PARTITIONING OF STEARIC-ACID SPIN LABELS INTO PHOSPHOLIPID-BILAYERS WITH DIFFERENT FLUIDITIES

The difference of spin label density between the phases with different fluidities was studied in multilamellar DPPC liposomes. ESR spectra of stearic acid spin labels of unprotonated form were observed. The spectral intensity due to the labels in lipid was measured on comparing to that due to the labels in the buffer solution. The intensity ratio of these two lines was calculated in both the gel and liquid crystalline phases. The label density of 16-SAL in the liquid crystalline phase is 2.8 times more concentrated than that in the gel phase. The values are 1.85 for 5-SAL and 5.6 for 12-SAL. The application of these values to the case when two regions with different fluidities coexist in a lipid bilayer was studied.