| Helix | A spiral, staircase-like, structure with a repeating pattern described by two simultaneous operations, rotation, and translation. It is the natural conformation of many biological polymers. |
| Hematin | An iron protoporphyrin differing from heme in that the central iron atom is in the ferric (Fe+++) rather than the ferrous (Fe++) state; the prosthetic group of methemoglobin. |