Abstract
A chlorophyll type pigment (F698) fluorescing maximally at 698 mmu at 77 degrees K has been observed in preparations of chlorophyll. This fluorescence is quenched by small amounts of naturally occurring materials, including plastoquinone and the ubiquinones, and by nitrobenzene, probably by formation of a nonfluorescent complex. Fluorescence quenching does not occur in the presence of carotenes, xanthophylls, or reduced plastoquinone and ubiquinone. The fluorescence is sharply temperature dependent, with a steep rise in intensity occurring at 165 degrees K. At 77 degrees K the fluorescence yield is between 0.8 and 1.0. The red absorption maximum of the pigment is at 675 mmu at room temperature and at 688 mmu at 77 degrees K. In vivo, a low temperature emission is also observed at 698 mmu, and this fluorescence is quenched by nitrobenzene. It is proposed that the pigment found in vitro is also the one responsible for emission at 698 mmu in vivo. A reaction of F698 with plastoquinone is suggested as the primary photochemical step in system II of photosynthesis.