Question

A typical amino acid with one amino group and one carboxylic acid group, such as serine (Figure 24.18 ), can exist in water in several ionic forms. (a) Suggest the forms of the amino acid at low \(\mathrm{pH}\) and at high \(\mathrm{pH}\). (b) Amino acids generally have two \(\mathrm{p} K_{a}\) values, one in the range of 2 to 3 and the other in the range of 9 to \(10 .\) Serine, for example, has \(\mathrm{p} K_{a}\) values of 2.19 and \(9.21 .\) Using species such as acetic acid and ammonia as models, suggest the origin of the two \(\mathrm{p} K_{a}\) values. (c) Glutamic acid is an amino acid that has three \(\mathrm{p} K_{a}\) 's: 2.10 , 4.07, and 9.47. Draw the structure of glutamic acid, and assign each \(\mathrm{p} K_{a}\) to the appropriate part of the molecule.

Short answer

At low pH, the amino acid primarily exists as a protonated species, with the amino group having a positive charge (-NH3+) and the carboxylic acid group remaining in its acidic form (-COOH). At high pH, the carboxylic acid group loses a proton and becomes deprotonated (-COO-), while the amino group remains protonated (-NH3+). The two pKa values correspond to the carboxylic acid group (2.19) and amino group (9.21). Glutamic acid has three pKa values, with the first being related to the main carboxyl group (2.10), the second to the side chain carboxylate group (4.07), and the third to the amino group (9.47).

Step-by-step solution

a) Suggesting the forms of the amino acid at low and high pH levels

An amino acid contains an amino group (-NH2) and a carboxylic acid group (-COOH). In an aqueous environment, the amino group can accept a proton and form an ammonium ion (-NH3+), and the carboxylic acid group can lose a proton and form a carboxylate ion (-COO-). At low pH (acidic conditions), amino acids are primarily protonated, meaning that the amino group will carry a positive charge (-NH3+), and the carboxylic acid group will not be deprotonated (-COOH). At high pH (basic conditions), amino acids are primarily deprotonated. The carboxylic acid group will lose its proton and form a carboxylate ion (-COO-), while the amino group will remain protonated due to its lower pKa (-NH3+).

b) Suggesting the origin of the two pKa values of amino acids

Amino acids generally have two pKa values: 1. One is in the range of 2 to 3 and is attributed to the carboxylic acid group (-COOH). This pKa arises from the dissociation of the hydrogen (proton) from the carboxylic acid group as a result of an acidic environment. This process forms a carboxylate ion (-COO-). 2. The other is in the range of 9 to 10 and is attributed to the amino group (-NH2). This pKa arises from the protonation of the amino group due to a basic environment. In this process, the amino group acts as a proton acceptor and forms an ammonium ion (-NH3+).

c) Drawing the structure of glutamic acid, and assigning each pKa to the appropriate part of the molecule

Glutamic acid has the following structure: \[HOOC-CH(NH2)-CH2-CH2-COOH\] It has three pKa values: 2.10, 4.07, and 9.47. 1. The first pKa value (2.10) is related to the carboxyl group (-COOH) at the end of the molecule. This pKa arises from the dissociation of the hydrogen (proton) from the carboxylic acid group, forming a carboxylate ion (-COO-). 2. The second pKa value (4.07) is due to the side chain carboxylate group (-COOH) present in the molecule. The proton dissociates from this group, forming -COO-. 3. The third pKa value (9.47) corresponds to the amino group (-NH2) at the other end of the molecule. The amino group acts as a proton acceptor in a basic environment, forming an ammonium ion (-NH3+).

Key concepts

Amino Acid Structure

Amino acids are the building blocks of proteins, and understanding their structure is crucial for unlocking their biological functions.

Each amino acid consists of a central carbon atom, known as the \( \alpha \) carbon, to which four different groups are attached: a hydrogen atom (H), a carboxyl group (\( -COOH \)), an amino group (\( -NH_2 \)), and a variable side chain represented as \( -R \). The \( -R \)-group, also called the side chain, is unique to each amino acid and determines its properties and role within proteins.

The \( \alpha \) carbon's tetrahedral geometry allows amino acids to exist in two mirror-image forms, referred to as L- and D-isomers. In nature, only L-amino acids are commonly found in proteins, showing the biological preference for this configuration.

Acid-Base Properties of Amino Acids

Amino acids are amphiprotic; they can donate or accept protons, acting as acids or bases, respectively. These acid-base properties are crucial for their behavior in different pH environments, and they are described by their \( pK_a \) values.

The \( pK_a \) value represents the pH at which half of the species is deprotonated (loses a hydrogen ion) and half is protonated (gains a hydrogen ion). For the carboxyl group (\( -COOH \)), a \( pK_a \) value around 2 to 3 indicates it’s the first to lose a proton under increasing pH levels. Meanwhile, the amino group (\( -NH_2 \)) typically has a higher \( pK_a \) value of 9 to 10, reflecting its tendency to accept a proton under decreasing pH levels.

In aqueous solutions, amino acids can exist as zwitterions, where the carboxyl group is deprotonated (\( -COO^- \)), and the amino group is protonated (\( -NH_3^+ \)), giving the molecule both positive and negative charges. This dual-natured existence plays a significant role in the overall charge of the molecule and affects its solubility, absorption, and reactivity.

Amino Acid Ionization States

The ionization states of amino acids are influenced by the pH of their environment. These states are significant as they impact the amino acids' functional capacity, particularly in enzyme catalysis and protein folding.

At a low pH (acidic environment), amino acids exist mainly in their cationic form, with the amino group accepting an extra proton to become \( -NH_3^+ \) and the carboxyl group remaining as \( -COOH \). Conversely, at high pH (alkaline environment), the carboxyl group releases a proton to form a carboxylate ion (\( -COO^- \)), while the amino group tends to remain as \( -NH_3^+ \) due to its higher \( pK_a \) value.

Each amino acid has a characteristic isoelectric point (pI), where it has no net charge. This is calculated by averaging the \( pK_a \) values of the carboxyl and amino groups. Understanding these ionization states is paramount for predicting amino acid behavior in different conditions, which is essential for practical applications like protein purification and drug design.