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 (Pchl d) was found in greening leaves at 77K. Photoactive Pchld forms showed phos phorescence with the major spectral bands at 920 and 970 nm and 2.5-3 msec lifetime. Photochemical formation of the primary non-fluorescent intermedia tes (NFI) was accompanied by quenching of Pchld fluorescence and phosphores cence, phosphorescence quenching being less efficient by a factor of two. T he primary chlorophyllide form Chld 684/676 that appeared as a result of NF I dark transformation showed phosphorescence with a maximum at 980 nm and 2 msec lifetime. Secondary chlorophyllide forms Chld 690/680 and Chld 695/68 5, the products of photochemical Chld 684/676 transformation and intermedia tes in biosynthesis of chlorophyll of the light harvesting complex, emitted phosphorescence with maxima at 990-995 and 1005-1010 nm and 1-2 msec lifet imes. Chlorophyll Chi 675/668, the major product of the non-photochemical " side" reaction of Chld 684/676 that probably underlies the biogenesis of ph otosystem II, exhibited phosphorescence with a maximum at 954-960 nm and 2 msec lifetime. Analysis of the difference spectra of the side reaction prod ucts revealed a minor 930 nm phosphorescence band that might correspond to pheophytin a formed via the "side" reaction. Triplet states of the intermed iates were not quenched by carotenoids, and the quantum yields of their pop ulation at 77K were not less than 10%. Under physiological conditions the t riplet intermediates might be involved in photochemical reactions, in parti cular, in singlet oxygen photogeneration and photodestruction of the photos ynthetic apparatus.