At what pH would the structure below be the predominant ionization state of Histidine? Consider the ionization state of all three ionizable functional groups in this amino acid.
The Correct Answer and Explanation is:
To determine the pH at which the predominant ionization state of histidine occurs, we need to consider the ionization states of its functional groups:
- Amino Group (-NH3+): This group typically ionizes at a pKa of around 9.0, meaning it is protonated and carries a positive charge (NH3+) at pH values below this value. Above pH 9.0, it will deprotonate to form NH2, losing its positive charge.
- Carboxyl Group (-COOH): The carboxyl group in histidine has a pKa around 1.8, meaning it will be protonated (COOH) at very low pH. As the pH rises above 1.8, the group deprotonates to form the carboxylate ion (-COO-), acquiring a negative charge.
- Imidazole Ring (Histidine’s Side Chain): The imidazole ring of histidine has a pKa around 6.0. This means at pH values below 6.0, the imidazole group will be protonated (positively charged), while at pH values above 6.0, it will be deprotonated (neutral).
Predominant Ionization State:
At a pH of around 7.0, the following occurs:
- The amino group will be protonated (NH3+).
- The carboxyl group will be deprotonated (-COO-).
- The imidazole ring will be mostly deprotonated (neutral), but can also have some protonated molecules, depending on the pH.
Thus, at pH 7.0, the neutral form of histidine, with the amino group protonated (+1), the carboxyl group deprotonated (-1), and the imidazole ring mostly neutral, would likely be the predominant ionization state.
In conclusion, the predominant ionization state of histidine is at a pH of approximately 7.0, where the amino group is positively charged, the carboxyl group is negatively charged, and the imidazole ring is neutral or weakly positively charged. This is the physiological pH at which histidine often resides in proteins, allowing the imidazole side chain to participate in hydrogen bonding and enzyme catalysis.
