Photosynthetic reaction center transients, P 435 and P 424, in Chromatium D

Abstract

1. The correlation of kinetic, saturation and potentiometric evidence shows that P 435 is a spectral component of the same reaction center as P883 in Chromatium D. 2. There is only one cytochrome oxidizing reaction center P883/P 435, since the spectrum of P 435 is the same independent of which cytochrome (C 553 or C 555) is oxidized and the P 435 reduction kinetics correlate with the oxidation kinetics of either cytochrome, depending upon which is being oxidized. 3. The oxidation half-time of P883/P 435 is less than 50 nsec. 4. The E m of P883/P 435 is +486 mV and that of its primary electron acceptor is −134 mV. 5. P 424 is observable only when the primary electron acceptor, X, is in the reduced state prior to the flash: (A) at room temperature in chromatophores between −318 and −145 mV and (B) at 77°K over the same potential range (first laser flash) or at higher potentials (subsequent laser flashes) under conditions in which the first laser flash permanently reduces X. 6. P424, P883/P 435, cytochrome C 553 and cytochrome C 555 are all associated with Thornber's subchromatophore Fraction A, but not Fraction B. 7. The rise half-time of P 424 in chromatophores is 50 nsec at room temperature and 200 nsec at 77°K. Its recovery half-time is 2.2 to 2.4 μsec at both temperatures in chromatophores or 16 msec in whole cells at 77°K. 8. P 424 might represent either an oxidation of bacteriochlorophyll spectrally modified by the presence of reduced primary electron acceptor or a light-induced reduction state of bacteriochlorophyll. 9. Thermodynamically, it is possible that this new species might be a link in the direct reduction of NAD +.