Interaction of Ascorbic Acid with Spin-Labeled Derivatives of Stearic Acid Embedded in Liposome Membranes

The interaction of ascorbic acid with nitroxyl radicals from spin labels of 5-doxyl stearic and 16-doxyl stearic acids embedded in phosphatidylcholine liposomal membranes was studied using electron paramagnetic resonance (EPR) spectroscopy. Ascorbic acid was shown to effectively restore the 5-doxyl stearic acid tag in liposomes. After 90 minutes of incubation with 10 mM ascorbic acid, the EPR signal from this label decreased by 70%. After 20 minutes, the signal decreased by only 50%. At the same time, there was a decrease in the EPR amplitude of 16-doxyl stearic acid by no more than 35% after incubation of the liposomes for 120 minutes at a concentration of 100 mM ascorbic acid. It is also significant that the signal of 16-doxyl stearic acid remained stable for more than 3 hours. During the reduction of 5-doxyl stearic acid, an ascorbate radical was formed, but this was not observed for 16-doxyl stearic acid. This is likely due to the limited diffusion of ascorbic acid into the bilayer. In the absence of active transport, ascorbic acid is able to penetrate the membrane and interact with radicals near the polar region, reaching the level of 5-doxyl stearic acid tags. These findings are important for our understanding of the antioxidant role of vitamin C in lipid membranes.

spin labels, ascorbic acid, liposomes, EPR spectroscopy, membrane permeability, nitroxyl radicals