of about 30° in the helix's axis. However, proline is often seen as the first residue of a helix, it is presumed due to its structural rigidity. At the other extreme, glycine also tends to disrupt helices because its high conformational flexibility makes it entropically expensive to adopt the relatively constrained α-helical structure. Table of standard amino acid alpha-helical propensities Estimated differences in free energy change, Δ(ΔG), estimated in kcal/mol per residue in an α-helical configuration, relative to alanine arbitrarily set as zero. Higher numbers (more positive free energy changes) are less favoured. Significant deviations from these average numbers are possible, depending on the identities of the neighbouring residues. {| class="wikitable sortable" |+Differences in free energy change per residue !rowspan=2| Amino acid !rowspan=2 class="unsortable"| 3-letter !rowspan=2 class="unsortable"| 1-letter !colspan=2| Helical penalty |- !kcal/mol !kJ/mol |- | Alanine | Ala | A | |- | Arginine | Arg | R | |- | Asparagine | Asn | N | |- | Aspartic acid | Asp | D | |- | Cysteine | Cys | C | |- | Glutamic acid | Glu | E | |- | Glutamine | Gln | Q | |- | Glycine | Gly | G | |- | Histidine | His | H | |- | Isoleucine | Ile | I | |- | Leucine | Leu | L | |- | Lysine | Lys | K | |- | Methionine | Met | M | |- | Phenylalanine | Phe | F | |- | Proline | Pro | P | |- | Serine | Ser | S | |- | Threonine | Thr | T | |- | Tryptophan | Trp | W | |- | Tyrosine | Tyr | Y | |- | Valine | Val | V | |} Dipole moment 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