bacteriochlorophylls and pheophytin-a

Ultrafast transient absorption (TA) spectroscopy was used to study electron transfer (ET) at 100 K in native (as isolated) reaction centers (RCs) of the green filamentous photosynthetic bacterium Chloroflexus (Cfl.) aurantiacus. The rise and decay of the 1028 nm anion absorption band of the monomeric bacteriochlorophyll a molecule at the B

Topics: Bacteriochlorophyll A; Chloroflexus; Photosynthetic Reaction Center Complex Proteins; Temperature

Femtosecond transient absorption spectroscopy in the range of 500-1040 nm was used to study electron transfer at 5 K in reaction centers of Rhodobacter sphaeroides R26 in which the bacteriopheophytins (BPhe) were replaced by plant pheophytin a (Phe). Primary charge separation took place with a time constant of 1.6 ps, similar to that found in native RCs. Spectral changes around 1020 nm indicated the formation of reduced bacteriochlorophyll (BChl) with the same time constant, and its subsequent decay in 620 ps. This observation identifies the accessory BChl as the primary electron acceptor. No evidence was found for electron transfer to Phe, indicating that electron transfer from BA- occurs directly to the quinone (QA) through superexchange. The results are explained by a model in which the free energy level of P+Phe- lies above that of P+BA-, which itself is below P*. Assuming that the pigment exchange does not affect the energy levels of P* and P+BA-, our results strongly support a two-step model for primary electron transfer in the native bacterial RC, with no, or very little, admixture of superexchange.

Topics: Bacteriochlorophylls; Cold Temperature; Electron Transport; Kinetics; Light-Harvesting Protein Complexes; Pheophytins; Photosynthetic Reaction Center Complex Proteins; Rhodobacter sphaeroides; Spectrophotometry

The major part (> 90%) of bacteriopheophytin a in reaction centers (RCs) of Rhodobacter sphaeroides was substituted by plant pheophytin a. In modified RCs the photochemical formation of P+Qa- occurs with with a quantum efficiency of 79%. The intermediary state P+I- displayed a recombination time constant of 1.5 ns, and the electron transfer from I- to Qa was characterized by a time constant of 540 ps. On the basis of spectral properties of P+I- for native and modified RCs, it was suggested that bacteriopheophytin, as well as bacteriochlorophyll monomers located in L protein branch, have a transition at 545 nm with approx. equal extinction coefficients. Accordingly, the state P+I- in modified RCs is proposed to consist of a thermodynamic mixture of P+BL- (approximately 80%) and P+Phe- (approximately 20%).

Topics: Bacteriochlorophylls; Electron Transport; Light-Harvesting Protein Complexes; Pheophytins; Photosynthetic Reaction Center Complex Proteins; Plant Proteins; Rhodobacter sphaeroides; Spectrophotometry; Time Factors