Energy transfer between protochlorophyllide molecules: Evidence for multiple chromophores in the photoactive protochlorophyllide-protein complex in vivo and in vitro

Abstract

The absorption and fluorescence properties of dark-grown bean leaves, of isolated prolamellar body membranes and of an isolated, active protochlorophyllide-protein complex (protochlorophyll holochrome) were examined at 77 °K. The leaf is characterized by two main forms of protochlorophyll(ide) absorbing at about 650 and 637 nm respectively and a minor form absorbing at 628 nm. PChl-650 †2 †2 Abbreviations used: PChl-650, PChl-637 and PChl-628, protochlorophyll forms having absorption maxima at 650 nm, 637 nm and 628 nm, respectively; PChl-H, protochlorophyll holochrome. and PChl-637 are photoconvertible to chlorophyllide a at room temperature, but PChl-628 is non-convertible. Prolamellar body membranes closely resemble the whole leaf in their spectroscopic properties, but the soluble protochlorophyll holochrome shows an absorption maximum at 637 nm and minor components at 650 nm and 628 nm. The fluorescence emission spectra at 77 °K of dark-grown bean leaves and prolamellar bodies show two main bands at 655 and 630 to 632 nm. The emission at 655 nm emanates from PChl-650, but it is activated by light absorbed both by PChl-650 and PChl-637. Energy absorbed by PChl-637 is transferred to PChl-650 with high efficiency. Fluorescence emitted at 630 nm originates from PChl-628. Protochlorophyll holochrome gave a fluorescence emission spectrum which resembled the spectra obtained with leaves and prolamellar bodies, except for an additional emission band at 643 nm. We concluded that at 77 °K the main form of protochlorophyllide in the holochrome (PChl-637) transfers a large proportion of its excitation energy to PChl-650. After the photoconversion of part of the protochlorophyllide in the holochrome, excitation energy is transferred to the newly formed chlorophyllide a, and the efficiency of transfer increases as the fraction of chlorophyllide a molecules increases. The observation of energy transfer among the chromophores of protochlorophyll holochrome indicates that a number of protochlorophyllide molecules are organized into a unit or attached to the single apoprotein molecule. The observed efficiencies of energy transfer are consistent with a model of protochlorophyll holochrome containing at least four chromophores.