The helical wheel and heptad repeats. (a) The seven residues that comprise two turns of the α helix can be represented by a helical wheel and are labeled a through g. (b) Residues in positions a and d lie on the same side of the helix; hydrophobic residues in these positions create an amphipathic helix and form the interface with a second helix to form a coiled-coil. (c) The packing of a and d residues of two α helices in a coiled-coil. Here, the hydrophobic residues are leucines.