This might relate to the concept of 'capping' alpha-helices, where the placement of beneficial charges at either end of a helix is shown to increase the overall stability of the protein. See the article1 published in Nature.
In general, by introducing preferential charges at the ends of alpha-helices you stabilize the inherent dipole moment created by the alignment of amide groups in the protein backbone.
Here is a different citation (found here):
In 1988, Richardson and Richardson observed that certain amino acid
residues are stastically favored as the amino-terminal caps for the
alpha helices in 45 globular proteins. The introduction of a
negatively charged side chain at the N-cap, which can neutralize the
partial dipole created by the unpaired amide protons, has been shown
to increase stability in T4 lysozyme.
I also believe the figure in the question is a bit misleading - as the illustrated charges may be intended on side-chain groups. I therefore assume that this helix is a part of a protein, and that the illustration simply wants to highlight the fact that you place charged side-chain groups on the ends of the helix (i.e. capping) within a protein structure.
- Serrano, L., Fersht, A. Capping and α-helix stability. Nature 342, 296–299 (1989). DOI: 10.1038/342296a0