Question

Identify the correct statement among the following:

• A. Amino acid, cysteine easily undergoes dimerization due to the presence of free SH group.
• B. All naturally occurring amino acids are optically active.
• C. Glutamic acid is the only amino acid that contains a –COOH group at the side chain.
• D. All naturally occurring amino acids except glycine contain one chiral centre.

Hint:

For amino acids, remember that glycine is the only amino acid without a chiral centre. Most amino acids, except for glycine, have one chiral carbon.

Answer 1

The Correct Option is A

The objective is to evaluate four statements concerning the properties of naturally occurring amino acids and identify the accurate one.

Concept Used:

A foundational knowledge of amino acid structure and characteristics is required, including:

1. Chirality and Optical Activity: Chirality in an amino acid is determined by its \(\alpha\)-carbon being bonded to four distinct groups. Chiral molecules typically exhibit optical activity, meaning they can rotate the plane of polarized light. Glycine is an exception, as its \(\alpha\)-carbon is bonded to two hydrogen atoms.
2. Classification of Amino Acids: Amino acids are categorized based on their side chain (R-group) nature. Acidic amino acids possess an additional carboxyl group (–COOH) in their side chain.
3. Chemical Properties of Side Chains: The unique chemical attributes of amino acids are dictated by their side chains. For instance, cysteine features a sulfhydryl (–SH) or thiol group, which is chemically reactive.

Step-by-Step Solution:

Each statement will be individually assessed for its validity.

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

This assertion posits that all amino acids, with the exception of glycine, possess precisely one chiral center. Examination is as follows:

• Glycine (Gly) has a hydrogen atom as its side chain, rendering its \(\alpha\)-carbon achiral.
• The majority of other amino acids, such as Alanine (Ala), have a single chiral center at the \(\alpha\)-carbon.
• However, consider Threonine (Thr) and Isoleucine (Ile).
• Threonine: The \(\alpha\)-carbon is chiral. Additionally, its side chain, –CH(OH)CH\( _3 \), contains a chiral carbon (the \(\beta\)-carbon). Thus, Threonine has two chiral centers.
• Isoleucine: The \(\alpha\)-carbon is chiral. Its side chain, –CH(CH\( _3 \))CH\( _2 \)CH\( _3 \), also contains a chiral carbon (the \(\beta\)-carbon). Consequently, Isoleucine also possesses two chiral centers.

Due to Threonine and Isoleucine having two chiral centers, this statement is deemed incorrect.

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

Optical activity is contingent upon a molecule being chiral. As established in Step 1, Glycine's structure is H\( _2 \)N–CH\( _2 \)–COOH. Its \(\alpha\)-carbon is bonded to two hydrogen atoms, making it achiral. Being achiral, Glycine cannot rotate the plane of polarized light and is therefore optically inactive. As 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 suggests that Glutamic acid is exclusively an acidic amino acid. A review of acidic amino acids is necessary.

• Glutamic acid (Glu): Its side chain is –CH\( _2 \)CH\( _2 \)COOH, containing a carboxyl group.
• Aspartic acid (Asp): Its side chain is –CH\( _2 \)COOH, which also contains a carboxyl group.

As 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."

Cysteine's (Cys) side chain is –CH\( _2 \)SH, a sulfhydryl or thiol group.

• Thiol groups are prone to oxidation.
• In an oxidation reaction, two cysteine molecules can interact. The hydrogen atom from each –SH group is removed, forming a covalent bond between the two sulfur atoms. This linkage is termed a disulfide bond (–S–S–).
• This reaction links two cysteine molecules, resulting in a dimer known as cystine.

\[ 2 \times \text{(Cysteine)} \xrightarrow{[\text{Oxidation}]} \text{Cystine} + 2\text{H}^+ + 2\text{e}^- \]

This dimerization is a biochemically significant reaction, as disulfide bridges are vital for stabilizing the three-dimensional structures of numerous proteins. Consequently, this statement is correct.

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