PRESENCE OF 2 CHLOROPHYLL-A' MOLECULES AT THE CORE OF PHOTOSYSTEM-I

The pigment composition in cells and thylakoid membranes of a cyanobacterium, and in leaves and intact or P700-enriched chloroplasts of higher plants, has been reinvestigated by solvent extraction/HPLC. In acetone extracts, the amount of chlorophyll (Chl) a' relative to Chl a or P700 was nearly twice that in chloroform extracts: the latter gave a result in line with the stoichiometry of Chl a'/P700 congruent-to 1 found by chloroform extraction (Kobayashi, M. et al. (1988) Biochim. Biophys. Acta 936, 81-89). By two-step extraction, first with chloroform then with acetone, a roughly equal amount of Chl a' was extracted in each step, while more than 90% of Chl a was extracted already in the first step. For chloroplasts heavily enriched in P700, the residue of the first extraction exhibited a Chl a'/Chl a molar ratio outreaching the equilibrium value (0.33), as verified by the second extraction/HPLC. These results evidence that two Chl a' molecules exist at the Photosystem (PS) I core, and just one of them is at a site to which only such a hydrophilic molecule as acetone is accessible. A preliminary discussion is given on the possible role of the two Chl a' molecules in PS I.