EPR spectra of spin-labeled phosphatidylcholine analogs in large unilamellar vesicles. Spectra of C5-SL-PC (A) and C16-SL-PC (C) were recorded in LUV consisting of POPC (a), POPC/cholesterol (70:30) (b), POPC/desmosterol (70:30) (c), and POPC/lanosterol (70:30) (d) at 30°C as described in Materials and Methods. The arrows show the outer hyperfine splitting (A), which is increased in the presence of steroids. Note that for C16-SL-PC, an additional component (see high field peak of the spectra) may indicate that a small part of this lipid is organized in a different ordered domain within the membrane. From the spectra of C5-SL-PC and C16-SL-PC, an order parameter (S, panel B) and a correlation time of rotation (τ, panel D), respectively, was calculated. Data are the average ± SE of estimate of 4–5 independent measurements.