98. The CO-NH moiety between two C-alpha atoms constitutes the Peptide bond. The measured bond lengths (Angstroms) are: C(alpha)-CO (1.51); C=O (1.24); C-N(1.33); N-C(alpha)1.46. In which of these, there is a decrease from the expected bond length due to partial double bond character of the peptide bond?
A. C(alpha)-Co
B. C=0
C. C-N
D. N-C(alpha)

Detailed Explanation:

A peptide bond is a covalent bond formed between two amino acids, linking the carboxyl group of one amino acid to the amino group of another. This bond plays a crucial role in protein structure and function. The peptide bond exhibits partial double bond character due to resonance, where electrons are delocalized across the C=O and C-N bonds. As a result, this bond restricts rotation and influences bond lengths.

The given bond lengths in the peptide bond are as follows:

• C(alpha)-CO: 1.51 Å

• C=O: 1.24 Å

• C-N: 1.33 Å

• N-C(alpha): 1.46 Å

Let’s break down each of these bond lengths in the context of the peptide bond’s partial double bond character.

A. C(alpha)-CO (1.51 Å)

The bond between C(alpha) and the carbonyl carbon (CO) is longer than the expected bond length due to the resonance structure that imparts some double bond character to the C=O bond. This bond is expected to be slightly longer than a typical single bond due to partial double bond character, which restricts rotation and shortens other bonds.

B. C=O (1.24 Å)

The C=O bond is a true double bond and exhibits the expected bond length for a carbonyl group. This is in line with the typical bond length observed for a C=O double bond (about 1.24 Å).

C. C-N (1.33 Å)

The C-N bond in the peptide bond exhibits partial double bond character. The actual bond length of 1.33 Å is shorter than what would be expected for a typical single C-N bond (around 1.47 Å), indicating that the peptide bond has partial double bond character, which shortens the C-N bond length.

D. N-C(alpha) (1.46 Å)

The N-C(alpha) bond is a single bond and typically has a bond length of 1.46 Å, which is close to the expected bond length for a C-N single bond. There is no significant decrease from the expected bond length due to partial double bond character, as it is a single bond.

Conclusion:

The decrease from the expected bond length due to the partial double bond character of the peptide bond is most evident in the C-N bond, where the bond length is shorter than expected for a typical C-N single bond due to resonance.

Final Answer:

C. C-N