<Tr> <Td> Valine </Td> <Td> Val </Td> <Td> V </Td> <Td> 0.61 </Td> <Td> 2.55 </Td> </Tr> <P> A helix has an overall dipole moment due to the aggregate effect of the individual microdipoles from the carbonyl groups of the peptide bond pointing along the helix axis . The effects of this macrodipole are a matter of some controversy . α - helices often occur with the N - terminal end bound by a negatively charged group, sometimes an amino acid side chain such as glutamate or aspartate, or sometimes a phosphate ion . Some regard the helix macrodipole as interacting electrostatically with such groups . Others feel that this is misleading and it is more realistic to say that the hydrogen bond potential of the free NH groups at the N - terminus of an α - helix can be satisfied by hydrogen bonding; this can also be regarded as set of interactions between local microdipoles such as C = O H − N . </P> <P> Coiled - coil α helices are highly stable forms in which two or more helices wrap around each other in a "supercoil" structure . Coiled coils contain a highly characteristic sequence motif known as a heptad repeat, in which the motif repeats itself every seven residues along the sequence (amino acid residues, not DNA base - pairs). The first and especially the fourth residues (known as the a and d positions) are almost always hydrophobic; the fourth residue is typically leucine--this gives rise to the name of the structural motif called a leucine zipper, which is a type of coiled - coil . These hydrophobic residues pack together in the interior of the helix bundle . In general, the fifth and seventh residues (the e and g positions) have opposing charges and form a salt bridge stabilized by electrostatic interactions . Fibrous proteins such as keratin or the "stalks" of myosin or kinesin often adopt coiled - coil structures, as do several dimerizing proteins . A pair of coiled - coils--a four - helix bundle--is a very common structural motif in proteins . For example, it occurs in human growth hormone and several varieties of cytochrome . The Rop protein, which promotes plasmid replication in bacteria, is an interesting case in which a single polypeptide forms a coiled - coil and two monomers assemble to form a four - helix bundle . </P> <P> The amino acids that make up a particular helix can be plotted on a helical wheel, a representation that illustrates the orientations of the constituent amino acids (see the article for leucine zipper for such a diagram). Often in globular proteins, as well as in specialized structures such as coiled - coils and leucine zippers, an α - helix will exhibit two "faces"--one containing predominantly hydrophobic amino acids oriented toward the interior of the protein, in the hydrophobic core, and one containing predominantly polar amino acids oriented toward the solvent - exposed surface of the protein . </P>

The interior of the alpha helix along the central axis contains
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