ELECTROCHEMICAL-BEHAVIOR OF ROSMANOL 9-ETHYL ETHER, A DITERPENE LACTONE ANTIOXIDANT ISOLATED FROM SAGE

The electrochemical behaviour and mechanism of the redox process of th e natural antioxidant rosmanol 9-ethyl ether, isolated from Saliva off icinalis L., were studied. The cyclic voltammograms of rosmanol 9-ethy l ether (R9EE), at characteristic pH values, and the electrochemical p arameters for all investigated pH values were measured. Three characte ristic pH regions, each with different behaviour of R9EE, were identif ied. In regions of pH < 4 and pH > 5 only one anodic peak appeared, wh ereas in the solutions of pH 4-5 two anodic peaks could be noted. The overall oxidation mechanism at pH < 4 is an e.H.e.H. oxidation mechani sm, which as a final product gives a quinonic molecule. The influence of pH on the second oxidation peak potential tends towards zero in acc ordance with the preceding dissociation of the one of phenolic groups, thus suggesting an e.e.H. mechanism at pH > 5. This means that at the pH values expected in plant cells, R9EE has an unexpected structure, making this substance a potent antioxidant. Electrochemical and spectr ophotometric measurements enabled us to establish an extremely low pK( a) value (4.35) for R9EE.