Exclusion of a cholesterol analog from the cholesterol-rich phase in modelmembranes

Vesicles of phosphatidylcholine/cholesterol mixtures show a wide compositio n range with coexistence of two fluid phases, the 'liquid disordered' (chol esterol-poor) and 'liquid ordered' (cholesterol-rich) phases. These systems have been widely used as models of membranes exhibiting lateral heterogene ity (membrane domains). The distributions of two fluorescent probes (a fluo rescent cholesterol analog. NBD-cholesterol, and a lipophilic rhodamine pro be, octadecylrhodamine B) in dimyristoylphosphatidylcholine/cholesterol ves icles were studied, at 30 degreesC and 40 degreesC. The steady-state fluore scence intensity of both probes decreases markedly with increasing choleste rol concentration. unlike the fluorescence lifetimes. The liquid ordered to liquid disordered phase partition coefficients K-p were measured, and valu es much less than unity were obtained for both probes, pointing to preferen ce for the cholesterol-poor phase. Globally analyzed time-resolved energy t ransfer results confirmed these findings. It is concluded that, in particul ar. NBD-cholesterol is not a suitable cholesterol analog and its distributi on behavior in phosphatidylcholine/cholesterol bilayers is in fact opposite to that of cholesterol. (C) 2001 Elsevier Science B.V. All rights reserved .