SYMMETRICAL STRUCTURAL FEATURES AND BINDING-SITE OF THE PRIMARY ELECTRON-DONOR IN THE REACTION-CENTER OF CHLOROBIUM

The protein binding interactions of the constituent bacteriochlorophyll a molecules of the primary electron donor, P-840, in isolated reaction centers from Chlorobium limicola f thiosulphatophilum and the electronic symmetry of the radical cation P-840(+.) were determined using near-infrared Fourier transform (FT) Raman spectroscopy excited at 1064 nm. The FT Raman vibrational spectrum of P-840 indicates that it is constituted of a single population of BChl a molecules which are spectrally indistinguishable. The BChl a molecules of P-840 are pentacoordinated with only one axial ligand on the central Mg atom, and the pi-conjugated C-2 acetyl and C-9 keto carbonyls are free of hydrogen-bonding interactions. The FT Raman spectrum of P-840(+.) exhibits a 1707 cm(-1) band attributable to a BChl a Cs keto carbonyl group vibrational frequency that has upshifted 16 cm(-1) upon oxidation of P-840; this upshift is exactly one-half of that expected for the one-electron oxidation of monomeric BChl a in vitro. The 16 cm(-1) upshift, thus, indicates that the resulting +1 charge is equally shared between two BChl a molecules. This situation is markedly different from that of the oxidized primary donor of the purple bacterial reaction center of Rhodobacter sphaeroides, (i) which exhibits a 1717 cm(-1) band that has upshifted 26 cm(-1), indicating an asymmetric distribution of the resulting +1 charge over the two constituent BChl a molecules, and (ii) whose H-bonding pattern with respect to the pi-conjugated carbonyl groups is asymmetric. Therefore, according to the Raman data of P-840 and P-840(+.) presented here, the primary electron donor of Chlorobium is a highly symmetric dimer of BChl a molecules; this symmetry is with respect to the protein interactions of P-840 and the positive charge distribution in P-840(+.) The above results and conclusions are consistent with the proposal of a homodimeric structure of the Chlorobium reaction center.