Stabilization of liposomal membranes by thermozeaxanthins: carotenoid-glucoside esters.

Thermozeaxanthins (TZS) are novel carotenoid-glucoside esters existing in the cell membranes of the thermophilic bacterium, Thermus thermophilus. The effect of TZS on membrane permeability was studied by measuring the leakage of the fluorescent dye from calcein-entrapped large unilamellar liposomes (LUVs). The LUVs were composed of a small portion (0.2-1.0 mol%) of TZS and phosphatidylcholine (PC) of various length and saturation degree of hydrocarbon chains. Incorporation of TZS in egg PC LUVs stabilized the liposomes in the temperature range from 30 to 80 degrees C, as only 2.6% of the entrapped calcein leaked out in contrast to 10% release from the egg PC liposomes without TZS. LUVs composed of dipalmitoylphosphatidylcholine (DPPC) or dioleoylphosphatidylcholine (DOPC) were stabilized by the incorporation of TZS at a temperature below 30 degrees C. Inclusion of TZS in LUVs composed of dimyristoylphosphatidylcholine, whose hydrocarbon chains are shorter than both DPPC and DOPC, did not stabilize the liposomes. About 90% of the entrapped dye was lost indicating defects of the liposomal membranes. Matching of the lipid bilayer thickness with the molecular length of TZS in the bilayers is discussed.