The peptide C=O bond of the nth residue of the backbone of alpha-helix points along and forms a hydrogen bond with the peptide N—H group of which among the following residue?

The peptide C=O bond of the nth residue of the backbone of alpha-helix points along and forms a hydrogen bond with the peptide N—H group of which among the following residue? Correct Answer (n+4)th

The peptide C=O bond of the nth residue of the backbone of alpha-helix points along and forms a hydrogen bond with the peptide N—H group of the (n+4)th residue.This arrangement results in the formation of a strong hydrogen bond, which has nearly optimum N—O distance (2.8 Armstrong). Moreover, in the core of the helix, the atoms are in van der Waals contact, because the core of the helix is tightly packed.