Problem 119 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following statements about proteins: 1\. all natural amino acids which are constituents of proteins are \(\alpha\)-amino acids 2\. \(\alpha\)-amino acids are all optically active and have the L-configuration 3\. an especially favourable conformation for the peptide linkage in proteins is the \(\alpha\)-helix arrangement. 4\. \(\alpha\)-amino acids are connected by ester linkages of these statements (a) 1 and 3 are correct (b) 1 and 2 are correct (c) 2 and 3 are correct (d) 2,3 and 4 are correct

Step-by-Step Solution

Answer

(a) 1 and 3 are correct.

Step 1: Analyze Statement 1

Statement 1 asserts that all natural amino acids, which are constituents of proteins, are \(\alpha\)-amino acids. Amino acids in proteins generally have the amino group attached to the \(\alpha\)-carbon, so this statement is correct.

Step 2: Analyze Statement 2

This statement claims that all \(\alpha\)-amino acids are optically active and have the L-configuration. Most \(\alpha\)-amino acids are optically active, but glycine is an exception as it is not chiral. Thus, Statement 2 is incorrect.

Step 3: Analyze Statement 3

Statement 3 states that a favourable conformation for the peptide linkage in proteins is the \(\alpha\)-helix arrangement. The \(\alpha\)-helix is indeed a common and stable structural motif in proteins, so this statement is correct.

Step 4: Analyze Statement 4

Statement 4 suggests \(\alpha\)-amino acids are connected by ester linkages. Proteins link amino acids through peptide bonds (amide linkages), not ester linkages, so this statement is incorrect.

Step 5: Determine Correct Option

After analyzing each statement's validity, choose the option that includes statements 1 and 3. Thus, (a) 1 and 3 are correct is the correct answer.

Key Concepts

Amino Acids

Amino acids are the building blocks of proteins. They play a crucial role in various biological processes. Each amino acid has a common structure: an amino group (\(-NH_2\)), a carboxyl group (\(-COOH\)), and a side chain that varies between different amino acids. The unique feature of amino acids found in proteins is that the amino group is attached to the \(\alpha\)-carbon, which is the carbon atom adjacent to the carboxyl group.

• The sequence and composition of amino acids determine the protein's structure and function.
• Amino acids can be classified based on the properties of their side chains, such as polar, nonpolar, acidic, or basic.

Understanding amino acids and their structures is vital for grasping how proteins are built and function within living organisms.

Alpha-Helix

The alpha-helix is one of the most common secondary structures found in proteins. It's a right-handed coil where each amino acid contributes to the overall helical shape through hydrogen bonds. These bonds occur between the carbonyl oxygen of one amino acid and the amide hydrogen of another amino acid, four residues earlier.

• This arrangement stabilizes the structure, making it highly favorable and recurrent in protein structures.
• The alpha-helix allows proteins to be compact while maintaining their structural integrity.

The predictability of the alpha-helix makes it a key component in understanding protein folding and dynamics.

Optical Activity

Most amino acids exhibit optical activity, a property where molecules can rotate the plane of polarized light. This phenomenon occurs due to the presence of a chiral center in amino acids, typically the \(\alpha\)-carbon, which bonds to four distinct groups. However, glycine is an exception, as it has two hydrogen atoms attached to this carbon, making it achiral and, therefore, not optically active.

• The L-configuration of amino acids is predominant in nature and refers to the orientation of the amino acids as they are found in proteins.
• Optical activity is crucial for understanding how amino acids and proteins interact with their environment.

The property of optical activity plays a fundamental role in the biosynthesis and functionality of proteins.

Peptide Bonds

Peptide bonds are the linchpins that hold amino acids together in proteins. They form when the carboxyl group of one amino acid reacts with the amino group of another, releasing a molecule of water. This results in a bond known as a peptide bond, or an amide linkage.

• Peptide bonds are covalent and remarkably stable, providing the backbone for protein structures.
• The sequence of amino acids linked by peptide bonds forms the primary structure of proteins.

The process of peptide bond formation is called polymerization, creating a long chain of amino acids known as a polypeptide, which then folds into a functional protein.