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 $p K_{\mathrm{a}}$ 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 part (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 pI of histidine. (d) The histidine group plays an important role in buffering blood (the pH of blood is about 7.4 ). Which conjugate acid-base pair shown in part (a) is responsible for maintaining the pH of blood?

Key Concepts

Conjugate Acid-Base Pair

A conjugate acid-base pair consists of two species that differ by a proton. The acid member can release a proton to form its conjugate base, while the base can accept a proton to revert to its acid form, making these pairs essential for maintaining pH through buffering mechanisms.

Buffering Capacity

Buffering capacity is the ability of a system to resist changes in pH when an acid or base is added. In biological systems, this function is achieved by conjugate acid-base pairs, which can donate or accept protons to help stabilize the pH within a narrow and optimal range.

Isoelectric Point (pI)

The isoelectric point is the specific pH at which an amino acid (or any amphoteric molecule) carries no net charge because the positive and negative charges are balanced. It is typically calculated as the average of the pKa values that flank the formation of the dipolar ion.

pKa

pKa is the negative logarithm of the acid dissociation constant, reflecting the strength of an acid. It indicates the pH at which half of the molecules of a given acidic group are deprotonated, serving as a critical parameter for predicting the ionization state of different functional groups in molecules like amino acids.

Stepwise Ionization

This concept refers to the sequential loss (or gain) of protons from a molecule's ionizable groups, each characterized by its own pKa value. In polyprotic acids like amino acids, the order of ionization is determined by the acidity of each group, with the strongest acid losing its proton first.

Dipolar Ion

A dipolar ion is a form of a molecule in which there is an internal balance of positive and negative charges, resulting in no net overall charge even though the molecule contains both cationic and anionic groups. This neutral yet internally charged state is key in amino acid chemistry.

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