Question

The polypeptide chain in proteins does not exhibit free rotation because of the partial double-bond character of the peptide bond. Explain this fact with resonance structures.

Short answer

The partial double-bond character of the peptide bond, due to resonance, restricts rotation, making the polypeptide chain more rigid.

Step-by-step solution

Understanding Peptide Bonds

A peptide bond is formed between the carboxyl group of one amino acid and the amino group of another. This bond links amino acids together in a polypeptide chain.

Concept of Resonance Structures

Resonance structures are different ways of drawing the same molecule, showing the delocalization of electrons. This means the actual structure of the molecule is a hybrid of all possible resonance structures.

Resonance in Peptide Bonds

In a peptide bond, the lone pair of electrons on the nitrogen can overlap with the carbonyl double bond (C=O), creating a resonance structure where the nitrogen has a partial double-bond character with the carbonyl carbon.

Partial Double-Bond Character

Because of the resonance, the peptide bond has partial double-bond character, which restricts rotation around the bond. This means the bond acts more like a double bond rather than a single bond.

Implications for Polypeptide Chains

The partial double-bond character of the peptide bond prevents free rotation, causing the polypeptide chain to be more rigid. This rigidity is important for the protein's three-dimensional structure.

Key concepts

resonance structures

Resonance structures are different depictions of the same molecule. These depictions highlight the delocalization of electrons within the molecule. Resonance gives a better understanding of the stability and reactivity of molecules. In resonance, electrons can shift positions, creating multiple forms that combine into a hybrid structure. This hybrid is more stable than any single form.

partial double-bond character

In peptide bonds, the lone pair of electrons on the nitrogen atom can interact with the carbonyl group. This interaction leads to the delocalization of electrons. As a result, the bond between the nitrogen and the carbonyl carbon has characteristics of both a single bond and a double bond, known as partial double-bond character. This dual nature restricts rotation around the peptide bond, making it more rigid.

polypeptide chain

A polypeptide chain is a sequence of amino acids linked by peptide bonds. The partial double-bond character within these bonds contributes to the structural rigidity of the chain. This rigidity is crucial as it helps maintain the protein's three-dimensional shape, impacting its function. Proteins rely on their precise shapes to perform specific biological tasks, and any change in shape can alter or inhibit their function.

amino acids

Amino acids are the building blocks of proteins. Each amino acid consists of an amino group, a carboxyl group, a hydrogen atom, and a unique side chain, all attached to a central carbon atom. When amino acids link via peptide bonds, their respective amine and carboxyl groups form these bonds, creating long chains called polypeptides. The unique sequences of amino acids in polypeptides determine a protein's structure and function.