SYNERGISTIC EFFECT OF CHLOROPHYLL-A ON THE OXIDATION PROPERTIES OF VITAMIN-E

The antioxidant effects of chlorophyll a (CHL) in the dark have been i nvestigated by kinetic studies in a model system. The oxidation of chl orophyll a, thermally induced at 60 degrees C by azo-bis-isobutyronitr ile (AIBN) has been examined in heptanol. The experimental rate equati on involves the chlorophyll a concentration raised to the nonintegral power 0.2. To explain this observation, the reaction scheme suggests t he reversible formation of a charge transfer complex between isobutyro nitrileperoxyl radicals AO(2)(.) and chlorophyll a (<((12'))under bar> <((-12'))under bar> AO(2)(.) + CHL (k-12') reversible arrow (k12') AO( 2) - CHL(.)(+) with k(12') = 2630 M(-1) s(-1) and k(-12') = 0.05 s(-1) (60 degrees C, heptanol). The induced oxidation of methyl linoleate w as performed at 40 degrees C and 60 degrees C in heptanol or in ethyl heptanoate, in the presence of antioxidants. In heptanol, chlorophyll a exhibited an exceptionally long induction period, thereby proving to be a chain-breaking antioxidant as good as BHT (2,6-di-t-butyl 4-meth yl phenol) or vitamin E. However, chlorophyll a was much less reactive than these two well-known phenolic antioxidants in ethyl heptanoate. The effects of the addition of chlorophyll a together with vitamin E t o preserve methyl linoleate were investigated at 60 degrees C by the s ame kinetic studies. Chlorophyll a (1 x 10(-4) M) synergistically enha nced the antioxidant effect of vitamin E (4 x 10(-4) M) in heptanol. T he addition of chlorophyll a decreased the rate of vitamin E consumpti on only when methyl linoleate was present, and this reduction did not increase with chlorophyll a concentration. It was noticed that such a synergy did not occur in ethyl heptanoate. The inhibition rate constan ts k(7) and k(12) for reaction between linolylperoxyl radicals and the two antioxidants have been measured at 60 degrees C in heptanol. Valu es of k(7) for vitamin E and k(12) for chlorophyll a were 1.6 x 10(-6) M(-1) s(-1) and 4 x 10(3) M(-1) s(-1), respectively. These relative r eactivities show that peroxyl radicals react almost quantitatively wit h vitamin E, so that the synergistic effect results from the complexat ion of tocopheroxyl radicals T-. by chlorophyll a (16) T-. + CHL --> T - CHL(.)(+) with k(16) congruent to 6 M(-1) s(-1) at 60 degrees C in h eptanol.