The peptide bond is formed during a condensation reaction between two amino acids. Amide ((CO)-NH-) group and carboxylic acid (-COOH) add up to release water molecule.

The peptide bond is planar because resonance is possible when all nonbonding electrons and empty orbitals are in the same plane i.e. like a double bond (sp2 hybridization). As resonance leads towards stability, it prefers to stay in trans planner configuration.

Why is peptide bond planar?

Resonance of peptide bond

There is a partial double bond character in a peptide bond. This is confirmed by analyzing the bond length of (CN) bond in ((CO)-NH-) which is 1.30 nm instead of 1.47 nm for (C-N) single bond i.e the molecule has ceased to rotate. It is mostly found in trans configuration which exhibits double bond characteristics i.e. (sp2 hybridization).

The resonance and bond length thus prove that a peptide bond is planar in nature.