Fluorescence Studies of Dehydroergosterol in Phosphatidylethanolamine/Phosphatidylcholine Bilayers

Abstract

Our previous fluorescence study has provided indirect evidence that lipid headgroup components tend to adopt regular, superlattice-like lateral distribution in fluid phosphatidylethanolamine/phosphatidylcholine (PE/PC) bilayers (Cheng et al., 1997, Biophys. J. 73:1967–1976). Here we have further studied this intriguing phenomenon by making use of the fluorescence properties of a sterol probe, dehydroergosterol (DHE). Fluorescence emission spectra, fluorescence anisotropy ( r), and time-resolved fluorescence intensity decays of DHE in 1-palmitoyl-2-oleoyl-PC (POPC)/1-palmitoyl-2-oleoyl-PE (POPE) mixtures were measured as a function of POPE mole fraction ( X PE) at 23°C. Deviations, including dips or kinks, in the ratio of fluorescence peak intensity at 375 nm/fluorescence peak intensity at 390 nm ( I 375/ I 390), fluorescence decay lifetime ( τ), or rotational correlation time ( ρ) of DHE versus PE composition plots were found at X PE ≈ 0.10, 0.25, 0.33, 0.65, 0.75, and 0.88. The critical values at X PE ≈ 0.33 and 0.65 were consistently observed for all measured parameters. In addition, the locations, but not the depth, of the dips for X PE < 0.50 did not vary significantly over 10 days of annealing at 23°C. The observed critical values of X PE coincide (within ±0.03) with some of the critical mole fractions predicted by a headgroup superlattice model proposing that the PE and PC headgroups tend to be regularly distributed in the plane of the bilayer. These results agree favorably with those obtained in our previous fluorescence study using dipyrenylPC and Laurdan probes and thus support the proposition that 1) regular arrangement within a domain exists in fluid PE/PC bilayers, and 2) superlattice formation may play a significant role in controlling the lipid composition of cellular membranes (Virtanen et al., 1998, Proc. Natl. Acad. Sci. USA. 95:4964–4969). The present data provide new information on the physical properties of such superlattice domains, i.e., the dielectric environment and rotational motion of membrane sterols appear to change abruptly as the lipid headgroups exhibit regular superlattice-like distributions in fluid bilayers.