Problem 122 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following statements about amino acids: (1) nitrous acid liberates nitrous oxide from amino acids (2) an important sensitive test for the detection of amino acids is the ninhydrin colour test (3) glycine is the only amino acid which does not possess a chiral centre. (4) all the natural amino acids that occur as constituents of proteins belong to the L-series

Step-by-Step Solution

Answer

Statements 2, 3, and 4 are true; Statement 1 is false.

Step 1: Identify True or False

First, analyze each statement to determine whether it is true or false. This will involve recalling some facts about amino acids.

Step 2: Evaluate Statement 1

Statement 1: 'Nitrous acid liberates nitrous oxide from amino acids.' This is false. Nitrous acid generally reacts with primary amines to form diazonium salts, not nitrous oxide.

Step 3: Evaluate Statement 2

Statement 2: 'An important sensitive test for the detection of amino acids is the ninhydrin colour test.' This is true. Ninhydrin reacts with amino acids to produce a colored compound, typically purple, which is a basis for detecting amino acids.

Step 4: Evaluate Statement 3

Statement 3: 'Glycine is the only amino acid which does not possess a chiral centre.' This is true. Glycine is the simplest amino acid and has two hydrogen atoms attached to its alpha carbon, making it achiral.

Step 5: Evaluate Statement 4

Statement 4: 'All the natural amino acids that occur as constituents of proteins belong to the L-series.' This is true. The natural amino acids that are incorporated into proteins during translation are L-amino acids.

Step 6: Compile the Results

Based on evaluation: Statements 2, 3, and 4 are true. Statement 1 is false.

Key Concepts

Ninhydrin Test

The ninhydrin test is a popular method for detecting amino acids. When amino acids are treated with ninhydrin, a chemical reaction occurs that usually results in a colored compound, most commonly a purple-blue hue. This makes the test particularly sensitive and useful for identifying amino acids in various samples.
It works because amino acids have amino groups that react with ninhydrin. During this reaction, the amino group is removed, and ninhydrin is reduced. As a result, an intensely colored dye called Ruhemann's purple is produced. Notably, while all amino acids produce some color with ninhydrin, the intensity and the specific shade can vary.
**Why Ninhydrin is Important:**

• Sensitivity: It can detect minuscule quantities of amino acids due to the intense color change.
• Versatility: Works on most amino acids, making it useful for diverse applications.
• Simplicity: The procedure does not require extensive equipment or materials.

Given these advantages, it's often employed in biochemical labs to analyze protein and amino acid content.

Chiral Center in Amino Acids

The concept of chirality is pivotal in understanding amino acids. A chiral center is an atom, most commonly a carbon, that is attached to four different groups. This results in molecules that can be non-superimposable mirror images of each other, much like left and right hands.
**Why Glycine is Special:**

• Single Hydrogen: Glycine's alpha carbon is bonded to two hydrogen atoms, making it the only naturally occurring amino acid without a chiral center.
• Simplicity: This lack of chirality simplifies glycine's structure, reducing its complexity in protein interactions.

This unique aspect of chirality affects how amino acids interact within proteins and how enzymes recognize and process them, influencing both biological function and structure.

L-series Amino Acids

Amino acids come in two forms or "series": L and D. These designations are based on the orientation of their molecular structure as compared to glyceraldehyde, a reference compound.
**L-series in Biology:**

• Protein Construction: Only L-amino acids are used to build proteins in organisms, making them critical to life.
• Natural Occurrence: In nature, proteinogenic amino acids are exclusively of the L-form, which is essential in ensuring consistency in living organisms.

The fact that proteins are made of L-series amino acids speaks to a fundamental choice in evolution, highlighting the importance of molecular orientation and structure in biological processes. This has significant implications for biochemical research and pharmaceutical development, where the distinction between L and D is crucial.