Problem 39 - A Complete Resource Book in Chemistry for JEE Main

Question

The \(\alpha\)-amino acid which contains an aromatic side chain is (a) proline (b) tyrosine (c) valine (d) tryptophan

Step-by-Step Solution

Answer

The b1-amino acids with aromatic side chains are tyrosine and tryptophan.

Step 1: Identify Aromatic Side Chain

An aromatic side chain contains a ring structure that is stable and exhibits resonance. Common examples include the benzene ring and other ring structures that follow Huckel's rule for aromaticity.

Step 2: Examine the Options

We need to identify which of the given amino acids has an aromatic side chain: (a) Proline - has a cyclic non-aromatic side chain. (b) Tyrosine - has an aromatic phenol (benzene) ring in its side chain. (c) Valine - has a non-aromatic branched alkyl chain. (d) Tryptophan - contains an indole ring which is aromatic.

Step 3: Conclusion

Among the options, tyrosine and tryptophan both contain aromatic rings. However, the question asks for an aromatic side chain singularly noted, so we must choose one that commonly is referred to in basic studies of amino acids with aromatic side chains.

Key Concepts

Aromatic Side Chains

An aromatic side chain is a key feature in some amino acids, characterized by its ring structure. Unlike simple chains, aromatic rings are planar and enjoy stability due to resonance. This means that electrons are delocalized, creating a more uniform electronic distribution around the ring. This property is what gives aromatic rings their unique scent and stability.

Not all amino acids have aromatic side chains. Those that do are distinct because of their functionality in proteins and enzyme activity. To determine if an amino acid has an aromatic side chain, you can look for:

• Ring structures, such as benzene rings.
• Stability provided by resonance.
• Following Huckel's rule, which indicates that the structure must have a certain number of π-electrons to be aromatic.

These features not only affect the chemical properties of the amino acids but also influence their role in biological systems. Understanding these can be crucial for studies in biochemistry and molecular biology.

Tyrosine

Tyrosine is one of the twenty standard amino acids used by cells to synthesize proteins. It is known for its aromatic side chain, which consists of a phenol group. The phenol group is essentially a benzene ring with a single hydroxyl group (OH).

Here are some features and functions of Tyrosine:

• It has a dual role due to its phenol group, making it polar and able to form hydrogen bonds.
• Tyrosine is a precursor to important neurotransmitters like dopamine, epinephrine, and norepinephrine.
• It plays a role in the synthesis of thyroid hormones.

Because of these properties, tyrosine is critical in metabolic pathways and plays a part in signal transduction within cells. Its ability to participate in enzyme catalysis and interaction with other molecules enhances its importance in biochemistry.

Tryptophan

Tryptophan is another standard amino acid with a significant aromatic side chain. Its side chain contains an indole group, a structure that is aromatic due to the fused benzene and pyrrole rings.

The characteristics and roles of Tryptophan are manifold:

• The indole ring imparts increased electron density, which plays a role in protein interactions.
• Tryptophan is a precursor to serotonin, a neurotransmitter that influences mood, sleep, and appetite.
• It also contributes to the production of melatonin, a hormone regulating sleep cycles.

This amino acid is essential, meaning it must be obtained through diet, as the body does not synthesize it. Tryptophan's ability to contribute to significant health and neurological functions make it an intriguing subject of study in nutrition and health sciences.