and y-ion series in tandem spectra from proteolytic digests.