In order to analyze peptides by using mass spectrometry, they need to be turned

Question

In order to analyze peptides by using mass spectrometry, they need to be turned into ions. Instead of making the radical cation, M^+, like is commonly done, it is easier to protonate them, to make an [M+H]^+. An important question that needs to be considered is, where is the proton? That information can be used to help determine the amino acids that are present in the peptide. One thing that has been found is that if the peptide contains the amino acids arginine (arg), histidine (his) or lysine (lys), the proton ends up on that amino acid (arg, his or lys). Explain why arg, his and lys in peptides are more basic than other amino acids. Suppose a peptide contains a combination of arg, his and/or lys. Rank the three amino acids according to their basicity, and explain why they have the order they do.

Explanation / Answer

These three amino acids are highly basic because of they have basic side chains. They are all protonated at neutral pH.

b) basiciry order is Arginine>lycine>histidine

Arigine has pK value of 12, so it is highly basic. next comes lycine then histidine. At pH 7, lycine and Argine is protonated completely but histidine not.

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