The lateral distribution of pyrene-labeled sphingomyelin and glucosylceramide in phosphatidylcholine bilayers

The lateral distribution of N-[10(1-pyrenyl)decanoyl]-sphingomyelin (PyrSPM) and N-[10(1-pyrenyl)decanoyl]-glucocerebroside (PyrGlcCer) was studied in multilamellar vesicles of 1,2-dipalmitoyl-, 1,2-dimyristoyl-, and 1-palmitoyl-2-oleoyl-phosphatidylcholine (DPPC, DMPC, and POPC, respectively) under anaerobic conditions by determining the excimer-to-monomer fluorescence intensity ratio (E/M) as a function of temperature. The E/M(T) curves for PyrSPM and PyrGlcCer in the three phosphatidylcholine matrices are qualitatively similar to the curves reported for 1-palmitoyl-2-[10-(1-pyrenyl)decanoyl]-phosphatidylcholine (PyrPC) in the same three matrix phospholipids (Hresko, R. C., I. P. Sugár, Y. Barenholz, and T. E. Thompson, 1986, Biochemistry, 25:3813-3823). However, there is independent evidence to suggest that sphingomyelin and glucocerebroside are organized in POPC, DPPC, and DMPC in a more complex manner than is PyrPC. In an effort to examine further the relationship between the lateral distribution of the labeled lipid and the shape of an E/M(T) curve, E/M vs. temperature simulations were carried out together with an analysis of the equation that relates E/M to the system parameters. The results indicate that information about the lateral distribution of the pyrene-labeled lipid can be obtained from an E/M(T) curve only for those systems in which the gel to liquid crystalline phase transition temperature of the matrix lipid is higher than that of the pyrene-labeled lipid. However, very little can be known about the system from an E/M(T) curve if the matrix lipid has the lower phase transition temperature.