Investigation of B-branch electron transfer by femtosecond time resolved spectroscopy in a Rhodobacter sphaeroides reaction centre that lacks the QA ubiquinone

Solid-State NMR Analysis of the Dynamics of Cofactors: Comparison of Heliobacterial and Purple Bacterial Reaction Centers

Photosynthetic reaction centers (RCs) serve as natural engines converting solar energy to chemical energy. Understanding the principles of efficient charge separation and light-induced electron transfer (ET) between the chlorophyll-type pigments might guide the synthesis for artificial photosynthetic systems. We present detailed insight into the dynamics at the atomic level using solid-state NMR techniques applied to the RCs of Heliobacillus (Hb.) mobilis (HbRCs) and the purple bacterium Rhodobacter (R.) sphaeroides (PbRCs). It is assumed that heliobacteria were among the first phototrophic organisms; therefore, their RC can be regarded as ancient. They are constructed homodimerically with perfect C2 symmetry, enabling ET over both branches of cofactors. Modern RCs of R. sphaeroides wild-type (WT) have higher redox power and are functionally highly asymmetric. The dynamics of the cofactors in both RCs has been explored using nuclear hyperpolarization, induced by the solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) effect. Based on the individual incorporation of 13C positions of the cofactors (through supplementation by 13C-δ-aminolevulinic acid), photo-CIDNP magic-angle spinning (MAS) NMR experiments provide access to the local dynamics of the cofactors along the ET path over a broad range of time scales. Theoretical analysis of the dynamic deformation of these macrocycles is also discussed in terms of function. The dynamics observed in HbRCs appears to be correlated to ET. The cofactors in PbRC are significantly less dynamic than those in the HbRC. Relevance for efficiency and redox properties are discussed.

Protein Vibration Effects on Primary Electron Transfer Dynamics in Rhodobacter sphaeroides Photosynthetic Reaction Center

Primary electron transfer (ET) in the chromophore subsystem in a bacterial reaction center (RC) is a unique process, and is coupled with the protein motion, which, like the ET, is caused by photoexcitation of these chromophores. ET is also coupled with dissipative processes, which are caused by interaction between chromophores and vibrations of its surrounding protein. We propose a new dynamics calculation method that accounts for both these effects of protein vibrations. Within this method, the photoinduced protein motion causes an addition of coherent component to the ET rate. We performed dynamics calculation using this method and parameters, which were determined from the ab initio wave functions of the chromophore subsystem and protein normal vibrational modes. We showed that it is this protein motion that causes oscillations in the time-dependencies of stimulated emission intensities and of absorption at 1020 nm. Moreover, the latter oscillations are related to the coherent component of the ET rate.

The polyphasic chlorophyll a fluorescence rise measured under high intensity of exciting light

Chlorophyll a fluorescence rise caused by illumination of photosynthetic samples by high intensity of exciting light, the O-J-I-P (O-I₁-I₂-P) transient, is reviewed here. First, basic information about chlorophyll a fluorescence is given, followed by a description of instrumental set-ups, nomenclature of the transient, and samples used for the measurements. The review mainly focuses on the explanation of particular steps of the transient based on experimental and theoretical results, published since a last review on chlorophyll a fluorescence induction [Lazár D (1999) Biochimica et Biophysica Acta 1412, 1-28]. In addition to 'old' concepts (e.g. changes in redox states of electron acceptors of photosystem II (PSII), effect of the donor side of PSII, fluorescence quenching by oxidised plastoquinone pool), 'new' approaches (e.g. electric voltage across thylakoid membranes, electron transport through the inactive branch in PSII, recombinations between PSII electron acceptors and donors, electron transport reactions after PSII, light gradient within the sample) are reviewed. The K-step, usually detected after a high-temperature stress, and other steps appearing in the transient (the H and G steps) are also discussed. Finally, some applications of the transient are also mentioned.

Rewiring photosynthesis: engineering wrong-way electron transfer in the purple bacterial reaction centre

The purple bacterial reaction centre uses light energy to separate charge across the cytoplasmic membrane, reducing ubiquinone and oxidizing a c-type cytochrome. The protein possesses a macroscopic structural two-fold symmetry but displays a strong functional asymmetry, with only one of two available membrane-spanning branches of cofactors (the so-called A-branch) being used to catalyse photochemical charge separation. The factors underlying this functional asymmetry have been the subject of study for many years but are still not fully understood. Site-directed mutagenesis has been partially successful in rerouting electron transfer along the normally inactive B-branch, allowing comparison of the kinetics of equivalent electron transfer reactions on the two branches. Both the primary and secondary electron transfer steps on the B-branch appear to be considerably slower than their A-branch counterparts. The effectiveness of different mutations in rerouting electron transfer along the B-branch of cofactors is discussed.