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.

Step 1: Understanding Optical Activity

Optical activity in amino acids comes from the presence of a chiral carbon. A chiral carbon is a carbon atom that has four different groups attached to it. An amino acid is optically inactive if it does not have any chiral centers.

Step 2: Evaluating Glycine Structure

Glycine is the simplest amino acid with the structure NH2-CH2-COOH. In glycine, the alpha carbon is attached to two hydrogen atoms, which means it does not have a chiral center because it lacks four different substituents.

Step 3: Identifying Optical Activity

Since glycine has no chiral center due to the repetition of hydrogen atoms attached to its alpha carbon, it is the only amino acid among the options that is optically inactive.

Key Concepts

Chiral Centers

A chiral center in an amino acid is a carbon atom that has four different groups attached to it. This unique configuration makes the molecule asymmetrical, allowing it to exist in two mirror-image forms, known as enantiomers. Enantiomers are like your left and right hands - similar in shape, but opposite in orientation. This characteristic is crucial for optical activity, which means the molecule can rotate plane-polarized light. In amino acids, the alpha carbon is typically where you find chirality. However, not all amino acids have chiral centers. If the alpha carbon has identical atoms or groups attached to it, there is no chirality, and thus, no optical activity. Key points about chiral centers:

• Essential for optical activity.
• Usually located on the alpha carbon in amino acids.
• Must have four different substituents to be chiral.

Glycine Structure

Glycine is the simplest amino acid with the chemical structure NH2-CH2-COOH. In glycine, the central carbon (also known as the alpha carbon) is bonded to:

• An amino group (NH2)
• A hydrogen atom (H)
• A carboxyl group (COOH)
• Another hydrogen atom (H)

Since the alpha carbon in glycine is bonded to two hydrogen atoms, it does not have four different groups attached, meaning glycine lacks a chiral center. This absence makes glycine special among amino acids because it is not optically active. Understanding glycine’s structure helps highlight why it's unique compared to other amino acids that typically have optical activity due to their chiral centers.

Amino Acid Stereochemistry

Amino acid stereochemistry refers to the spatial orientation of atoms in an amino acid molecule. This arrangement is important for understanding how amino acids interact with biological systems, given that most proteins and enzymes can be sensitive to the configuration of molecules. Most naturally occurring amino acids are chiral, meaning they have two possible stereochemical forms: the L and D isomers. In proteins, the L-isomer is predominantly found. These forms are not superimposable, and their unique shapes allow for specific interactions in biological systems. For amino acids with a chiral center, the spatial arrangement directly affects their behavior and interaction:

• L- and D- forms cannot be interchanged by simple rotation.
• The specific 3D form contributes to the amino acid's properties.
• Many biological processes are stereospecific, favoring one isomer over the other.

Understanding stereochemistry is fundamental in biochemistry, pharmacology, and nutrition, as the specific structure of amino acids determines their role and function in the body.