Figure 5. Definition of helicoidal coordinate system.

The black vertical $z$-axis is surrounded by a cylinder, around which a helical dynamin tube winds. Its green center line with radius $R$ and pitch $\mathrm{2}\mathrm{}\pi \mathrm{}p$ is given by (Equation 1). On this space curve we define the (right-handed) Darboux-frame ${\displaystyle \{\mathit{T},\mathit{N},\mathit{L}\}}$, consisting of a tangent vector ${\displaystyle \mathit{T}}$ (black), normal vector ${\displaystyle \mathit{N}}$ (blue) and co-normal vector ${\displaystyle \mathit{L}}$ (red), which are given by (Equation 2). In the ${\displaystyle \mathit{N}}$-${\displaystyle \mathit{L}}$-plane we can then define the circular cross-section of the filament and place rings of CG beads at the correct distance from the green filament axis. The direction along ${\displaystyle \mathit{N}}$ points towards the enclosed cylinder, but it is easy to rotate it by an angle $\phi$ around the ${\displaystyle \mathit{T}}$ axis. This is how one may rotate the beads representing the PH domain (red) off the underlying substrate.