Figure 1. Illustration of the geometric measures characterizing a αβ2 unit.
The position of the helix is defined with respect to a co-ordinate system anchored on the β-sheet. s1 is the mean vector representing the two strands, while s2 is a vector perpendicular to s1 in the plane containing the sheet. The orientation of the helix axis h is defined according to a polar co-ordinate system: the azimuthal angle θ is the angle between s1 and the projection of h in the plane defined by s1 and s2, while the elevation angle ψ is the angle between this plane and h. D measures the distance between the helix and the sheet (see text for details).