Practice question

Aromatic amino acids exhibit larger masses due to phenyl rings and contribute distinctive residues in peptide spectra. Phenylalanine molecular weight is 165.07 Da, residue mass after dehydration is 147.07 Da after subtracting water. Tyrosine residue mass is 163 Da, tryptophan 186 Da, histidine 137 Da. Observing mass difference of 147 Da between successive fragment ions indicates phenylalanine at that position during de novo sequencing. High-resolution mass spectrometry distinguishes phenylalanine from oxidized methionine which is isobaric near 147.03 Da, requiring accurate mass measurement for confident assignment in proteomics, critical for protein identification accuracy.