Problem 171 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following statements is correct? [2012] (a) All amino acids except glycine are optically active (b) All amino acids are optically active (c) All amino acids except lysine are optically active (d) All amino acids except glutamic acid are optically acitve

Step-by-Step Solution

Answer

(a) All amino acids except glycine are optically active.

Step 1: Understanding the Optical Activity

Optical activity in amino acids is determined by the presence of a chiral center, which is when a carbon atom is bonded to four different groups. This chirality causes the molecule to rotate plane-polarized light. Most amino acids have a chiral center at the alpha carbon, making them optically active.

Step 2: Identifying the Exception

Glycine is the simplest amino acid with the structure NH₂CH₂COOH. It does not have a chiral center because it has two hydrogen atoms attached to the alpha carbon, instead of four different groups. Therefore, it is not optically active.

Step 3: Review the Statements

(a) states all amino acids except glycine are optically active. This aligns with our understanding as glycine lacks a chiral center. (b), (c), and (d) incorrectly suggest other exceptions or claim all are optically active, which we know isn't true since glycine is not optically active.

Step 4: Conclusion

Based on the analysis, the correct statement is (a) All amino acids except glycine are optically active, because glycine is the only amino acid without a chiral center, making it not optically active.

Key Concepts

Chirality in Molecules

Chirality is an essential concept in chemistry, especially when it comes to understanding the properties of molecules. A molecule is considered chiral if it cannot be superimposed on its mirror image. This uniqueness arises due to the presence of a chiral center, typically a carbon atom bonded to four different groups.

The structural arrangement of atoms around this chiral center offers the molecule its optical activity, which allows it to rotate plane-polarized light in different directions. Chirality is not just a quirky feature; it plays a crucial role in the behavior and interaction of molecules, particularly in biological systems.

Imagining chirality is like thinking of a pair of gloves. Just like your left hand won't fit a right glove perfectly, a chiral molecule and its mirror image do not overlay, creating its "handedness."

Understanding chirality helps us delve deeper into how molecules function, react, and are utilized in biological contexts, making it a cornerstone of stereochemistry.

Amino Acids with Chiral Centers

Amino acids are often described as the building blocks of proteins, and most of them exhibit optical activity due to the presence of a chiral center. In amino acids, this chiral center is typically located at the alpha carbon, which is directly bonded to four different groups: an amine group (NH₂), a carboxyl group (COOH), a hydrogen atom (H), and a variable side chain (R).

This distinct arrangement ensures that the majority of amino acids are chiral and, thus, optically active. This optical activity means that they can rotate plane-polarized light to the left or right, a feature that is imperative for their function in biological organisms.

These chiral centers are essential for the diverse roles amino acids play in forming proteins and enzymes. They dictate how these molecules interact with one another, influencing the overall structure and function of proteins in living organisms. Almost every natural amino acid, except glycine, possesses such a chiral center, which permits this characteristic behavior.

Glycine's Lack of Optical Activity

Glycine is the simplest amino acid and is unique because it lacks the optical activity characteristic of most other amino acids. This absence stems from its structure: the alpha carbon in glycine is bonded to two hydrogen atoms instead of four different groups.

Because the alpha carbon is attached to identical hydrogen atoms, it lacks a chiral center. Without this structure, glycine cannot exhibit optical activity—meaning it does not rotate plane-polarized light.

Notably, glycine's absence of chirality does not impede its function in proteins, although this lack of handedness distinguishes it from its peers. Its simplicity allows glycine to fit into various places within protein structures where more complex, chiral amino acids might not easily go. This adaptability underscores why glycine, despite its simplicity, remains an essential component in the repertoire of amino acids.