Histidine is one of the 20 amino acids found in proteins....

Histidine is one of the 20 amino acids found in proteins. Shown here is a fully protonated histidine molecule where the numbers denote the $\mathrm{p} K_{\mathrm{a}}$ values of the acidic groups. (a) Show stepwise ionization of histidine in solution. (Hint: The $\mathrm{H}^{+}$ ion will first come off from the strongest acid group followed by the next strongest acid group and so on.) (b) A dipolar ion is one in which the species has an equal number of positive and negative charges. Identify the dipolar ion in (a). (c) The pH at which the dipolar ion predominates is called the isoelectric point, denoted by pI. The isoelectric point is the average of the $\mathrm{p} K_{\mathrm{a}}$ values leading to and following the formation of the dipolar ion. Calculate the pl of histidine. (d) The histidine group plays an important role in buffering blood (see Chemistry in Action on p. 732 ). Which conjugate acidbase pair shown in (a) is responsible for this action?

Key Concepts

Acid-Base Equilibria in Amino Acids

Amino acids contain both acidic (carboxyl) and basic (amino) functional groups, which can donate or accept protons, respectively. The equilibrium between different protonation states is governed by the acid dissociation constants (pKa values) of these groups, and these equilibria are crucial in determining the behavior of amino acids in various pH environments.

Stepwise Ionization

Stepwise ionization refers to the sequential loss or gain of protons from an amino acid in solution as the pH changes. Each functional group in an amino acid has its own pKa, and the ionization occurs in the order of the groups’ acid strengths—starting with the group having the lowest pKa (the strongest acid) followed by the next, and so on. Understanding this process is critical when analyzing the overall charge of the molecule at different pH values.

Dipolar Ion

A dipolar ion, also known as a zwitterion, is a species that contains both positive and negative charges while having an overall neutral charge. In amino acids, this state typically occurs when the amino group is protonated and the carboxyl group is deprotonated. The dipolar form plays a special role in biological systems due to its unique solubility and reactivity properties.

Isoelectric Point (pI)

The isoelectric point is the pH at which an amino acid or protein carries no net electric charge, due to the balance of positive and negative charges. It is usually calculated as the average of the pKa values of the groups involved in forming the neutral, dipolar ion. This property is important for understanding protein behavior in electrophoresis and other separation techniques.

Buffering Action of Amino Acids

Buffering action in amino acids involves the ability to resist pH changes by means of the acid-base equilibrium of their ionizable groups. In biological systems such as blood, specific amino acid residues (acting as conjugate acid-base pairs) can accept or donate protons, thereby stabilizing the pH. This buffering mechanism is essential for maintaining the proper physiological conditions necessary for various metabolic processes.

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