CHOLESTEROL AND ERGOSTEROL SUPERLATTICES IN 3-COMPONENT LIQUID-CRYSTALLINE LIPID BILAYERS AS REVEALED BY DEHYDROERGOSTEROL FLUORESCENCE

We have examined the fractional sterol concentration dependence of deh ydroergosterol (DHE) fluorescence in holesterol/dimyristoyl-L-alpha-ph osphatidylcholine (DMPC), DHE/ergosterol/DMPC and holesterol/dipalmito yl-L-alpha-phosphatidylcholine (DPPC) liquid-crystalline bilayers, Flu orescence intensity and lifetime exhibit local minima (dips) whenever the total sterol mole fraction, irrespective of the DHE content, is ne ar the critical mole fractions predicted for sterols being regularly d istributed in hexagonal superlattices. This result provides evidence t hat all three of these naturally occurring sterols (e.g., cholesterol, ergosterol, and DHE) can be regularly distributed in the membrane and that the bulky tetracyclic ring of the sterols is the cause of regula r distribution. Moreover, at the critical sterol mole fractions, the s teady-state anisotropy of DHE fluorescence and the calculated rotation al relaxation times exhibit distinct peaks, suggesting that membrane f ree volume reaches a local minimum at critical sterol mole fractions. This, combined with the well-known sterol condensing effect on lipid a cyl chains, provides a new understanding of how variations in membrane sterol content change membrane free volume. In addition to the fluore scence dips/peaks corresponding to hexagonal superlattices, we have ob served intermediate fluorescence dips/peaks at concentrations predicte d by the centered rectangular superlattice model. However, the 22.2 mo l% dip for centered rectangular superlattices in DHE/ergosterol/DMPC m ixtures becomes diminished after long incubation (4 weeks), whereas on the same time frame the 22.2 mol% dip in DHE/cholesterol/DMPC mixture s remains discernible, suggesting that although all three of these ste rols can be regularly distributed, subtle differences in sterol struct ure cause changes in lateral sterol organization in the membrane.