What is the net charge of the tripeptide at pH 11.5?

Question

Consider the following pKa values for glycine, arginine, and lysine. What is the approximate net charge of the tripeptide Gly-Arg-Lys at pH 11.5?

Free amino acid pKa of alpha-carboxyl group pKa of alpha-amino group pKa of side chain

Glycine 2.4 9.8 ---

Arginine 1.8 9.0 12.5

Lysine 2.2 9.1 10.5

Answer 1

According to convention, amino groups is on the left and carboxyl group is on the right, so we have...

H2N-glycine - arg - lys-COOH

Glycine terminal:

H₃N⁺------9.8----NH₂ so @ pH 11.5 it will be NH₂ (no charge)

Lysine terminal:

COOH----2.2---COO^- so @ pH 11.5 it will be COO^- (-1 charge)

Lysine side chain:

H₃N⁺---10.5---NH₂ so @ pH 11.5 it will be NHJ₂ (no charge)

Arginine side chain:

H₂N⁺ ---12.5---NH so @ pH11.5 it will be NH(no charge)

Adding up all the charges we have...

0 + -1 + 0 + 0 = -1 net charge on the tripeptide