A photochemically active subchromatophore fragment isolated from Chromatium chromatophores after treatment with Triton X-100 shows absorbance decreases at 890 (reaction center bacteriochlorophyll, Bchl) and 275 mμ (ubiquinone) with rise times ≤5 X 10−5 sec and half-decay times of 0.25 sec. The light minus dark difference spectrum between 220 and 920 mμ consists essentially of changes attributed to the photooxidation of the reaction center Bchl P890 and photoreduction of endogenous ubiquinone. The effects of pH of the suspending medium in the range of 3-14 and that of heating the fragments between 40 and 80° are very similar for both P890 and ubiquinone reactions. Both reactions are independent of temperature from room temperature down to 77°K. Quantum requirement measurements yielded an average of 6 quanta required for each molecule of P890 oxidized or ubiquinone reduced. Chemical difference spectra of the subchromatophore fragments showed they contain P890 at a ratio of 1:9 to the bulk Bchl. The subchromatophore fragment also contains 3.3 mole % cytochrome relative to the total Bchl. Ubiquinone is present at a high concentration relative to the bulk Bchl, approximately 1:2. The photooxidation of P890 and photoreduction of ubiquinone are both abolished when sufficient ferricyanide or dithionite is present. Presumably when P890 is chemically oxidized or when ubiquinone is reduced, the coupled redox reaction is prevented. When the reduced forms of dyes such as N,N,N’,N’-tetramethyl-p-phenylenediamine (TMPD), 2,6- dichlorophenolindophenol (DPIP), or phenazine methosulfate (PMS) are present, the P890 absorbance either becomes smaller (with a faster decay rate) or disappears completely, but the initial absorbance change due to ubiquinone reduction, which is presumed to be coupled to P890 photooxidation, is unaffected. The disappearance of the P890 absorbance change is caused by an efficient transfer of electrons from the reduced dye to the photooxidized P890, which turns over very rapidly so that the transient cannot be seen. In the presence of reduced dyes, a slower secondary ubiquinone reduction can also be observed, which takes place following the initial fast transfer of electrons from P890 to ubiquinone. The P890-ubiquinone reaction has the properties of a primary photochemical reaction in the bacterial system. © 1968, American Chemical Society. All rights reserved.
