Problem 45 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following amino acid is optically inactive? (a) phenyl amine (b) asparagine (c) glutamic acid (d) glycine

Step-by-Step Solution

Answer

Glycine is optically inactive due to the lack of a chiral center.

Step 1: Understand Optical Activity

Optical activity refers to a substance's ability to rotate the plane of polarized light. For an amino acid to be optically active, it must have a chiral center, which means it has four different groups attached to a central carbon atom.

Step 2: Identify Chiral Centers in Amino Acids

Amino acids typically have a central (alpha) carbon atom connected to an amino group, a carboxylic acid group, a hydrogen atom, and a side chain (R group). For an amino acid to be optically active, the R group must differ from the other three groups. Review the structure of each given amino acid to identify the presence of a chiral center.

Step 3: Examine Glycine's Structure

Glycine has a central carbon atom bonded to two hydrogen atoms (in addition to the amino and carboxyl groups). Therefore, it lacks a chiral center as it does not have four different groups attached to it, making it optically inactive.

Step 4: Review Other Options

Phenyl amine, asparagine, and glutamic acid each have a chiral alpha carbon with different R groups, ensuring they are optically active. Glycine is unique in lacking a chiral center due to having two identical hydrogen atom groups.

Key Concepts

Amino Acids

Amino acids are the building blocks of proteins and play a crucial role in biological processes. They contain:

• An amino group (-NH₂)
• A carboxyl group (-COOH)
• A hydrogen atom
• A variable side chain (R group) that determines the amino acid's unique properties

The general formula for an amino acid can be written as \( ext{NH}_2 ext{-CHR-COOH} \), where "R" represents the unique side chain. The properties of the R group can affect everything from the amino acid's function to its solubility in water.
Although there are only 20 standard amino acids, they combine in various ways to give rise to the vast diversity of proteins found in living organisms. Each amino acid can influence the protein's structure and function in unique ways. Most importantly, when considering optical activity, the presence of a chiral center directly impacts whether an amino acid is optically active or inactive.

Chiral Center

A chiral center in an amino acid is a carbon atom that has four different groups attached to it, making it an asymmetrical center. This asymmetry allows for optical activity, where the molecule can rotate the plane of polarized light.
Not all amino acids have a chiral center, but when they do, they exist in two different spatial configurations known as enantiomers. These enantiomers are non-superimposable mirror images of each other.

• A chiral center is often found at the alpha carbon, the central carbon in amino acids.
• The presence of a chiral center in amino acids typically makes them optically active.

In a biological context, most naturally occurring amino acids are found in the L-configuration, one of the two possible enantiomeric forms.
For an amino acid to be optically active, it needs this chiral center. This means there must be four distinct groups connected to the alpha carbon. Identifying these groups is crucial to understanding whether the amino acid can engage with polarized light.

Glycine

Glycine is a unique amino acid with the simplest structure among the 20 standard amino acids.

• Its side chain consists of a single hydrogen atom, making it structurally different from its counterparts.
• The general formula of glycine is \( ext{NH}_2 ext{-CH}_2 ext{-COOH} \).
• Due to its simplicity, glycine lacks a chiral center, as the alpha carbon is bonded to two hydrogen atoms instead of four distinct groups.

This absence of a chiral center means glycine is optically inactive, meaning it does not rotate the plane of polarized light.
The lack of optical activity in glycine makes it unique, as most other amino acids do possess this property. It highlights the diversity and special characteristics found within amino acids, each contributing differently to protein structure and function. As glycine lacks chirality, it often functions in flexible regions of proteins and can adapt to spaces where more rigid amino acids cannot fit.