DEHYDROERGOSTEROL STRUCTURAL ORGANIZATION IN AQUEOUS-MEDIUM AND IN A MODEL SYSTEM OF MEMBRANES

The aggregation of Delta(5,7,9(11),22)-ergostatetraen-3 beta-ol (dehyd roergosterol or DHE), a fluorescent analog of cholesterol, was studied by photophysical techniques. It was concluded that the aqueous disper sions of DHE consist of strongly fluorescent microcrystals, and no evi dence for self-quenching in micellar-type aggregates was found. The or ganization of DHE in model systems of membranes (phospholipid vesicles ) is strongly dependent on the vesicle type. In small unilamellar vesi cles, no evidence for aggregation is obtained, and the fluorescence an isotropy is rationalized on the basis of a random distribution of fluo rophores. On the contrary, in large unilamellar vesicles (LUVs), a ste eper concentration depolarization was observed. To explain this, a mod el that takes into account transbilayer dimer formation was derived. T his was further confirmed from observation of excitonic absorption ban ds of oxa-1,3-diazol-4-yl-amino)-23,24-bisnor-5-cholen-3 beta-ol (NBD- cholesterol) in LUV, which disappear upon sonication. It is concluded that, in agreement with recent works, sterol aggregation is a very eff icient process in large vesicles (and probably in natural membranes), even at very low concentrations (similar to 5 mol%).