Problem 4 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Among the following compounds most basic amino acid is: (a) Asparagine (b) Lysine (c) Serine (d) Histidine

Step-by-Step Solution

Answer

Lysine is the most basic amino acid.

Step 1: Understand Basic Nature of Amino Acids

The basic nature of an amino acid is determined by the presence of basic functional groups in its side chain. Amino acids like lysine and histidine contain additional amino groups which can accept protons, making them basic.

Step 2: Analyze Each Option

- **(a) Asparagine**: Contains a neutral amide group in its side chain, making it non-basic. - **(b) Lysine**: Contains an additional amino group in its side chain, making it highly basic. - **(c) Serine**: Contains a hydroxyl group, which is neutral and not basic. - **(d) Histidine**: Contains an imidazole group, which can accept protons and is considered basic, but less so than lysine.

Step 3: Compare Basicity

Lysine, with its longer side chain terminating in an amino group, provides more opportunity for accepting protons compared to histidine's imidazole ring. This makes lysine the most basic amino acid among the options.

Step 4: Conclude the Solution

Based on the side chain characteristics, lysine is identified as the most basic amino acid among the given compounds since it has a higher propensity to accept protons compared to asparagine, serine, and histidine.

Key Concepts

Amino Acid Structure

Amino acids are the building blocks of proteins and play a crucial role in many biological processes. Each amino acid consists of a central carbon atom (or alpha carbon) that is bonded to four groups:

• An amino group (\( \text{NH}_2 \))
• A carboxyl group (\( \text{COOH} \))
• Hydrogen atom
• A distinct side chain, often denoted as \( \text{R} \)

The side chain (\( \text{R} \)) varies among amino acids and determines their properties and functions. This diversity in side chains is why amino acids can have acidic, basic, or neutral characteristics. For instance, some side chains contain additional functional groups that influence the acid-base behavior of the amino acid. Basic amino acids, like lysine, possess one or more extra amino groups in the side chain, which can significantly alter their chemical behavior.
Understanding the structure of amino acids is essential to grasp why they behave differently in biological systems.

Proton Acceptance

Proton acceptance is a key aspect of the basicity of amino acids. A basic compound has the ability to accept protons (\( \text{H}^+ \)) in a chemical reaction. This characteristic is primarily related to the functional groups present in the molecule.

Amino acids, such as lysine and histidine, have groups in their side chains that make them capable of accepting protons and thus exhibiting basic properties.
The amino group (\( \text{NH}_2 \)) in lysine, for example, can accept a proton to form a positively charged ammonium group (\( \text{NH}_3^+ \)).
This ability to accept protons enhances their participation in numerous cellular activities where they often act as buffer systems.
Lysine’s long side chain terminating in an additional amino group provides many sites for protonation, making it more basic than histidine or other amino acids without such groups.

Functional Groups in Amino Acids

The functional groups in amino acids are vital to determining their chemical behavior, particularly with regard to their basicity or acidity. These functional groups include the carboxyl group, amino group, and unique side chains that differentiate one amino acid from another.

Here are some key functional groups found in amino acids:

• Amino Group: Found in all amino acids, it has the potential to accept protons, especially in basic amino acids like lysine and arginine.
• Carboxyl Group: Present in all amino acids, typically donating protons and thus contributing to acidity.
• Side Chains: Varies among amino acids, containing elements like sulfur, hydroxyl, or additional amine or imidazole groups affecting their interaction in biological systems.

In compounds like histidine, the imidazole ring in its side chain can also accept protons, contributing to its basic nature. However, the presence of an auxiliary amino group in lysine's side chain allows it to be classified as the most basic among standard amino acids.