Question

Predict the predominant ionized forms of the following amino acids at pH 7 : glutamic acid, leucine, threonine, histidine, and arginine.

Short answer

Glutamic acid: -COO⁻ (side chain), Leucine: -NH₃⁺, -COO⁻, Threonine: -NH₃⁺, -COO⁻, Histidine: neutral side chain, Arginine: protonated side chain.

Step-by-step solution

Determine the pKa values

Identify the pKa values of the ionizable groups (side chains, amino group, and carboxyl group) for each amino acid. For example, glutamic acid has a side chain pKa of approximately 4.1.

Compare pH with pKa values

Compare the given pH (7) with each pKa value to determine the ionization state. If pH > pKa, the group will be deprotonated. If pH < pKa, the group will be protonated.

Analyze Glutamic Acid

For glutamic acid, the pKa of the carboxyl side chain is 4.1, and pH 7 is greater than 4.1. Thus, the carboxyl side chain will be deprotonated (-COO⁻). The amino group (pKa ~ 9.7) will be protonated (-NH₃⁺).

Analyze Leucine

Leucine does not have an ionizable side chain, but the amino group (pKa ~ 9.6) will be protonated (-NH₃⁺), and the carboxyl group (pKa ~ 2.4) will be deprotonated (-COO⁻).

Analyze Threonine

Threonine has a hydroxyl group in its side chain, which isn’t ionizable at physiological pH. Therefore, its amino group will be protonated (-NH₃⁺) and the carboxyl group will be deprotonated (-COO⁻).

Analyze Histidine

Histidine has a side chain with a pKa around 6.0. At pH 7, which is higher than its pKa, the side chain will be mostly deprotonated (neutral). The amino group will be protonated (-NH₃⁺) and the carboxyl group will be deprotonated (-COO⁻).

Analyze Arginine

Arginine has a side chain with a pKa around 12.5, which is much higher than pH 7. Thus, the side chain will remain protonated (-NH₂⁺═C(NH₂)NH₂). The amino group will be protonated (-NH₃⁺) and the carboxyl group will be deprotonated (-COO⁻).

Key concepts

pKa values of amino acids

Amino acids have specific pKa values for their ionizable groups. These pKa values help us predict how the amino acids behave at different pH levels.
Each amino acid has an amino group (with a pKa around 9-10) and a carboxyl group (with a pKa around 2-3). Some amino acids also have ionizable side chains with their own distinct pKa values.
For example:

• Glutamic acid has a side chain with a pKa ~4.1.
• Histidine has a side chain with a pKa ~6.0.
• Arginine has a side chain with a pKa ~12.5.

These values are crucial for understanding how amino acids ionize in different environments. When the pH is below the pKa value, the group is protonated. When the pH is above the pKa value, the group is deprotonated.

Ionization state of amino acids

At a given pH, the ionization state of amino acids can be predicted using their pKa values. If the pH > pKa, the ionizable group will be deprotonated. If the pH < pKa, the group will be protonated.
For example, at pH 7:

• Glutamic acid: Its side chain (pKa ~4.1) will be deprotonated (-COO⁻) since 7 > 4.1.
• Leucine: No ionizable side chain, but its carboxyl group (pKa ~2.4) will be deprotonated (-COO⁻), and its amino group (pKa ~9.6) will be protonated (-NH₃⁺).

Thus, knowing the pKa values helps determine whether a group carries a charge. This is vital for understanding how amino acids interact and form structures in proteins.

Amino acid side chains

Side chains give amino acids their unique properties. Some can ionize depending on the pH, influencing the amino acid's overall charge and behavior.
Examples include:

• Histidine: Has an imidazole side chain with a pKa around 6.0, so it can be protonated or deprotonated around physiological pH. At pH 7, it is mostly deprotonated (neutral).
• Arginine: Has a guanidinium side chain with a high pKa (~12.5), keeping it protonated (-NH₂⁺═C(NH₂)NH₂) at pH 7.

The charge of these side chains affects how amino acids interact within proteins. You can predict these interactions better by knowing the pKa values and the pH of the environment.