What property of the peptide bond causes it to be rigid and nearly planar?

The rigidity and nearly planar nature of the peptide bond is primarily due to its partial double bond character. In a peptide bond, the carbonyl carbon (C=O) and the nitrogen (N-H) share electrons in such a way that the bond has characteristics of both a single bond and a double bond. This phenomenon arises from resonance; the electrons involved in the bonding can delocalize between the carbon, oxygen, and nitrogen atoms.

This partial double bond character restricts rotation around the bond, leading to a planar structure where the atoms attached to the peptide bond are often in a fixed arrangement. This planarity is crucial for the stability of protein structures, as it influences the folding patterns and ultimately the function of the protein.

The other options do not directly contribute to the rigidity and planarity of the peptide bond. Hydrogen bonding, while significant in stabilizing secondary structures like alpha-helices and beta-sheets, does not affect the bond itself. Single covalent bonding is a characteristic of many types of bonds, but it does not imply the rigidity or planarity seen in peptide bonds. Electrostatic interactions generally pertain to interactions between charged or polar groups and do not influence the local structure of the peptide bond.