Reversible photochemical holeburning in Rhodopseudomonas viridis reaction centers

The change in absorption of the band assigned to the primary electron donor in R. viridis reaction centers (P960) has been measured at 1.4–2.1 K following narrow bandwidth excitation at many wavelengths within the P960 absorption band. In all cases nearly the entire band was photobleached (full width at half-maximum about 400 cm −1). This result can be explained by either an ultra-fast decay process from the initially excited state or a substantial difference in the equilibrium nuclear configuration between the ground and excited state of the donor. The latter may arise if the excited state of the donor has substantial charge transfer character. The results suggest a charge separation step which precedes formation of a bacteriopheo-phytin b anion.