On the Helical Crystals of Cholesterol Monohydrate

We review in this short perspective the history of cholesterol crystals and crystal structures. We address in particular the helical crystals that form in vitro and in pathology from environments rich in bile acids or from phospholipid membranes. We review the known mechanisms leading to crystals with chiral morphology, from screw-dislocation mediated growth to mechanisms involving asymmetric mechanical strain. We propose a mechanism for cholesterol helical crystal development based on the monoclinic cholesterol monohydrate crystal structure. We suggest that curvature arises in few layers thick crystals due to the tension induced between the hydrophobic layer and the ice-like H-bonded lattice of the water molecules with the cholesterol hydroxy groups. Helicity would ensue through a combination of the curvature and the fast growth of a thin ribbon in one crystal direction.