Searching For Protein Modifications.
Protein modifications can also be explored by database searching. For instance, a phosphopeptide can be correlated to a peptide sequence in the database with an additional mass increment due to the phosphogroup (80 Da). The algorithm used for peptide sequence tag, allows detection of mass difference on either side of tag sequence. For instance, the tag sequence and C-terminus of the peptide could agree with the database entry, but the mass of fragment with the N-terminus could be larger by 80 Da. This would suggest that there is modification due to a phosphogroup, yielding a mass difference of 80 Da between the N-terminus of peptide and the start of the sequence tag.