Dipole Moment
A helix has an overall dipole moment caused by the aggregate effect of all the individual dipoles from the carbonyl groups of the peptide bond pointing along the helix axis. This can lead to destabilization of the helix through entropic effects. As a result, α helices are often capped at the N-terminal end by a negatively charged amino acid, such as glutamate, in order to neutralize this helix dipole. Less common (and less effective) is C-terminal capping with a positively charged amino acid, such as lysine. The N-terminal positive charge is commonly used to bind negatively charged ligands such as phosphate groups, which is especially effective because the backbone amides can serve as hydrogen bond donors.
Read more about this topic: Alpha Helix
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