Based on the titration curve of an unknown amino acid, shown below, can the side chain of the amino acid readily exchange protons with water?

Question

Quizplus · Accepted Answer

11ecba2d_0e69_2890_a3bf_3d1722ba5610_MD0008_00 Acidic and basic chemical groups, also known as ionizable groups, have intrinsic tendencies to exchange protons with water. This tendency is described by the pK_a, or the pH at which half of the ionizable groups of interest are protonated. pK_a values can be determined by titration, which is typically performed by gradually adding a base to an acidic solution of the molecule of interest. As the pH of the solution approaches the pK_a of a chemical group, the system acts as a buffer and resists changes in pH by accepting protons from water or donating protons to it. On a titration curve, the region near a pK_a (called the buffering region) is relatively flat because the addition of base changes the pH by only a small amount. All amino acids have ionizable carboxyl and amino groups with pK_a values near 2 and 9.5, respectively. Therefore, all amino acid titration curves have two buffering regions, one near each pK_a. Seven of the standard amino acids (R, K, Y, C, H, E, and D) also have ionizable side chains, each with a unique pK_a. The titration curve for each of these amino acids has a third buffering region associated with the side chain pK_a. Because the titration curve in the question shows only two buffering regions, the amino acid has only two pK_a values. Therefore, its side chain is not ionizable and cannot exchange protons with water. (Choices A and B) The titration curve corresponds to an amino acid with a non-ionizable side chain, which cannot exchange protons with water. It has only two equivalence points and two buffering regions. (Choice C) The final pH of a titration is determined by the pH of the base (or acid) that is gradually added during the titration. It is not related to the molecule that is titrated. Educational objective: All amino acids have at least two pK_a values that correspond to their carboxyl and amino groups. Amino acids with ionizable side groups (R, K, Y, C, H, E, and D) have three pK_a values. Titration curves show the region surrounding each pK_a as relatively flat portions of the curve.

Based on the Titration Curve of an Unknown Amino Acid