Chlorophyll Destruction by the Bisulfite-Oxygen System

Destruction of chlorophyll, as determined by the loss in absorbance at 665 nm, occurred in two in vitro systems in the presence of bisulfite in 76% ethanol. The first system required light and O(2) in addition to bisulfite and exhibited an optimum pH of 4. Chlorophyll functioned as a photosensitizer and there was little chlorophyll destruction occurring above pH 5. With 286 mueinsteins m(-2) irradiation, approximately 80% of the chlorophyll was destroyed in three minutes. In the second system, chlorophyll destruction in the presence of bisulfite occurred in the dark and required Mn(2+), O(2), and glycine. Destruction of chlorophyll in this system was much more rapid than in the light system with approximately 70% destruction occurring in two seconds. In both systems, chlorophyll destruction was linked to bisulfite oxidation. The free radical scavengers hydroquinone, butylated hydroxytoluene, 1,2-dihydroxybenzene-3,5-disulfonic acid, and alpha-tocopherol were effective in inhibiting the destruction of chlorophyll in both systems. The singlet O(2) scavengers, 2,5-dimethylfuran and 1,3-diphenylisobenzofuran, were ineffective inhibitors and beta-carotene only slightly effective when tested in the light system. The evidence suggests that in these two systems chlorophyll was destroyed by free radicals, probably superoxide radical, which was produced during the aerobic oxidation of bisulfite.