Electron Spin Resonance Studies on the Interaction between Liposomal Membrane and Triton X-100

Abstract

The interaction between Triton X-100 and phosphatidylcholine liposomes was studied by the spin labeling technique and turbidity measurement. The order parameter (S) of spin labeled stearic acid and the turbidity both decreased upon addition of Triton X-100. The effect of Triton X-100 depended on the species of phosphatidylcholine. In egg phosphatidylcholine liposomes, the effect of Triton X-100 was biphasic. In the range of low Triton/phospholipid molar ratios (below 0.4) there was a gradual decrease in the S value without any appreciable decrease in the turbidity. This suggests that Triton X-100 fluidizes the membrane without forming small fragments. Above a molar ratio of 0.4 there was a drastic decline in the turbidity, indicating that the membrane was solubilized. In dimyristoylphosphatidylcholine liposomes, biphasic behavior was not observed. The S value and the turbidity decreased noticeably upon addition of a little Triton X-100. The sensitivity of dimyristoylphosphatidylcholine liposomes to Triton X-100 was much greater than that of egg phosphatidylcholine liposomes. Phase separation between phosphatidylcholine-rich regions and Triton-rich regions may occur in view of the S values reached upon the addition of Triton X-100 to both phosphatidylcholine liposomes. This was also indicated by the spectral change of spin labeled phosphatidylcholine liposomes caused by Triton X-100.