The Bruker Lecture. Identification and characterization of the primary donor in bacterial photosynthesis: a chronological account of an EPR/ENDOR investigation

A chronological account is presented of the work that has led to the identification and characterization of the primary donor, D, in photosynthetic reaction centres (RCs) of Rb. sphaeroides. The observation that the EPR linewidth of D+ is ca. 40% smaller than that of the bacteriochlorophyll (BChl+) monomer in vitro led to the hypothesis that D+ is a dimer in which the electron is shared between two bacteriochlorophylls. This was convincingly proved by ENDOR experiments that showed the spin densities in BChl+ to be, on the average, twice as large as in D+. The dimer model was confirmed a decade later, when the structure of the reaction centre (RC) was determined by X-ray diffraction. The electronic structure of D+ was investigated by ENDOR both in solutions and in single crystals of RCs. The results showed that in the native dimer the spin density is asymmetrically distributed, favouring the A (also called the L) half by ca. 2:1. Electron densities were also obtained in two mutant RCs in which either one or the other BChl was changed to bacteriopheophytin (BPhe). The unpaired spin in these mutants (heterodimers) resides on the BChl half. The spin density distribution is significantly different in the two heterodimers and is also different from the spin density distribution in the respective halves of the homodimer. These differences point to the effect of the protein environment on the spin density. Molecular orbital calculations (RHF-INDO/SP) are in good agreement with the experimentally determined spin densities. The concluding section presents speculations on the possible advantages of a dimer over a monomer in the primary charge-separation process.