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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:

Two alpha helices

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  • $\begingroup$ I am not sure about the thermal stability but carboxy-terminal amidation can protect from carboxypeptidase digestion. That is just biochemical stability. Can you indicate where exactly you read about what you are asking (add links if you can or paste an excerpt) $\endgroup$ – WYSIWYG Sep 12 '15 at 13:31
  • $\begingroup$ It was an exercise sheet: goo.gl/x1xiaj - Unfortunately, there are no online solutions, but in class, they told us that charged alpha helices are less thermally stable than uncharged ones (see question). $\endgroup$ – ste Sep 13 '15 at 13:30
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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.

enter image description here

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
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  • $\begingroup$ OP doesn't know what the concept behind it was and that is why they asked, don't add parts that make it seem as if they were asking for confirmation.. (Edit review) $\endgroup$ – Safdar Sep 3 at 17:18

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