Phosphorescence of intermediates of the terminal stage of chlorophyll biosynthesis in plants

Phosphorescence (radiative triplet state deactivation) of the intermediates involved in chlorophyll a photobiosynthesis from protochlorophyllide (Pchld) was found in greening leaves at 77K. Photoactive Pchld forms showed phosphorescence with the major spectral bands at 920 and 970 nm and 2.5-3 msec lifetime. Photochemical formation of the primary non-fluorescent intermediates (NFI) was accompanied by quenching of Pchld fluorescence and phosphorescence, phosphorescence quenching being less efficient by a factor of two. The primary chlorophyllide form Chld 684/676 that appeared as a result of NFI dark transformation showed phosphorescence with a maximum at 980 nm and 2 msec lifetime. Secondary chlorophyllide forms Chld 690/680 and Chld 695/685, the products of photochemical Chld 684/676 transformation and intermediates in biosynthesis of chlorophyll of the light harvesting complex, emitted phosphorescence with maxima at 990-995 and 1005-1010 nm and 1-2 msec lifetimes. Chlorophyll Chl 675/668, the major product of the non-photochemical "side" reaction of Chld 684/676 that probably underlies the biogenesis of photosystem II, exhibited phosphorescence with a maximum at 954-960 nm and 2 msec lifetime. Analysis of the difference spectra of the side reaction products revealed a minor 930 nm phosphorescence band that might correspond to pheophytin a formed via the "side" reaction. Triplet states of the intermediates were not quenched by carotenoids, and the quantum yields of their population at 77K were not less than 10%. Under physiological conditions the triplet intermediates might be involved in photochemical reactions, in particular, in singlet oxygen photogeneration and photodestruction of the photosynthetic apparatus.