Figure 2

HSE. Given the positions of 3 consecutive C _α atoms (A, B, C), the approximate side-chain direction $\overrightarrow{V}$ _b can be computed as the sum of $\overrightarrow{AB}$ and $\overrightarrow{CB}$. The plane perpendicular to $\overrightarrow{V}$ _b cuts the sphere centered at B in an upper and a lower hemisphere.