Antioxidant Activity of Natural Antioxidants versus Reactive Oxygen Species in Emulsion Systems and Its Spin-Dynamics

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In the present study, a kinetic investigation was progressed on the antioxidant reactions versus reactive oxygen species for natural antioxidants, such as vitamin E and catechins, in the emulsion phase by using time-resolved methods based on the fluorescence or ESR detection.1.The second-order rate constant for the scavenging reaction of the model free-radical in vitamin E (VE) emulsion systems was firstly obtained by using the fluorescence-detected stopped-flow method. The result indicates that the rate constants in emulsion systems are different from that in the homogeneous solution. This kinetic approach could also be applicable to the scavenging reaction of other radicals, such as galvinoxyl and DPPH, in emulsion systems.2.For investigating the dynamics of singlet oxygen (^1O_2) in the emulsion systems, the emission (at 1275 nm) from ^1O_2 was measured directly by using a time-resolved near-infrared fluorescence spectrophotometer. In the VE emulsion system, the life-time of ^1O_2 increases with an increase of the concentration of VE, while the VE usually shows 10^7 - 10^8 M^<-1>s^<-1> as the ^1O_2 quenching rate constant in homogeneous solutions. The result suggests that the ^1O_2 dynamics occurs mainly near the oil-droplet of VE in this system.3 The investigation on the spin-dynamics for the short-lived intermediate radials generated by the laser excitation of natural vitamin E and hydroperoxide is progressed by using the time-resolved ESR and transient absorption methods.The results of the present study give the time-resolved tools for investigating the kinetics and the spin-dynamics of the free-radicals and ^1O_2 in emulsion phase, which is a model of biological systems, colloids, food, and drinks. The obtained information for emulsion systems will clarify the unique dynamics in inhomogeneous systems.