Amino Acids

To determine the dipeptide "x," we need to analyze the amino acids "y" and "z" obtained upon its hydrolysis:

1. Information about "y": The amino acid "y" on treatment with aqueous HNO₂ produces lactic acid. This transformation suggests that "y" is likely to be alanine. The reaction of alanine with HNO₂ results in the formation of lactic acid through the loss of the amino group and subsequent rearrangement.
2. Information about "z": The amino acid "z," upon heating, forms the given cyclic molecule. The structure presented corresponds to a diketopiperazine formed by condensation of glycine. Such a cyclization is characteristic of glycine.
3. Identifying the Dipeptide:
   • Since "y" is alanine and "z" is glycine, the dipeptide "x" must be alanine-glycine.

Hence, based on the transformations and information given, the dipeptide "x" is identified as alanine-glycine.

The problem asks us to evaluate four statements about the properties of naturally occurring amino acids and identify the one that is correct.

Concept Used:

To solve this problem, we need a fundamental understanding of amino acid structure and properties, including:

1. Chirality and Optical Activity: An amino acid is chiral if its -carbon is bonded to four different groups. Chiral molecules are typically optically active, meaning they rotate the plane of polarized light. Glycine is an exception as its -carbon is bonded to two hydrogen atoms.
2. Classification of Amino Acids: Amino acids are classified based on the nature of their side chain (R-group). Acidic amino acids contain an additional carboxyl group (–COOH) in their side chain.
3. Chemical Properties of Side Chains: The unique chemical properties of amino acids are determined by their side chains. For example, cysteine has a sulfhydryl (–SH) or thiol group, which is chemically reactive.

Step-by-Step Solution:

We will analyze each statement individually to determine its validity.

Step 1: Analyze the statement "All naturally occurring amino acids except glycine contain one chiral centre."

This statement claims that apart from glycine, all other amino acids have exactly one chiral center. Let's examine this.

• Glycine (Gly) has a hydrogen atom as its side chain, making its -carbon achiral.
• Most other amino acids, like Alanine (Ala), have one chiral center at the -carbon.
• However, let's consider Threonine (Thr) and Isoleucine (Ile).
• Threonine: The -carbon is chiral. Its side chain, –CH(OH)CH , also contains a chiral carbon (the -carbon). So, Threonine has two chiral centers.
• Isoleucine: The -carbon is chiral. Its side chain, –CH(CH )CH CH , also contains a chiral carbon (the -carbon). So, Isoleucine also has two chiral centers.

Since Threonine and Isoleucine have two chiral centers, this statement is incorrect.

Step 2: Analyze the statement "All naturally occurring amino acids are optically active."

Optical activity requires a molecule to be chiral. As established in Step 1, the amino acid Glycine has the structure H N–CH –COOH. Its -carbon is bonded to two hydrogen atoms, so it is not a chiral center. Because Glycine is achiral, it cannot rotate the plane of polarized light and is therefore optically inactive. Since Glycine is a naturally occurring amino acid, this statement is incorrect.

Step 3: Analyze the statement "Glutamic acid is the only amino acid that contains a –COOH group at the side chain."

This statement claims that only Glutamic acid is an acidic amino acid. Let's review the acidic amino acids.

• Glutamic acid (Glu): Its side chain is –CH CH COOH. It contains a carboxyl group.
• Aspartic acid (Asp): Its side chain is –CH COOH. It also contains a carboxyl group.

Since both Aspartic acid and Glutamic acid are naturally occurring amino acids with a –COOH group in their side chains, this statement is incorrect.

Step 4: Analyze the statement "Amino acid, cysteine easily undergoes dimerization due to the presence of free SH group."

The side chain of Cysteine (Cys) is –CH SH. This –SH group is a sulfhydryl or thiol group.

• Thiol groups are known to be easily oxidized.
• In an oxidation reaction, two cysteine molecules can react with each other. The hydrogen atom from each –SH group is removed, and a covalent bond is formed between the two sulfur atoms. This bond is called a disulfide bond (–S–S–).
• This reaction links the two cysteine molecules, forming a dimer called cystine.

This dimerization is a very important biochemical reaction, as disulfide bridges are crucial for stabilizing the three-dimensional structures of many proteins. Therefore, this statement is correct.

The only correct statement among the options is: Amino acid, cysteine easily undergoes dimerization due to the presence of free SH group.