Thermal stability of charged and uncharged alpha helices (the concept of alpha-helix capping)

Question

I just learned that charged alpha helices are less thermally stable than uncharged ones. The only difference I see between them would be a larger dipole moment in the charged helix, but how would this contribute to stability?

Here a picture for clarification:

Answer 1

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.

Reference

1. Serrano, L., Fersht, A. Capping and α-helix stability. Nature 342, 296–299 (1989). DOI: 10.1038/342296a0