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| A: Simplified view down a coiled-coil of antiparallel alpha-helices. Calpha-Cbeta vectors and Calpha-Cbeta bonds are shown, with the positions of the heptad repeat labeled "a"-"g" for the helix at right. Note that the bonds pinwheel clockwise on the lefthand helix and counterclockwise on the righthand one, so that those bonds point toward one another on the top face of the helix contact (the incoming, or tip-to-tip, side) and extend back-to-back on the bottom face (the outgoing, or knuckle-touch, side). The definition of the "heptad angle" is indicated for position "d"; in this case, the indicated angle is negative, because it comes before the helix midline in the N-to-C sequence direction. B: Schematic drawing of how two hands can mimic the side-chain contact geometry of a pair of antiparallel helices, with the tip-to-tip and knuckle-touch sides of the contact labeled. Helix backbone direction is shown by perspective arrows, and overall orientation matches A. (Note that the left hand uses the fingers in reverse order, in order to stand in for a right-handed helix. Note also that positions "d" and " d'" are not at the same depth.)
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