Of all the different bond types found in proteins, which has partial double bond character? Choose one: A. A hydrogen bond B. An ionic bond C. A peptide bond D. A disulfide bond
The correct answer and explanation is:
Correct Answer: C. A peptide bond
Explanation (Approx. 300 words):
Proteins are composed of amino acids linked together by peptide bonds, which are covalent bonds formed through a dehydration reaction between the carboxyl group of one amino acid and the amino group of another. Among the various bond types found in proteins—hydrogen bonds, ionic bonds, disulfide bonds, and peptide bonds—the peptide bond uniquely exhibits partial double bond character.
This partial double bond character arises due to resonance between the carbonyl group (C=O) and the nitrogen (N-H) in the peptide linkage. Specifically, the lone pair of electrons on the nitrogen can delocalize into the carbonyl carbon, creating a resonance hybrid where the electrons are shared between the carbon, oxygen, and nitrogen atoms. This electron delocalization causes the peptide bond to have characteristics of both a single and a double bond.
As a result of this partial double bond character, the peptide bond is planar and rigid, meaning there is limited rotation around the C–N bond. This rigidity is crucial for maintaining the specific shapes of proteins, influencing their secondary structures such as alpha helices and beta sheets.
In contrast:
- Hydrogen bonds are weak, non-covalent interactions involving a hydrogen atom and an electronegative atom, such as oxygen or nitrogen. They do not involve electron sharing like covalent or resonance structures.
- Ionic bonds (or salt bridges) occur between oppositely charged side chains but are purely electrostatic interactions with no double bond character.
- Disulfide bonds are covalent bonds between two cysteine residues but are simple single covalent bonds without any resonance or double bond character.
Therefore, the peptide bond (C) is the correct answer as it is the only one among the choices that displays partial double bond character due to resonance.