Question

Which of the following peptides have a net positive charge at pH 7? (a) Gly- Ser-Lys, (b) Pro-Leu-Ile, (c) PheTyr-Asp.

Short answer

At pH 7, the peptide Gly-Ser-Lys has a net positive charge of +1, while the peptides Pro-Leu-Ile and Phe-Tyr-Asp have a net charge of 0. Therefore, the peptide with a net positive charge at pH 7 is Gly-Ser-Lys.

Step-by-step solution

Identify the ionizable side chains in each peptide

First, we need to identify the ionizable side chains present in each peptide. Take a look at the side chains of each amino acid and determine if they can accept or donate a proton. (a) Gly-Ser-Lys: Gly (G) and Ser (S) do not have ionizable side chains, but Lys (K) does. (b) Pro-Leu-Ile: Pro (P), Leu (L), and Ile (I) do not have ionizable side chains. (c) Phe-Tyr-Asp: Phe (F) does not have an ionizable side chain, but Tyr (Y) and Asp (D) do.

Determine the pKa values of the ionizable side chains

Now that we have identified the ionizable side chains, we need to find their pKa values: Lys (K) - ε-amino group: pKa = 10.53 Tyr (Y) - phenolic group: pKa = 10.07 Asp (D) - carboxyl group: pKa = 3.65

Calculate the net charge of each peptide at pH 7

At pH 7, we can determine if the ionizable side chain will be protonated or deprotonated based on its pKa value: (a) Gly-Ser-Lys: Lys (K) has a pKa of 10.53, which is greater than 7, so its ε-amino group will be protonated, and the net charge of the peptide will be +1. (b) Pro-Leu-Ile: None of these amino acids have ionizable side chains, so the net charge of the peptide will be 0. (c) Phe-Tyr-Asp: Tyr (Y) has a pKa of 10.07, which is greater than 7, so its phenolic group will be protonated. Asp (D) has a pKa of 3.65, which is less than 7, so its carboxyl group will be deprotonated. The net charge of the peptide will be 0, as the positive and negative charges cancel out.

Identify the peptides with a net positive charge at pH 7

Considering the net charges calculated in Step 3, we can conclude that: (a) Gly-Ser-Lys has a net positive charge of +1 at pH 7. (b) Pro-Leu-Ile has a net charge of 0 at pH 7. (c) Phe-Tyr-Asp has a net charge of 0 at pH 7. So, the peptide with a net positive charge at pH 7 is Gly-Ser-Lys.