Molecular structures of cholesterol and analogues. (A) Structures of cholesterol and cholesterol trisnorcholenic acid are shown in full. Only the steroid nucleus around the A/B fusion is shown for the remaining compounds to emphasize differences in the planar orientation of the C3 hydroxyls and geometry of the A/B ring fusion. The free carboxyl group (pKa = 5.5) at the end of the side chain in cholesterol trisnorcholenic acid is shown in its ionized form, which likely predominates under our experimental conditions (pH 7.2). (B) The structure of cholesterol enantiomer (ent-cholesterol) is shown in full to emphasize that this molecule is a mirror image of cholesterol. For all compounds, trivial names are provided. For IUPAC nomenclature, please refer to Table I.