Cholesterol monohydrate nucleation in ultrathin films on water

The growth of a cholesterol crystalline phase, three molecular layers thick at the air-water interface, was monitored by grazing incidence x-ray diffr action and x-ray reflectivity. Upon compression, a cholesterol film transfo rms from a monolayer of trigonal symmetry and low crystallinity to a trilay er, 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 hydrox yl 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 c holesterol derivative stigmasterol transforms, upon compression, directly i nto a crystalline trilayer in the rectangular lattice. These results may co ntribute to an understanding of the onset of cholesterol crystallization in pathological lipid deposits.