Effects of zwitterionic detergents on the electronic structure of the primary donor and the charge recombination kinetics of P+QA- in native and mutant reaction centers from Rhodobacter sphaeroides

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Abstract

The effects of detergents on the electronic structure of the oxidized primary donor P+ and the time constant τAP of the P+QA- charge recombination at ambient temperatures have been investigated in native and mutant reaction centers (RCs) from Rhodobacter sphaeroides. It is shown that N-lauryl-N,N-dimethyl-3-ammonio-1-propane sulfonate (SB12) induces a transition to a second distinct conformation of the RC. In the case of the wild type and the mutant FY(M197), in which a hydrogen bond is introduced to the 2-acetyl group of the dimer half of P that is associated with the M-subunit of the RC, the conformational change causes a more asymmetric spin density distribution between the two bacteriochlorophyll moieties of P+ in favor of the L-half. For both types of RCs the time constant τAP depends on the SB12/RC ratio as does the position of the long-wavelength band of P, λmax. The increase of τAP by ∼30 ms and the shift of λmax from ∼866 nm to ∼851 nm are indicative for the conformational change. In addition, a smaller linear increase of τAP with increasing SB12/RC ratio is superimposed on the variation of τAP due to the conformational change. Similar effects of SB12 on the optical spectra as well as on τAP are also observed for the two heterodimer mutants HL(L173) and HL(M202), in which one of the bacteriochlorophylls of P is replaced by a bacteriopheophytin. There are no clear indications for a correlation of τAP with the localization of the positive charge in P+. Furthermore, it is concluded from the dependence of τAP on the SB12/RC ratio that the single-site mutations do not affect the standard free energy difference of the two conformations to a measurable extent.

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