Question

​​​​​​​Mass spectrometry is a very accurate means of determining the mass and sequence of peptides, however some inconsistencies occur due to incomplete reactions and similar structures.

a. Name two amino acids whose mass can’t be distinguished by mass spectrometry.

b. If a bond between Ser and Val is not hydrolyzed, what amino acid could be mistakenly identified?

c. If a bond between two Gly residues is not hydrolyzed, what amino acid could be mistakenly identified?

Answer 1

a.

Answer: Leucine and isoleucine

Leucine and isoleucine are isomers of each other. This means that they have different structures but have the same molecular formula and thus the same molecular mass. Since mass spectrometry differentiates on the basis of m/z ratio, thus these two amino acids cannot be differentiated.

b.

Answer: Tryptophan

When a serine and a valine are bonded via peptide bond, the molecular mass of this dipeptide is same as that of Tryptophan.

Calculation:

When a peptide bond is formed, there is elimination of water. Thus we have to subtract the molecular mass of a water molecule = 18

Residue mass of Serine = 87

Residue mass of Valine = 99

Residue mass of Serine-Valine = 87 + 99 = 186

Residue mass of tryptophan = 186

Thus, it is clear that the moleculas mass of tryptophan and the dipeptide are equal. Thus, they cannot be differentiated with mass spectrometry unless the dipeptide of Serine and Valine is hydrolyzed.

c.

Answer: Asparagine

When two Glycine are bonded via peptide bond, the molecular mass of this dipeptide is same as that of Asparagine.

Calculation:

When a peptide bond is formed, there is elimination of water. Thus we have to subtract the molecular mass of a water molecule (18) from the molecular mass of each amino acid to get the residue mass.

Residue mass of Glycine = 57

Residue mass of Glycine-Glycine = 57 + 57 = 114

Residue mass of Asparagine = 114

Thus, it is clear that the moleculas mass of Asparagine and the dipeptide are equal. Thus, they cannot be differentiated with mass spectrometry unless the dipeptide of Glycine-Glycine is hydrolyzed.

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