QA Binding in Reaction Centers of the Photosynthetic Purple Bacterium Rhodobacter sphaeroides R26 Investigated with Electron Spin Polarization Spectroscopy

Abstract

The relation between quinone (QA) binding and electron transport in reaction centers (RCs) of photosynthetic purple bacteria is investigated, using electron spin polarization (ESP) X-band (9 GHz) EPR as a tool to probe for structural changes resulting from charge separation and stabilization and from replacing the native QA molecule with other quinones. We present a study of possible changes in QA-binding that might be responsible for the remarkably prolonged lifetime of the charge-separated state at cryogenic temperatures for RCs of Rhodobacter sphaeroides R26 cooled under illumination [Kleinfeld, D., et al. (1984) Biochemistry 23, 5780-5786]. It is shown that this effect is not caused by a major reorientation of the chromophores. Furthermore, we studied the effects of structurally different quinones functioning as primary electron acceptor in different purple bacteria. With simulations of ESP X-band spectra of the spin-polarized secondary radical pair P.+QA.+- in menaquinone-reconstituted, Zn(2+)-substituted RCs of Rb. sphaeroides R26, we show that quinone reconstitution is highly selective for site and orientation. Furthermore, we find that a very small exchange interaction between P.+ and QA.+- (magnitude of JPQ approximately 1 microT) is needed to account accurately for the observed relative line intensities at X-band, without affecting the accuracy of the simulations of reported ESP K-band spectra [Füchsle, G., et al. (1993) Biochim. Biophys. Acta 1142, 23-35; Van der Est, A., et al. (1993) Chem. Phys. Lett. 212, 561-568]. This pronounced influence of small values for JPQ on the X-band ESP line shape results from cancellation effects of absorptive and emissive contributions to the spectrum, such that small shifts can be observed. The exchange interaction has opposite sign for the native, ubiquinone-containing RC [viz. JP.UQ = (-0.8 +/- 0.2) microT] and the menaquinone-substituted RC [JP.MK = (+0.3 +/- 0.2) microT]. The implications of these observations for electron-transport theory are discussed.

Time-Resolved High-Frequency and Multifrequency EPR Studies of Spin-Correlated Radical Pairs in Photosynthetic Reaction Center Proteins

“…This result is consistent with other experiments that have shown large changes in the P86/QA-recombination or QA-to QB electron transfer rates can occur without large changes in the donor/acceptor geometries (Franzen et al, 1990;van den Brink et al, 1994;Tiede et aI., 1996). Thus, the reorganization energy for the electron transfer process between p+ 86511 QA and P+865HQA-is a dominant factor that alters the electron transfer rate.…”

Radicals and Radical Pairs in Photosynthesis

“…While for PS I a drastic change of the quinone orientation is observed after substitution with duro-and naphthoquinone no such change is observed in bacterial RC. The result for bacterial RC is consistent with the result of van den Brink et al (366). Van der Est et al (373) suggested weaker hydrogen bonds as an interpretation of the strong variation of the quinone orientation in PS I.…”

“…The spectra reported by van den Brink et at. (366) for RC frozen under light and frozen in the dark are virtually identical. Therefore, the authors concluded that no major light-induced structural change in the orientation of QA with respect to P,,, occurs as a result of the charge transfer.…”

“…The simulations obtained for the P+ (QXFe) spectra for quinone-replaced RC, even under inclusion of possible singlet-triplet mixing in the primary radical pair, are much worse than what is possible for the case of Zn-substituted RC with the native QA (see e.g. Prisner et al (100) and van den Brink et al (366). A likely reason for part of the discrepancies between experiment and simulation for P+ (QiFe) spectra is the neglect of spin relaxation in the model used.…”

Pulsed EPR experiments on radical pairs in photosynthesis: Comparison of the donor‐acceptor distances in photosystem I and bacterial reaction centers

“…Additionally a large isotropic coupling J of more than 10 pT was found in the analysis of the experimental data [8]. However, an isotropic coupling of this magnitude is not consistent with the transient EPR spectra in different frequency bands [9].…”