If you colour those residues you see that what is at the same face of the
helix in figure 69 a,
is more or less vertically above each other in the flat plot of figure 69 b.
Explain how figure 69 b was made. Explain this, in 3D, to the assistants
before continuing.
Many ways to explain this. The best would be to take a flat, elongated sheet
of paper, roll it up, draw a helix on the outside surface, and let it become
flat again.
In figure 69 a cut was made between the two aspartic acids 2 and 13. Whatever
is at the one side of the cut is on one edge in figure 69 b, and vice versa.