Figure 1.

Description of the helical contour of the BOC model. (a) n: unit vector parallel to the helix axis. P: pitch (P < 0 for a left-handed helix and P > 0 for a right-handed helix). d: diameter. θ: rise angle between the i-th bond vector υ_i and the plane perpendicular to n. (b) Axis view. α: azimuthal angle. ${\upsilon }_i^P$: projection of υ_i on the plane perpendicular to n. (c) Dihedral angle for three consecutive bonds $\overline{\mathit{\text{OP}}}$, $\overline{\mathit{\text{PQ}}}$, and $\overline{\mathit{\text{QR}}}$. The arrow labeled ‘Viewing Dir.’ provides the counterclockwise sense (positive dihedral angle up to 180°) in the rotation of the plane PQR relative to plane OPQ with $\overline{\mathit{\text{PQ}}}$ as the rotation axis.