Initial Charge Separation in the Reaction Center of Rhodobacter sphaeroides

3 Physik-Department derTechnischen Universitat, W 8000 Miinchen 2, FRG Present address Central Research and Development Dept., 

Electron transfer within the bacterial photosynthetic reaction center (RC) has been shown by a variety of methods to only occur along one branch of the chromophores (the so-called L or A branch) (Kirmaier and Holten, 1987 Parson, 1987). Recent room temperature kinetic data support a model in which the electronically excited primary donor (a dimer of bacteriochlorophylls, P) decays with a time constant of 3.5 ps (in Rhodobacter (Rb,) sphaeroides) to populate a very short-lived intermediate containing P+ and a reduced bacteriochlorophyll (B ) molecule (Holzapfel et al, 1989,1990). Subsequently the electron is transferred to a bacteriopheophytin (H) with a time constant of 0.9 ps. The existence of the state P+H is generally accepted but there is presently debate over the existence of the 

Subsaturating photobleaching spectra were taken on chromatophores as described previously (Farchaus et al, 1990a). Room temperature absorption spectra were taken on a Shimadzu UV-160 spectrophotometer or an Aminco DW2a spectrophotometer. Low temperature absorption and linear dichroism (LD) spectra (lOK) were taken on a previously described apparatus (Breton, 1985) of RCs immobilized in polyacrylamide gels and squeezed uniaxially (Breton, 1988). 

Sub-saturating photobleaching spectra of purified chromatophores isolated from A) Wild type cells, B) FM210 cells and C) LM210 cells. All three were grown semi-aerobically. The chromatophores were suspended in 50 mM MOPS, 100 mM KCl containing 9 iM valinomycin and 1 mM ascorbate. 

The LD spectra (Fig. 3) of the three RCs are essentially the same. There are no changes in the angles of the transition moments of P, the monomer Bchls and the Bphe molecules beyond the limits of accuracy of the technique ( 5°). Within the limits, though, there is an indication that the angle between the C2 axis and the Qy transition of the Bphe molecules might be somewhat smaller for the mutants. 

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The Role of Tyrosine M210 in the Initial Charge Separation in the Reaction Center of Rhodobacter sphaeroides... 

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