Charge recombination at low temperature in photosynthetic bacteria reaction centers: Evidence for two conformation states

Abstract

The kinetics of the decay of state P +Q − A at low temperature for bacterial reaction centers present a complex wavelength dependence around the crossing points of the light-induced difference spectrum (approx. 801 and approx. 757 nm). This phenomenon is observed for chromatophores of Rhodospirillum rubrum G9 and Rhodobacter sphaeroides R26 or 241 as well as for isolated reaction centers of the later species. For untreated reaction centers or chromatophores, the kinetics of the absorbance changes measured at wavelengths slightly above 801 nm are about three times faster than the ones measured below 801 nm. These results are taken as evidence of two conformational states for the reaction centers. Each of these two states has a characteristic decay rate, 11.5 and 36.5 ms respectively, in the case of R26 reaction centers, and a slightly different light-induced spectrum. These two states are in the ratio 60/40, the more abundant being the faster one. They are not influenced by the pH, the presence of the secondary acceptor or the removal of the accessory bacteriochlorophyll molecule of the non photoactive branch. These two conformational states are still present in LM particles, although the kinetics of their charge recombination process are slowed down by a factor of 2.5. Preillumination of reaction centers at room temperature, which lengthens the charges recombination process at low temperature (Kleinfeld, D., Okamura, M.Y. and Feher, G. (1984) Biochemistry 23, 5780–5786) affects only the kinetics of the slow decaying state. This light pretreatment leads also to a ratio of 0.5 between the fast and the slow decaying states, compared to 1.5 for dark-adapted reaction centers. The non-exponential decay of the P +Q − A state measured in the long-wavelength band at low temperature is explained by the superposition of the decays of these two conformational states of the reaction center.