Stability study of lecithin liposomes during storage using ESR

The technique of electron spin resonance is employed to study the stability of liposomes by measuring the decay of the encapsulated volume as a function of the incubation time. This volume at first remained constant, next it decreased to 50% of its initial value in a characteristic time t50%, then it decreased further to a final residual value. The characteristic time varied from 1.2 to 23 days, it was larger for egg yolk than for soybean samples, and for both materials it increased with the lysolecithin to lecithin ratio. The oxidation damage associated with the appearance of hydroperoxides increased with the incubation time, and was twice as large for unilamilar vesicles than it was for multilamilar vesicles. The oxidative decay was three times faster for soybean than for egg yolk samples, but the amount of oxidative damage after the time t50% was only 10-15% greater for the soybean case. The studies were carried out using the Tempone nitroxide radical ion, and the broadening agent tris(oxalato)chromate(III).