The growth of a cholesterol crystalline phase, three molecular layers thick at the air-water interface, was monitored by grazing incidence x-ray diffraction and x-ray reflectivity. Upon compression, a cholesterol film transforms from a monolayer of trigonal symmetry and low crystallinity to a trilayer, composed of a highly crystalline bilayer in a rectangular lattice and a disordered top cholesterol layer. This system undergoes a phase transition into a crystalline trilayer incorporating ordered water between the hydroxyl groups of the top and middle sterol layers in an arrangement akin to the triclinic 3-D crystal structure of cholesterol · H2O. By comparison, the cholesterol derivative stigmasterol transforms, upon compression, directly into a crystalline trilayer in the rectangular lattice. These results may contribute to an understanding of the onset of cholesterol crystallization in pathological lipid deposits.
Rapaport, H., Kuzmenko, I., Lafont, S., Kjaer, K., Howes, P. B., Als-Nielsen, J., … Leiserowitz, L. (2001). Cholesterol monohydrate nucleation in ultrathin films on water. Biophysical Journal, 81(5), 2729–2736. https://doi.org/10.1016/S0006-3495(01)75915-2