Assuming pKa values of 3.8 for the a-COOH and 8.5 for the a-amino groups and that the numbers in parentheses indicate the pKa values of side chain groups, at what pH would electrophoresis provide the best resolution of the following dipeptides?
His (6.0) – Ser
Gly – Lys (10.0)
I think I'm missing the theory behind this. If the side chain and the pH are both acidic, does that mean it's better?
To determine the pH at which electrophoresis would provide the best resolution for the given dipeptides, we need to consider the charges on the amino acid residues in each dipeptide at different pH values.
In electrophoresis, charged molecules migrate towards electrodes based on their charge and pH of the solution. The migration speed depends on the net charge of the molecule, which in turn depends on the pKa values of the functional groups in the amino acids and the pH of the solution.
In the case of dipeptides, the resolution depends on whether the charge on the dipeptide is greater or smaller than the charge on the buffer at a particular pH. For best resolution, we want the charges to be most different.
Now let's consider the individual components of each dipeptide:
1. His (6.0) – Ser:
- Histidine (His) has a side chain (imadazole group) with a pKa value of 6.0. At pH lower than 6.0, the imidazole group will be positively charged (protonated), and above pH 6.0, it will be neutral (deprotonated). This means that His will be positively charged below pH 6.0 and neutral above pH 6.0.
- Serine (Ser) does not have any charged side chains and remains uncharged at all pH values.
2. Gly – Lys (10.0):
- Glycine (Gly) does not have any charged side chains and remains uncharged at all pH values.
- Lysine (Lys) has a side chain (amino group) with a pKa value of 10.0. At pH lower than 10.0, the amino group will be positively charged (protonated), and above pH 10.0, it will be neutral (deprotonated). This means that Lys will be positively charged below pH 10.0 and neutral above pH 10.0.
To analyze resolution, we need to compare the charge states of the two dipeptides at different pH values. For best resolution, we want the two dipeptides to have the most different charge states.
In the case of His-Ser, His will be positively charged below pH 6.0, while Ser remains uncharged at all pH values. This is the only charged state in this dipeptide, so it will have its highest resolution at pH values lower than 6.0.
In the case of Gly-Lys, Gly is uncharged at all pH values, while Lys will be positively charged below pH 10.0. This means that the dipeptide will have its highest resolution at pH values higher than 10.0.
Therefore, electrophoresis will provide the best resolution for His-Ser at pH values lower than 6.0 and for Gly-Lys at pH values higher than 10.0.