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

Question

Which one of the following statements is correct? (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 Optical Activity

Optical activity refers to a molecule's ability to rotate the plane of polarized light. This property is due to the presence of a chiral center in the molecule, typically a carbon atom bound to four different groups.

Step 2: Identifying Amino Acid Chirality

Most amino acids have a central carbon atom (the alpha carbon) bonded to four different groups: a hydrogen atom, a carboxyl group (-COOH), an amino group (-NH2), and a unique side chain (R group), making them chiral and optically active.

Step 3: Exception to Chirality

Glycine is the only amino acid lacking a chiral center because its R group is a hydrogen atom, resulting in two hydrogens attached to the alpha carbon. Therefore, glycine is not optically active.

Step 4: Evaluating the Statements

Since glycine is the only common amino acid that is not optically active due to the absence of a chiral center, the correct statement must reflect this exception.

Step 5: Conclusion

The statement 'All amino acids except glycine are optically active' accurately reflects the exception to the optical activity of amino acids.

Key Concepts

Amino Acid Chirality

Amino acid chirality is a fascinating topic. In chemistry, chirality essentially means that a molecule cannot be superimposed onto its mirror image. To better picture this, imagine your hands: they are mirror images but not identical when placed over each other. Most amino acids are chiral because of their structure. They have a central alpha carbon attached to four different groups:

• A hydrogen atom
• A carboxyl group \((-COOH)\)
• An amino group \((-NH_2)\)
• A unique side chain, known as the R group

These distinct group attachments create what we call a "chiral center," leading to the optical activity of amino acids. This optical activity is identified by their ability to rotate plane-polarized light, a property that arises specifically from their chirality.

Chiral Center in Amino Acids

A chiral center is a carbon atom in the amino acid structure that is bonded to four different groups. This arrangement gives rise to molecules being optically active, meaning they can rotate the plane of polarized light. In amino acids, the chiral center is usually the alpha carbon. The unique combination of different groups around the alpha carbon allows each amino acid to exist in two forms:

• L-form (Levo) - naturally occurring in proteins
• D-form (Dextro) - found sometimes in bacterial cell walls and antibiotics

The chiral center is crucial for the optical properties of amino acids and is significantly important in biological systems because it affects the way amino acids react and function in proteins. The distinct spatial orientation also affects the chemical properties and roles they play in living organisms.

Glycine as Non-chiral Amino Acid

Glycine is unique among the naturally occurring amino acids. It stands out because it lacks a chiral center. The reason is simple: the side chain of glycine is a hydrogen atom. This means the alpha carbon is bonded to two hydrogen atoms, a carboxyl group, and an amino group. The symmetry due to the two hydrogen atoms means glycine cannot rotate plane-polarized light and hence is not optically active. As it doesn't possess the usual structural characteristics of a chiral molecule, glycine represents the sole exception amongst the standard amino acids found in proteins where chirality is absent. Understanding glycine's structural nuances helps explain why it doesn't partake in the optical activity that other amino acids exhibit.

Amino Acid Structure

The structure of amino acids is fundamental to understanding their behavior and functions. All amino acids share a core structure which includes:

• An amino group \((-NH_2)\)
• A carboxyl group \((-COOH)\)
• An alpha carbon
• A distinctive side chain or R group

This standard structure is what makes amino acids the building blocks of proteins. The variation comes in the form of the side chains (R groups), which differ among the 20 standard amino acids, giving each a unique set of properties and roles in protein structures and functions. These side chains can be polar, non-polar, acidic, or basic, influencing how amino acids interact with each other and other molecules. The interplay of these properties drives the protein folding process and, ultimately, determines the protein's functionality in biological systems.