Fig. 3. (a) Formulae of octamers 4–6. Diffraction pattern (b) and crystals (c) of 4. Crystal structures of 5 (d) and 6 (e and f). In (d), some side chains have been omitted for clarity. Hydrogen bonds (dashed grey lines) connect two stacked helices of 6 at their C-termini (f). Carbon, nitrogen, and oxygen atoms are shown in green, blue, and red, respectively. Calcium is shown as purple balls. Hydrogen atoms have been omitted for clarity. C- and N-termini and helix handedness are indicated in cartoons.