Electron Spin Resonance of Chlorophyll and the Origin of Signal I in Photosynthesis

Abstract

essential that in vitro esr observations be interpreted in terms of the chlorophyll species actually present. Chlorophyll is able to act both as electron donor and electron acceptor in chargetransfer complexes. The central Mg atom of chlorophyll is coordinatively unsaturated when it has the coordination number 4, and at least one of the Mg axial positions must always be occupied by an electron donor group. In the absence of other nucleophiles, the ketone C-O function in Ring V of one chlorophyll molecule serves as donor to the Mg atom of another, forming chlorophyll dimers, (Chl2), or oligomers, (ChlW)3. Extraneous nucleophiles (bases) can compete for the coordination site at Mg, with disruption of the chlorophyllchlorophyll interactions, to form chlorophyll-ligand adducts, (Chl-L). The nature of the nucleophile determines whether the chlorophyll-nucleophile adduct is monomeric or polymeric. Bifunctional ligands, such as dioxane, pyrazine, 1, diazobicyclo(2.2.2.)octane, and, in particular, water can cross-link chlorophyll molecules or chlorophyll dimers by coordination to Mg to form large (chlorophyll-nucleophile) micelles (to be published). The chlorophyll species present in a particular experiment are very sensitive to temperature, adventitious nucleophiles such as water, and solvent, factors not always taken into account in previous work. EXPERIMENTAL METHODSChlorophyll samples were dried, prior to solution preparation, by codistillation with CCL (36). The solvents used for in vitro measurements were first dried over Linde 3A molecular sieve and degassed under reduced pressure. Solutions were prepared and oxidant was added in a nitrogen-filled dry box or on the vacuum line.Irradiations were performed with a 150-W Varian Eimac lamp. Infrared components were removed by 2 cm of water and 2 dichroic infrared-rejecting filters. All irradiations were by red light with a Corning 2404 sharp cut-off filter. All in vitro measurements were made in 4-mm quartz esr tubes at -170'C; in vivo measurements were made at room temperature on concentrated slurries of cells held in a Varian water cell (V4548).625

Primary acceptor in bacterial photosynthesis: obligatory role of ubiquinone in photoactive reaction centers of Rhodopseudomonas spheroides.

“…The primary electron donor is well characterized and is thought to be a specialized bacteriochlorophyll dimer (1)(2)(3)(4)(5). The primary* electron acceptor, on the other hand, has so far eluded definitive identification.…”

EPR, ENDOR, and Special TRIPLE measurements of P•+ in wild type and modified reaction centers from Rb. sphaeroides

“…sphaeroides 2.4.1 (see for example Feher et al 1975;Norris et al 1971;Artz et al 1997). The spectrum of the four mutant RCs at pH 8.0 were fitted yielding the same gvalue and different Gaussian linewidths.…”

“…1c) that interact with the unpaired electron of P •? , the EPR spectrum shows just a single, unresolved line with a linewidth DB 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 •?…”

“…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;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 •?…”

“…(*14 G at room temperature) due to the dimeric character of P •? (Norris et al 1971;McElroy et al 1972;Feher et al 1975;Lendzian et al 1993).…”