Treatment of freeze-dried spinach chloroplasts with 15% acetone in hexane at 0° removes carotenoids and the majority of the chlorophyll, but leaves the P700 in the membrane residue. Subsequent treatment of the residue with the detergent Triton X-100 solubilizes a small rod-to-ellipsoidal- shaped particle (about 150 × 60 Å) which contains P700 in the ratio of one P700 per 30 chlorophyll molecules and is thought to represent the basic structural unit of photosystem 1. The particles contain small amounts of cytochromes f and b6, but no β-carotene. Titration with ferriferrocyanide redox buffers reveals an E0′ of +480 mV. The particles so obtained, called high P700 (HP700) particles, exhibit a light-induced bleaching at 698 nm which represents the oxidation of P700. A secondary band is observed in the light-induced spectrum at 680 nm, and this band appears to be produced by a red shift of some accessory chlorophyll molecule closely associated with P700. Chemical oxidation of the particle with ferricyanide produces a similar difference spectrum. The kinetics of P700 bleaching reveal a markedly slower rate of oxidation in the light compared with a photosystem 1 particle prepared by the action of Triton X-100 alone (TSF-1). This difference most likely reflects a lowered efficiency of energy transfer between the residual light-harvesting chlorophyll and the reaction center P700. This conclusion is supported by the nature of the HP700 fluorescence at -196°. Compared with TFS-1 preparations, the HP700 particle shows decreased fluorescence at 730 nm (related to P700) and an increased fluorescence at 680 nm. The HP700 particle exhibits a light-induced electron paramagnetic resonance spectrum whose kinetics resemble those of bleaching at 698 nm. The particles photoreduce nicotinamide-adenine dinucleotide phosphate slowly with added crude ferredoxin and ascorbate-2,6-dichlorophenolindophenol under anaerobic conditions. © 1969, American Chemical Society. All rights reserved.
