Question

Glutamic acid is another of the amino acids found in proteins (Chapter 27). Glutamic acid has two carboxyl groups, one with \(\mathrm{p} K_{\mathrm{a}} 2.10\) and the other with \(\mathrm{p} K_{\mathrm{a}} 4.07\). [NH3+]C(CCC(=O)O)C(=O)O Glutamic acid (a) Which carboxyl group has which \(\mathrm{p} K_{\mathrm{a}}\) ? (b) Account for the fact that one carboxyl group is a considerably stronger acid than the other carboxyl group.

Short answer

Answer: In glutamic acid, the main chain carboxyl group (-COOH) is stronger due to its lower pKa value of 2.10. The difference in acidity between the two carboxyl groups of glutamic acid can be attributed to the inductive effect. The main chain carboxyl group is closer to the positively charged amino group (NH3+), which donates positive charge and electrons to the nearby carboxyl group, stabilizing it and enhancing its acidity. On the other hand, the side chain carboxyl group (-CH2-CH2-COOH) with a pKa value of 4.07 is weaker due to being further away from the amino group, causing less inductive effect.

Step-by-step solution

Understand the structure of glutamic acid

Glutamic acid is an amino acid that contains two carboxyl groups, one in the side chain and the other in the main chain. The structure of glutamic acid is shown below: H3N+-CH(COOH)-CH2-CH2-COOH

Identify the carboxyl groups

In the structure of glutamic acid, we can see two carboxyl groups: 1. The carboxyl group in the main chain: -COOH 2. The carboxyl group in the side chain: -CH2-CH2-COOH

Determine which carboxyl group has which pKa value

We know that the lower the pKa value, the stronger the acid. From the given pKa values, it can be inferred that: 1. The carboxyl group with pKa value 2.10 is stronger and it is the main chain carboxyl group (-COOH). 2. The carboxyl group with pKa value 4.07 is weaker and it is the side chain carboxyl group (-CH2-CH2-COOH).

Explain the difference in acidity of the two carboxyl groups

The difference in acidity between the two carboxyl groups can be attributed to the inductive effect, which is the electron-withdrawing or electron-donating effect of neighboring atoms in a structure. In the case of glutamic acid, the main chain carboxyl group (-COOH) has a positively charged amino group (NH3+) that donates positive charge and electrons to the nearby carboxyl group. This stabilizes the carboxyl group and enhances its acidity. On the other hand, the side chain carboxyl group (-CH2-CH2-COOH) is further away from the amino group causing less inductive effect, which means it is a weaker acid compared to the main chain carboxyl group.

Key concepts

Amino Acids in Proteins

Amino acids are the fundamental building blocks of proteins, complex molecules that perform a myriad of functions in living organisms. Glutamic acid is one such amino acid, and it's unique in that it contains two carboxyl groups—essentially providing two opportunities for it to participate in acid-base reactions within a protein's structure.

Understanding how these carboxyl groups interact is crucial for grasping protein structure and function. In proteins, the amino acids are linked via peptide bonds to form a long chain, which then folds into a specific three-dimensional structure. The position of a carboxyl group within the amino acid—whether it is part of the main chain or the side chain—affects its potential interactions and, in turn, the protein's properties and behaviors.

Carboxyl Group Acidity

The carboxyl group, denoted as -COOH, is a functional group commonly found in organic acids. The presence of this group defines the acidic nature of many biological molecules, including amino acids. Its acidity stems from the carboxyl group's ability to donate a hydrogen ion (H+) to solutions, becoming a negatively charged carboxylate ion (COO-).

The strength of an acid is expressed by its pKa value—the lower the value, the stronger the acid. A lower pKa means the carboxyl group more readily donates its proton, effectively behaving as a stronger acid. This is essential in various biological processes, such as enzyme catalysis and buffering systems in blood.

Inductive Effect on Acidity

The inductive effect refers to the influence of the distribution of electrons in a molecule on the acidity of carboxyl groups. It plays a pivotal role when comparing the acidity of different carboxyl groups within the same molecule, such as glutamic acid.

In glutamic acid, the main chain carboxyl group is closer to a positively charged amino group. This proximity leads to an electron-withdrawing effect, stabilizing the negative charge that results when the carboxyl group donates its proton. Consequently, this inductive effect increases the acidity, reflected in a lower pKa value. In contrast, the side chain carboxyl group is farther away from any strongly electron-withdrawing groups; hence, it experiences a lesser inductive effect and is a weaker acid, indicated by a higher pKa value.

This distinction in acidity due to the inductive effect has significant implications in the behavior of amino acids within proteins and is a crucial point for students to understand when studying protein chemistry.