H-bonding at the N-cap of an alpha-helix

Residues 1 through 5 H-bond from the CO to the NH of residues 5 through 9. These H-bonds are colored white.

That leaves the NH groups of residues 1-4 without H-bond partners in the backbone of the helix. In this capping motif, the NH group of residue 1 H-bonds with the sidechain of residue of residue 4 (Glu), and the NH group of residue 4 H-bonds with the sidechain of residue 1 (Ser). The NH groups of residues 2 and 3 will presumably make H-bonds with water.