The accessory bacteriochlorophyll: a real electron carrier in primary photosynthesis.

Abstract

The primary electron transfer in reaction centers ofRhodobacter sphaeroides is studied by subpicosecond absorption spectroscopy with polarized light in the spectral range of 920-1040 nm. Here the bacteriochlorophyll anion radical has an absorption band while the other pigments of the reaction center have vanishing ground-state absorption. The transient absorption data exhibit a pronounced 0.9-ps kinetic component which shows a strong dichroism. Evaluation of the data yields an angle between the transition moments of the special pair and the species related with the 0.9-ps kinetic component of 26 ± 8. This angle compares favorably with the value of 29°expected for the reduced accessory bacteriochlorophyll. Extensive transient absorbance data are fufly consistent with a stepwise electron ransfer via the accessory bacteriochlorophyll.In the primary processes of bacterial photosynthesis, absorbed light energy is stored via an electron transfer within the reaction center (RC). While the molecular structure of two bacterial reaction centers has been known for a number of years (1-3), the detailed molecular mechanism of the electron transfer is still the subject of intense investigations (4-20). There is general agreement that the first electron transfer process starts at a pair of bacteriochlorophyll (BChl) molecules-the special pair P-which acts as the primary donor. The

Primary Electron Transfer Kinetics in Membrane-Bound Rhodobacter sphaeroides Reaction Centers: A Global and Target Analysis

“…The species-associated difference spectra (SADS) ( Figure 6) were consistent with the expected spectral properties of the intermediate states specified in the scheme. In particular, Figure 6C shows the 810 and 860 nm bleach, together with the positive band at 1020 nm, that would be expected for the radical pair state P + B L - (5,8,9,12,18). Note the difference of the 1020 nm band with the SADS of Figure 6D,E in the overlay Figure 6F.…”

“…The most favourable fit to the full set of experimental data from 690-1060 nm required five lifetimes. A combination of the model for RC kinetics from (5,6,8,9,12,18) and the idea of radical pair relaxation (5,18,(38)(39)(40) gave a five-component kinetic scheme ( Figure 5) that offered a satisfactory description of the data. The SADS of the two excited states and three radical pair states involved in this scheme were consistent with what is known about the spectra of these states.…”

“…The first concerns the much debated role of the accessory BChl B L , and specifically the proposal that the P + B L -radical pair state is formed as a true intermediate during electron transfer from P* to P + H L - (5,6,8,9,12,18). The analysis presented in this paper is entirely consistent with the view that the P + B L -state is formed as a detectable intermediate, with in our case a lifetime of 1.9 ps in Q A -reduced, membrane-bound wild type Rb.…”

“…First, a wavelength dependence of the kinetics of the bandshift of the B absorption band has been observed (11). An additional time component has been assigned in this spectral region, and also around 1020 nm (12), that was attributed to the formation of B L -as an intermediate in the electron transfer process (8,9). Furthermore, fluorescence emission from P* has been found to decay with multiple time constants that extend from a few to hundreds of picoseconds (7,(12)(13)(14)(15)(16)(17)(18)(19)(20).…”

Tuning electron transfer by ester‐group of chlorophylls in bacterial photosynthetic reaction center

“…So far there is no report about the different B A ester-group orientations in bRC. In general, the formation of B À A is detected at 1020 nm in optical absorption spectra [27].…”