The natural antioxidant rosmarinic acid spontaneously penetrates membranes to inhibit lipid peroxidation in situ

► Rosmarinic acid protects lipids efficiently against hydrophilic radicals. ► 1 mol% of rosmarinic acid inserts spontaneously into preformed membranes. ► Rosmarinic acid does not alter the membrane structure and properties. ► Rosmarinic acid resides preferentially among the polar headgroups of lipids. ► 1 mol% of membrane-inserted rosmarinic acid fully inhibits lipid peroxidation in situ.