Question

draw the titration curve for Met-Arg-Tyr-Val (pH vs ml HCl) and draw the form(s) found on the first plateau.

Answer 1

The peptide titration curves are known by their stair-like curve, each step (plateau) in the stair represents the pKa being surpassed by the changing pH. At pH near 0 we have an excess of protons in the medium, this means every acid/base will be protonated, the acids will be COOH and the bases will be NH3+, when we reach the pKa value of one of the acid/base groups we will lose the proton from such group, if it is an acid we will obtain COO- and if it is a base we will obtain NH2. All this process also occur vice versa, if we start by pH near 14 all the groups will be deprotonated, and each time we reach a pKa value we are going to protonate the group in question.

First let's identify the acid/base groups in each residue of our peptide:

- Met: Only the N terminal with a pKa of 9.21

- Arg: Only the R group witha pKa of 12.48

- Tyr: This one has no ionizing groups

- Val: Only the C terminal with a pKa of  2.32

Now, let's order them from the most alkaline to the most acidic:

pKa1: 12.48

pKa2: 9.21

pKa3: 2.32

Let us draw our graph with a plateau at such pH values:

Now, we have to draw the peptide ionization form found in the first plateau, the one in pH 12.48. In this case every group will be deprotonated except the R group from arginine:

From left to right, the amino acids have the same sequence as they were given in the exexrcise, note that every amino and carboxyl group is deprotonated, except the R group of arginine (the only NH3+)