Mechanism of the Allomerization of Chlorophyll: Inhibition of the Allomerization by Carotenoid Pigments

Autooxidation of Chlorophyll, Free Radicals, Carotenoids, Photosynthesis It is shown that the allomerization of chlorophylls a and b is effectively inhibited by carotenoid pigments. In the light of this finding, two possible mechanisms are considered for the allomerization. One assumes the addition of singlet oxygen to the 9,10-double bond of the chlorophyll (Chi) enolate anion to yield a C-10 hydroperoxide or a dioxetane derivative. The other assumes a free-radical chain reaction involving Chi enolate anion, triplet oxygen, Chi C-10 radicals and peroxide radicals, and Chi C-10 hydroperoxide. The observation that the allomerization and its inhibition by carotenoids occur under carefully controlled dark conditions precluding singlet oxygen formation provides substantial support to the latter mechanism. Further evidence for the free-radical mechanism is obtained by observing the increase of the allomerization rate when air is replaced with pure oxygen from a container. The rate increases to about two-fold in the methanolic Chi solution containing no carotenoid but remains close to zero in the Chi solution containing an equimolar amount of ^-carotene. The relevance of the results to photosynthesis and cancer research is briefly discussed.