Structure of Rhodopseudomonas viridis reaction centers. Absorption and photoselection at low temperature

Abstract

The polarization excitation spectrum of Rhodopseudomonas viridis reaction centers is similar to those obtained with Rps. sphaeroides R-26 and 2-4-1 reaction centers. However, the absorption spectrum and photoinduced absorption changes are much better resolved, especially at low temperature, than in bacteriochlorphyll (BChl) a-containing reaction centers. This allows the separation of band shifts from bleaching in the light-induced difference spectra. In particular, the absorption band bleaching observed at 850 nm is shown to occur perpendicular to the 1000 nm long-wavelength band. This lends strong support to the assignment of the 850 nm band to the second excitonic component of the special pair. ‘Voyeur’ BChl absorption changes (presumably band shifts) upon photoformation of state P + Q − 1 are difficult to analyze quantitatively. The polarization values for these absorption changes are not easily reconcilable simply with absorption band shifts. We suggest that the relative orientation between the voyeur BChl transition moments and the long-wavelength transitions changes after charge separation. This reorientation may be responsible for the slow rate of back reaction between P + and Q − 1. At 4.2 K two bands are apparent in the long-wavelength absorption band. We propose that this structure is due to a charge-transfer state between the special pair and B, the primary BChl acceptor. Finally, we show that the low extinction coefficient of the absorption band at 808 nm, and the appearance of the long-wavelength band (1300 nm) for the neutral BChl molecule left after photooxidation of one molecule of the special pair, can be explained in terms of quantum mechanics.