Order parameters of the POPC-d31 acyl chain in sphingomyelin/POPC mixtures, normalized with respect to the corresponding order parameters of POPC-d31 alone. (×) EYSM/POPC-d31, 2:1 mol/mole, 42°C; (▪) BBSM/POPC-d31, 2:1 mol/mole, 42°C; (•) BBSM/POPC-d31, 1:2 mol/mole, 42°C; and (○) BBSM/POPC-d31, 1:2 mol/mole, 21°C.