Problem 4 (12 pts) The mass difference between Lysine (K) and Glutamine (Q) is only 0.0364 u. These residues are hardly distinguishable.

Problem 4 (12 pts)

The mass difference between Lysine (K) and Glutamine (Q) is only 0.0364 u. These residues are hardly distinguishable.

1) Amino groups including the e-amino group found in the side chain of K (but not found in Q) can be chemically modified by acetylation. In this reaction, an acetyl group (CH3- CO) replaces a single hydrogen atom in these amino groups.

Calculate the mass added to a peptide by each acetyl group. Show your calculation. (4 pts)

Atomic mass of H (1), C (12), O (16), N (14)

2) The sequence analysis of a peptide gave the following result: (2HN)-A-L-X1-P-X2-N-X3-X4-S-(COOH),

For the 4 residues labeled X1 to X4, it was not possible to discriminate between K and Q.

The m/z ratios before and after acetylation are the following: Before treatment: m/z = 507.8 (double-charged peptide) After treatment: m/z= 1181.6 (single-charged peptide)

Determine the number of K and Q residues in this peptide. Show your calculation. (6 pts)

3) Is it possible to find a K residue inside a tryptic peptide? Explain (2 pts)