Editing Alpha helix (section)

== Dipole moment ==
A helix has an overall [[Molecular dipole moment|dipole moment]] due to the aggregate effect of the individual microdipoles from the [[carbonyl]] groups of the peptide bond pointing along the helix axis.<ref>{{cite journal | vauthors = Hol WG, van Duijnen PT, Berendsen HJ | s2cid = 4147335 | year = 1978 | title = The alpha helix dipole and the properties of proteins  | journal = Nature | volume = 273 | issue = 5662 | pages = 443–446 | doi = 10.1038/273443a0| pmid = 661956 |bibcode = 1978Natur.273..443H }}</ref> 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 {{nowrap|1=C=O···H−N}}.<ref>{{cite journal | vauthors = He JJ, Quiocho FA | title = Dominant role of local dipoles in stabilizing uncompensated charges on a sulfate sequestered in a periplasmic active transport protein | journal = Protein Science | volume = 2 | issue = 10 | pages = 1643–7 | date = October 1993 | pmid = 8251939 | doi = 10.1002/pro.5560021010 | pmc=2142251}}</ref><ref>{{cite journal | vauthors = Milner-White EJ | title = The partial charge of the nitrogen atom in peptide bonds | journal = Protein Science | volume = 6 | issue = 11 | pages = 2477–82 | date = November 1997 | pmid = 9385654 | pmc = 2143592 | doi = 10.1002/pro.5560061125 }}</ref>