Figure 1. Visual representation of parameters used in coiled-coil fitting.

Geometrical meanings of the superhelical radius (R₀), the helical radius (R₁), the superhelical frequency (ω₀), the helical frequency (ω₁), chain axial offset (ΔZ_off), chain superhelical phase offset (Δφ₀), and starting helical phase (Δφ₁) are shown. The green tube represents the interfacial axis. Orange curves depict local helical axes, which in a coiled coil form a superhelix. The gray tube represents the helical curve, which passes through C_α atoms. Orange balls show the inward-facing points on the helical curves (not necessarily corresponding to locations of atoms) defined as points with helical phase equal to π. The distance along the interfacial axis between an inward-facing point on one helix and its closest counterpart on the opposite helix is defined as ΔZ_off, with sign indicating the order of the two points, relative to N→C of the first helix. The depicted case is an anti-parallel coiled coil with a positive ΔZ_off of 2.4 Å.