After reduction of P•+ by an exogenous cytochrome c 2, P can be excited again, leading to the transfer of a second electron to QB •− in a process that is coupled to the uptake of two protons. The generated hydroquinone QBH2 then carries the electrons and protons to the cytochrome bc 1 complex in a cycle that generates the proton gradient needed for the creation of energy-rich compounds. Fig. 1 (a) Cofactors in the bacterial photosynthetic RC from Rb. sphaeroides (PDB entry 1M3X; Camara-Artigas et al. 2002). (b) Structure of the primary donor of the RC from Rb. sphaeroides with the two BChl

a molecules PL and PM (phytyl chain truncated), and the three mutated residues His L168, Asn L170, and Asn M199 (PDB entry

1M3X; Camara-Artigas et al. 2002). (c) BI 6727 in vivo Molecular structure of bacteriochlorophyll a (BChl a) with IUPAC Momelotinib nmr numbering; the two methyl groups 21 and 121 and the β-protons 7, 8, 17, and 18 are indicated The two BChls that form P overlap at the ring A position with a separation distance of 3.5 Å (see e.g., Allen et al. 1987; Yeates et al. 1988; Ermler et al. 1994; Stowell et al. 1997). Due to the close contact, the two BChls are electronically coupled and the wavefunction of the unpaired electron is distributed over the conjugated systems of both macrocycles. This has been shown by some of the earliest spectroscopic measurements on the RC, in which a dimeric structure was postulated for the primary donor (“special pair hypothesis”)(Norris et al. 1971; 1975; Feher et al. 1975). Electron paramagnetic resonance, EPR, and its advanced multiple resonance methods (ENDOR/TRIPLE) are well-suited for the detailed characterization of the electronic structure of P•+ by mapping the spin density distribution over the conjugated system. In wild type, the distribution most is asymmetric with more of the spin density being located on the L-side of P (PL) than the M-side (PM)(Geßner et al. 1992; Lendzian et al. 1993; Rautter et al. 1994; 1995; 1996; Artz et al. 1997; Müh et al. 2002; Lubitz et al. 2002). Due to the large number of protons in the BChl macrocycle

(Fig. 1c) that interact with the unpaired electron of P•+, the EPR spectrum shows just a single, unresolved line with a linewidth ΔB pp (peak-to-peak) of 9.6 G (Norris et al. 1971; McElroy et al. 1972; Feher et al. 1975). The linewidth is reduced as compared to that of monomeric BChl a •+ (~14 G at room temperature) due to the dimeric character of P•+ (Norris et al. 1971; 1975; McElroy et al. 1972; Feher et al. 1975; Lendzian et al. 1993). Details of the spin density distribution can be obtained by determination of the hyperfine couplings (hfcs) via electron nuclear double resonance, ENDOR (Kurreck et al. 1988; Möbius et al. 1982). If the radical–protein complex rotates fast enough to average out all anisotropic contributions of the hfc (and g) tensors only isotropic interactions remain.