What is the net charge of the tripeptide PEW at a pH of 8?
a) -2 b) -1 c) 0 d) +1 e) +2
The correct answer and explanation is:
The net charge of the tripeptide PEW at pH 8 is -1.
To determine the net charge, we need to look at the ionizable groups on the amino acids and their pKa values. The tripeptide PEW consists of the following amino acids: Proline (P), Glutamic acid (E), and Tryptophan (W).
- Proline (P): Proline is a non-polar amino acid, and its side chain is not ionizable. Therefore, it does not contribute to the charge at any pH.
- Glutamic acid (E): Glutamic acid has a carboxyl group (-COOH) in its side chain that can deprotonate to become a carboxylate ion (-COO⁻). The pKa of the carboxyl group in glutamic acid is around 4.2. At pH 8, which is above the pKa of glutamic acid, the carboxyl group will be deprotonated, giving it a negative charge (-1).
- Tryptophan (W): Tryptophan is an aromatic amino acid, and its side chain does not ionize in the pH range of 8. It does not have a functional group that contributes to the charge at this pH.
The amino group at the N-terminus and the carboxyl group at the C-terminus will also influence the charge of the peptide:
- Amino group (N-terminus): The pKa of the amino group is around 9. At pH 8, the amino group will be partially protonated (NH₃⁺), contributing a +1 charge.
- Carboxyl group (C-terminus): The pKa of the carboxyl group is around 3.2. At pH 8, the carboxyl group will be deprotonated, contributing a -1 charge.
Now, summing the charges:
- Glutamic acid contributes -1.
- Tryptophan contributes 0.
- Proline contributes 0.
- The N-terminal amino group contributes +1.
- The C-terminal carboxyl group contributes -1.
Therefore, the net charge of the tripeptide PEW at pH 8 is:
+1 (from the N-terminus) + (-1 from glutamic acid) + (-1 from the C-terminus) = -1.
So the correct answer is b) -1.