Density maps for different structural types of MTs: (a) MT 11-3 (b) MT 12-3 (c) MT 13-3 (d) MT 14-3 (e) MT 15-4 (f) MT 16-4. The secondary structural features are clearly resolved in the density maps. MT 13-3 has perfectly straight PFs (Fig 2c). In contrast, super-twists exist for other types of MTs. For MT 11-3, 12-3, 15-4, and 16-4, the density maps display a minor right-handed supertwist (Fig 2a - 2c, and 2e - 2f). For MT 14-3, the density map shows a slight left-handed supertwist (Fig 2d).