Comparing the 𝐩 K𝐚 Values of Alanine and Polyalanine The...

Comparing the $\mathbf{p} K_{\mathbf{a}}$ Values of Alanine and Polyalanine The titration curve of alanine shows the ionization of two functional groups with $\mathrm{p} K_{\mathrm{a}}$ values of 2.34 and 9.69 corresponding to the ionization of the carboxyl and the protonated amino groups, respectively. The titration of di-, tri-, and larger oligopeptides of alanine also shows the ionization of only two functional groups, although the experimental $\mathrm{p} K_{\mathrm{a}}$ values are different. The trend in $\mathrm{p} K_{\mathrm{a}}$ values is summarized in the table. $$\begin{array}{lr}\text { Amino acid or peptidep } \boldsymbol{K}_{\mathbf{1}} \mathbf{p} \boldsymbol{K}_{\mathbf{2}} \\ \hline \text { Ala } & 2.349 .69 \\\text { Ala-Ala } & 3.128 .30 \\\text { Ala-Ala-Ala } & 3.398 .03 \\ \text { Ala- (Ala) }_{n^{-}} \text {Ala, } n \geq 4 & 3.427 .94 \\\hline\end{array}$$ (a) Draw the structure of Ala-Ala-Ala. Identify the functional groups associated with $\mathrm{p} K_{1}$ and $\mathrm{p} K_{2}$ (b) Why does the value of $\mathrm{p} K_{1}$ increase with each additional Ala residue in the oligopeptide? (c) Why does the value of $\mathrm{p} K_{2}$ decrease with each additional Ala residue in the oligopeptide?

Comparing the $\mathbf{p} K_{\mathbf{a}}$ Values of Alanine and Polyalanine The titration curve of alanine shows the ionization of two functional groups with $\mathrm{p} K_{\mathrm{a}}$ values of 2.34 and 9.69 corresponding to the ionization of the carboxyl and the protonated amino groups, respectively. The titration of di-, tri-, and larger oligopeptides of alanine also shows the ionization of only two functional groups, although the experimental $\mathrm{p} K_{\mathrm{a}}$ values are different. The trend in $\mathrm{p} K_{\mathrm{a}}$ values is summarized in the table.
$$\begin{array}{lr}\text { Amino acid or peptidep } \boldsymbol{K}_{\mathbf{1}} \mathbf{p} \boldsymbol{K}_{\mathbf{2}} \\
\hline \text { Ala } & 2.349 .69 \\\text { Ala-Ala } & 3.128 .30 \\\text { Ala-Ala-Ala } & 3.398 .03 \\
\text { Ala- (Ala) }_{n^{-}} \text {Ala, } n \geq 4 & 3.427 .94 \\\hline\end{array}$$
(a) Draw the structure of Ala-Ala-Ala. Identify the functional groups associated with $\mathrm{p} K_{1}$ and $\mathrm{p} K_{2}$
(b) Why does the value of $\mathrm{p} K_{1}$ increase with each additional Ala residue in the oligopeptide?
(c) Why does the value of $\mathrm{p} K_{2}$ decrease with each additional Ala residue in the oligopeptide?

Key Concepts

pKa and Acid-Base Equilibria

pKa is the negative logarithm of the acid dissociation constant (Ka) and is a measure of the strength of an acid in solution. In the context of peptides, pKa values indicate the tendency of terminal and side chain functional groups to donate or accept protons. These values can shift depending on molecular context, such as peptide length and the local environment of the ionizable groups.

Inductive and Electrostatic Effects in Peptides

This concept explains how the electronic environment of ionizable groups is influenced by adjacent atoms or groups in a molecule. When additional residues are added to a peptide chain, the local electron distribution can change due to inductive effects from nearby groups, leading to shifts in pKa values. These effects explain why the pKa of a terminal group may increase or decrease as the peptide is elongated, as electron-withdrawing or -donating effects alter the stability of the ionized form.

Functional Group Ionization

This refers to the ability of specific groups within a molecule to lose or gain protons (H+), which is characterized by their pKa values. In amino acids and peptides, the ionizable groups typically include the carboxyl and amino groups, each contributing uniquely to the acid-base behavior of the molecule. Understanding which group is ionizing at which pKa value is crucial for predicting the overall charge and behavior of the molecule during titration.

Amino Acid Structure and Peptide Bonds

This concept involves understanding the basic structure of amino acids and how they are linked by peptide bonds to form peptides and proteins. It includes recognizing the backbone elements common to all peptides, such as the amino (–NH3+) and carboxyl (–COOH) groups, and how these groups react during polymerization to form the peptide chain with terminal functional groups that can ionize.

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