1.Histidine has three ionizable functional groups. Write the equilibrium equations for its three ion-izationsand assign the proper pKa for each ionization. Draw the structure of histidine in each ionization state.What is the net charge on the histidine molecule in each ionization state? 2.Which structure drawn in (1) corresponds to theionization state of histidine at pH 1, 4, 8, and12?Note that the ionization state can be approximated by treating each ionizable group independently. 3.What is the net charge of histidine at pH 1, 4, 8, and 12? For each pH, will histidine migrate to-ward the anode (+) or cathode (-) when placed in an electric field?
Ionization State of Histidine.Each ionizable group of an amino acid can exist in one of two states, charged or neutral. The electric charge on the functional group is determined by the relationship between its pKa and the pH of the solution. This relationship is described by the Henderson-Hasselbalch equation.
1.Histidine has three ionizable functional groups. Write the equilibrium equations for its three ion-izationsand assign the proper pKa for each ionization. Draw the structure of histidine in each ionization state.What is the net charge on the histidine molecule in each ionization state?
2.Which structure drawn in (1) corresponds to theionization state of histidine at pH 1, 4, 8, and12?Note that the ionization state can be approximated by treating each ionizable group independently.
3.What is the net charge of histidine at pH 1, 4, 8, and 12? For each pH, will histidine migrate to-ward the anode (+) or cathode (-) when placed in an electric field?
The Correct Answer and Explanation is :
- Ionization Equilibria of Histidine:
Histidine has three ionizable functional groups:
- The α-amino group (–NH₂)
- The α-carboxyl group (–COOH)
- The imidazole side chain (–C₃H₃N₂, a basic functional group)
Each of these groups has a specific pKa and undergoes ionization in solution.
Equilibrium equations and pKa values:
- α-amino group: The pKa for this group is around 9.2. The equilibrium equation is: [
\text{NH}_3^+ \rightleftharpoons \text{NH}_2 + \text{H}^+
] - α-carboxyl group: The pKa for this group is around 1.8. The equilibrium equation is: [
\text{COOH} \rightleftharpoons \text{COO}^- + \text{H}^+
] - Imidazole side chain: The pKa for the imidazole group is around 6.0. The equilibrium equation is: [
\text{C}_3\text{H}_4\text{N}_2 \rightleftharpoons \text{C}_3\text{H}_4\text{N}_2^+ + \text{H}^+
]
Ionization States and Net Charge:
- Neutral form: When histidine is neutral, the amino group is protonated (( \text{NH}_3^+ )), the carboxyl group is deprotonated (( \text{COO}^- )), and the imidazole group is neutral (( \text{C}_3\text{H}_4\text{N}_2 )).
- Net charge = ( 0 )
- Fully deprotonated: When the amino group, carboxyl group, and imidazole are all deprotonated.
- Net charge = ( -1 )
- Ionization States at Different pH Values:
- pH 1: At a very low pH (pH 1), the carboxyl group will be protonated as ( \text{COOH} ), the amino group will be protonated as ( \text{NH}_3^+ ), and the imidazole group will be protonated as ( \text{C}_3\text{H}_4\text{N}_2^+ ). Hence, the structure will have three positive charges.
- Net charge = ( +2 )
- pH 4: At pH 4, the carboxyl group will be deprotonated (( \text{COO}^- )), the amino group will be protonated (( \text{NH}_3^+ )), and the imidazole group will remain neutral.
- Net charge = ( +1 )
- pH 8: At pH 8, the amino group will be protonated (( \text{NH}_3^+ )), the carboxyl group will be deprotonated (( \text{COO}^- )), and the imidazole group will be neutral.
- Net charge = ( 0 )
- pH 12: At pH 12, both the carboxyl group and imidazole will be deprotonated, leaving the amino group as ( \text{NH}_2 ).
- Net charge = ( -1 )
- Migration in Electric Field:
- At pH 1, histidine will have a net positive charge and will migrate toward the cathode (-).
- At pH 4, histidine will have a net positive charge and will migrate toward the cathode (-).
- At pH 8, histidine will be neutral and will not migrate in the electric field.
- At pH 12, histidine will have a net negative charge and will migrate toward the anode (+).
This behavior is based on the general principle that positively charged species move toward the cathode, and negatively charged species move toward the anode when subjected to an electric field.