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

First, we will identify the ionizable groups in each amino acid. By doing that, we can determine which amino acids have the potential to contribute positively or negatively to the overall net charge of the peptide. (a) Gly-Ser-Lys - Glycine (Gly, G): amino group (NH2), carboxyl group (COOH) - Serine (Ser, S): amino group (NH2), carboxyl group (COOH) - Lysine (Lys, K): amino group (NH2), carboxyl group (COOH), and side chain group (NH3+) (b) Pro-Leu-Ile - Proline (Pro, P): amino group (NH2), carboxyl group (COOH) - Leucine (Leu, L): amino group (NH2), carboxyl group (COOH) - Isoleucine (Ile, I): amino group (NH2), carboxyl group (COOH) (c) Phe-Tyr-Asp - Phenylalanine (Phe, F): amino group (NH2), carboxyl group (COOH) - Tyrosine (Tyr, Y): amino group (NH2), carboxyl group (COOH) - Aspartic acid (Asp, D): amino group (NH2), carboxyl group (COOH), and side chain group (COOH)

Now, we will determine the charges of the ionizable groups based on their pKa values. The pKa values of the common ionizable groups are approximately as follows: - NH2 (amino group): pKa ~ 9.0 - COOH (carboxyl group): pKa ~ 2.0 - Lys side chain group (NH3+): pKa ~ 10.5 - Asp side chain group (COOH): pKa ~ 3.9 Using the pKa values, we can estimate the charge of the ionizable groups at pH 7. If the pH is less than the pKa value, the ionizable group has a proton and keeps its positive charge. If the pH is greater than the pKa value, the group is deprotonated and acquires a negative charge. - At pH 7, the amino group (NH2) has a positive charge: pH < pKa (7 < 9.0) - At pH 7, the carboxyl group (COOH) has a negative charge: pH > pKa (7 > 2.0) - At pH 7, Lys side chain group (NH3+) has a positive charge: pH < pKa (7 < 10.5) - At pH 7, Asp side chain group (COOH) has a negative charge: pH > pKa (7 > 3.9)

Now, we will calculate the net charges of each peptide based on the charges of the ionizable groups: (a) Gly-Ser-Lys - Net charge = Gly_amino (+) + Gly_carboxyl (-) + Ser_amino (+) + Ser_carboxyl (-) + Lys_amino (+) + Lys_carboxyl (-) + Lys_sidechain (+) - Net charge = +1 - 1 + 1 - 1 + 1 - 1 + 1 - Net charge = +1 (b) Pro-Leu-Ile - Net charge = Pro_amino (+) + Pro_carboxyl (-) + Leu_amino (+) + Leu_carboxyl (-) + Ile_amino (+) + Ile_carboxyl (-) - Net charge = +1 - 1 + 1 - 1 + 1 - 1 - Net charge = 0 (c) Phe-Tyr-Asp - Net charge = Phe_amino (+) + Phe_carboxyl (-) + Tyr_amino (+) + Tyr_carboxyl (-) + Asp_amino (+) + Asp_carboxyl (-) + Asp_sidechain (-) - Net charge = +1 - 1 + 1 - 1 + 1 - 1 - 1 - Net charge = -1

Based on our calculations, peptide (a) Gly-Ser-Lys has a net positive charge of +1 at pH 7. The other peptides, (b) Pro-Leu-Ile and (c) Phe-Tyr-Asp, have net charges of 0 and -1, respectively. So, the peptide with a net positive charge at pH 7 is: (a) Gly-Ser-Lys.