Kinetic Radical-scavenging Activity of Platonin, a Cyanine Photosensitizing Dye

Abstract

Platonin is known to possess antioxidant activity. However, the kinetics of the radical-scavenging activities of this compound remain unknown. The aim of this study was to investigate the radical-scavenging activities of platonin by the induction period method in the polymerization of methyl methacrylate (MMA), initiated by thermal decomposition of 2,2′-azobis (isobutyronitrile) (AIBN) (a carbon-centered radical, R●), and benzoyl peroxide (BPO) (an oxygen-centered radical, PhCOO●), under nearly anaerobic conditions. The number of moles of R● or PhCOO● radicals trapped by platonin calculated with respect to 1 mole of inhibitor moiety unit (stoichiometric factor, n) was determined, and this showed that the n of fully oxidized platonin was 4. The inhibition rate constant (k_inh) of platonin showed a wide range of 0.8×10³ M^-1s^-1 to 1.6×10⁴ M^-1s^-1. To clarify the interaction between platonin and thiols, 2-mercapto-1-methylimidazole (MMI) was used as a representative thiol, because glutathiones were unsuitable due to their limited solubility in MMA. MMI in the presence of platonin showed neither catalytic activities nor synergistic activities. Platonin possesses radical-scavenging activities and acts as an antioxidant. On the basis of our experimental results, the radical-scavenging mechanism is discussed.