stigmasterol and cholesterol-acetate

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 x H(2)O. 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.

Topics: Air; Cholesterol; Cholesterol Esters; Crystallization; Lipid Bilayers; Membrane Lipids; Models, Chemical; Molecular Conformation; Stigmasterol; Surface Properties; Thermodynamics; Water; X-Ray Diffraction

Most work reporting the sterol composition of living organisms has not been done quantitatively, although good quantitative data are available for fatty acids and many other cellular components using an internal-standard method that compensates for errors during gas chromatographic analysis. In this paper, we report on the use of 7-stigmastenyl acetate as an internal standard for sterol analysis in two species of phytoplankton and oysters produced with two different diets. This internal-standard method provides an internal standard for this entire process of analysis, not just the gas chromatographic analysis. When analyzing 50-microgram samples of cholesterol acetate after hydrolysis and acetylation, about 30% of the sample was lost, resulting in a 30% error using the older external-standard method. Using the internal-standard method, the analysis error was less than 2%. Losses of sterol during analysis apparently are greater with plant and animal samples than with pure sterol standards. This internal-standard method was shown to be extremely useful, especially for samples with less than 500 micrograms of sterol. Finally, the standard error in sterol analysis is much lower when the internal-standard method is used, allowing statistical distinctions that are not possible otherwise. Use of 7-stigmastenyl acetate as an internal standard offers several advantages over the use of cholestane.

Topics: Animals; Cholesterol; Cholesterol Esters; Ostreidae; Phytoplankton; Reference Standards; Sterols; Stigmasterol