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);…
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?
Select an option to attempt
Correct Answer: Option C
Explanation
Partial double-bond character in the peptide bond:In the peptide bond (C-N), resonance delocalises the lone pair of the nitrogen into the adjacent carbonyl C=O:Consequences of this partial double-bond character:• The C–N bond is shorter than a typical single bond: observed 1.33 Å vs. expected ~1.47 Å (a C-N single bond in amines).• The C=O bond is longer than a typical double bond: observed 1.24 Å vs. expected ~1.20 Å.• The 6 atoms of the peptide group (Cα–C(=O)–N–H–Cα) are coplanar (partial double-bond restricts rotation).So the bond where the decrease from expected length is observed due to partial double-bond character is the C–N bond (expected ~1.47 Å, observed 1.33 Å).Answer: C–N (Option 3).