Question

11.5. In a MS-MS fragmentation experiment you detect the following set of y-ions arising from a long peptide.

a) What is the partial peptide sequence you can determine from these data?

120.13

257.27

314.32

427.48

583.67

b) The terminal residue is observed to be 19 mass units larger than expected. Why?

Answer 1

a) Difference in the mass values gives us the mass of amino acids -

These masses are 18 units less than that of actual amino acid because when amino acids polymerize to form peptides, water is eliminated

120.13 - 0 = 120.13 - Threonine

257.27 - 120.13 = 137.14 - Histidine

314.32 - 257.27 = 57.05 - Glycine

427.48 - 314.32 = 113.16 - Leucine/Isoleucine

583.67 - 427.48 = 156.19 - Arginine

So, the sequence is Threonine-Histidine-Glycine-Leucine-Arginine or Threonine-Histidine-Glycine-Isoleucine-Arginine

b) The terminal residue is observed to be 19 units higher because in that case the complete CO2- (Acid) group is present so it does not lose water plus one extra hydrogen coming from formation of NH3+ . Thus, 19 units extra