In your tandem mass-spectrometry of a pure protein, you focused a fragment with an m/z of 1,268 through the process & into the second mass-spec and found y-ion peaks of 1,137 and 1,022. The mass in Daltons for the possible relevant amino acids are provided: Y (163), N (114), W (186), D (115), G (57), L (113) and M (131). What is the order of the first two amino acid residues in the 1,268 fragment from N-terminal to C-terminal?

• A. N-M.
• B. N-Y.
• C. M-D.
• D. M-L.
• E. D-N.

Answer: (C)

Verified step by step guidance

1. Understand the problem: You are given a fragment with an m/z of 1,268 and y-ion peaks of 1,137 and 1,022. You need to determine the order of the first two amino acid residues from N-terminal to C-terminal in this fragment.
2. Calculate the mass difference between the fragment and the first y-ion peak: Subtract the m/z of the y-ion peak (1,137) from the m/z of the fragment (1,268) to find the mass of the first amino acid residue.
3. Calculate the mass difference between the first and second y-ion peaks: Subtract the m/z of the second y-ion peak (1,022) from the m/z of the first y-ion peak (1,137) to find the mass of the second amino acid residue.
4. Match the calculated mass differences to the given amino acid masses: Compare the calculated mass differences to the provided masses of amino acids (Y, N, W, D, G, L, M) to identify the amino acids corresponding to each mass difference.
5. Determine the order of the amino acids: Based on the identified amino acids from the mass differences, determine the order from N-terminal to C-terminal in the fragment.