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

Abstract

The effects of detergents on the electronic structure of the oxidized primary donor P⁺ and the time constant τ_AP of the P⁺Q ⁻_A 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 τ_AP depends on the SB12/RC ratio as does the position of the long-wavelength band of P, λ_max. The increase of τ_AP by ∼30 ms and the shift of λ_max from ∼866 nm to ∼851 nm are indicative for the conformational change. In addition, a smaller linear increase of τ_AP with increasing SB12/RC ratio is superimposed on the variation of τ_AP due to the conformational change. Similar effects of SB12 on the optical spectra as well as on τ_AP 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 τ_AP with the localization of the positive charge in P⁺. Furthermore, it is concluded from the dependence of τ_AP 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.