Question

Draw a structural formula for the form of each amino most prevalent at \(\mathrm{pH} \mathrm{} 10.0\). (a) Leucine (b) Valine (c) Proline (d) Aspartic acid

Short answer

Question: Determine the structural formula for the most prevalent form of each amino acid, Leucine, Valine, Proline, and Aspartic acid, at a pH of 10.0. Answer: (a) Leucine: HN-CH(CH2CH(CH3)2)COO- (b) Valine: HN-CH(CH(CH3)2)COO- (c) Proline: HN(CH2CH2-CH)-COO- (Note: The nitrogen atom of the amino group is part of the side-chain ring) (d) Aspartic acid: HN-CH(CH2COO-)COO-

Step-by-step solution

(Step 1: Identify Ionizable Groups of Each Amino Acid)

To properly draw the structures of the amino acids Leucine, Valine, Proline, and Aspartic acid, we need to identify the ionizable groups (amino, carboxyl, and any ionizable side chain) of each amino acid and their pKa values. (a) Leucine: Nonpolar aliphatic side chain with a pKa of 2.36 for the Carboxyl group, and 9.60 for the Amino group. (b) Valine: Nonpolar aliphatic side chain, pKa 2.32 for the Carboxyl group and 9.62 for the Amino group. (c) Proline: Nonpolar aliphatic side chain, pKa 1.99 for the Carboxyl group and 10.96 for the Amino group. (d) Aspartic acid: Polar acidic side chain, pKa 1.88 for the Carboxyl group, 9.60 for the Amino group, and 3.65 for the side chain.

(Step 2: Determine Ionization Status of Each Amino Acid at pH 10.0)

Compare the pH value of 10.0 to each amino acid's pKa values and determine if the acidic groups (carboxyl group and acidic side chain) will be ionized and if the basic groups (amino group) will be protonated. (a) Leucine: At pH 10.0, the carboxyl group (pKa = 2.36) will be ionized, and the amino group (pKa = 9.60) will not be protonated. (b) Valine: At pH 10.0, the carboxyl group (pKa = 2.32) will be ionized, and the amino group (pKa = 9.62) will not be protonated. (c) Proline: At pH 10.0, the carboxyl group (pKa = 1.99) will be ionized, and the amino group (pKa = 10.96) will be protonated. (d) Aspartic acid: At pH 10.0, the carboxyl group (pKa = 1.88) will be ionized, the amino group (pKa = 9.60) will not be protonated, and the side chain (pKa = 3.65) will be ionized.

(Step 3: Draw the Structural Formula for Each Amino Acid)

Finally, let's draw the structural formula for each amino acid considering their ionization status at pH 10.0. (a) Leucine: HN-CH(CH2CH(CH3)2)COO- (b) Valine: HN-CH(CH(CH3)2)COO- (c) Proline: HN(CH2CH2-CH)-COO- (Note: The nitrogen atom of the amino group is part of the side-chain ring) (d) Aspartic acid: HN-CH(CH2COO-)COO-

Key concepts

Ionizable Groups in Amino Acids

Amino acids, the building blocks of proteins, contain specific ionizable groups that can gain or lose protons (H⁺) in response to the pH of their environment. The core ionizable groups in all amino acids include the amino group (—NH₃⁺) and the carboxyl group (—COOH). Some amino acids also feature side chains with their own ionizable groups, such as the beta-carboxyl group in aspartic acid.

At low pH values, these groups tend to be protonated (carry extra H⁺ ions), while at high pH values, they become deprotonated (lose H⁺ ions). The presence of these ionizable groups contributes to the amino acid's ability to function in different roles within protein structures and biological processes. For example, aspartic acid can release or accept protons in enzymatic reactions thanks to its ionizable side chain.

Amino Acid Structure

The general structure of an amino acid consists of a central carbon atom known as the alpha (α) carbon, attached to four different groups: a hydrogen atom, an amino group (—NH₂ in its unprotonated state), a carboxyl group (—COOH), and a distinctive side chain (R-group) which determines the properties and classification of each amino acid.

For instance, leucine and valine have nonpolar aliphatic side chains, contributing to their hydrophobic character. Proline features a unique secondary amine where the R-group forms a cyclic structure with the amino nitrogen. Aspartic acid, on the other hand, has a polar acidic side chain which significantly affects its behavior at different pH levels.

pKa Values of Amino Acids

The pKa value of an amino acid is a measure of its tendency to either retain or release a proton. It indicates the pH at which the ionizable group is 50% ionized. Each ionizable group in an amino acid has a specific pKa value. At a pH below the pKa, the group tends to be protonated; above the pKa, it tends to be ionized.

For example, at pH 10.0, you can predict the ionization state of amino acids based on their pKa values. The carboxyl groups of leucine and valine have low pKa values (around 2.3), meaning they will be ionized in environments with a pH higher than their pKa values. In contrast, proline's amino group, with a pKa of 10.96, will remain protonated at pH 10.0. Understanding pKa values is crucial for predicting the behavior of amino acids in various pH conditions, which is important for biochemistry and molecular biology applications.