Radical-pair energetics and decay mechanisms in reaction centers containing anthraquinones, naphthoquinones or benzoquinones in place of ubiquinone

Abstract

In reaction centers from Rhodobacter sphaeroides (formerly called Rhodopseudomonas sphaeroides), light causes an electron-transfer reaction that forms the radical pair state (P+I−, or PF) from the initial excited singlet state (P∗) of a bacteriochlorophyll dimer (P). Subsequent electron transfer to a quinone (Q) produces the state P+Q−. Back electron transfer can regenerate P∗ from P+Q−, giving rise to ‘delayed’ fluorescence that decays with approximately the same lifetime as P+Q−. The free-energy difference between P+Q− and P∗ can be determined from the initial amplitude of the delayed fluorescence. In the present work, we extracted the native quinone (ubiquinone) from Rps. sphaeroides reaction centers, and replaced it by various anthraquinones, naphthoquinones, and benzoquinones. We found a rough correlation between the halfwave reduction potential (E12) of the quinone used for reconstitution (as measured polarographically in dimethylformamide) and the apparent free energy of the state P+Q− relatively to P∗. As the E12 of the quinone becomes more negative, the standard free-energy gap between P+Q− and P∗ decreases. However, the correlation is quantitatively weak. Apparently, the effective midpoint potentials (Em) of the quinones in situ depend subtly on interactions with the protein environment in the reaction center. Using the value of the Em for ubiquinone determined in native reaction centers as a reference, and the standard free energies determined for P+Q− in reaction centers reconstituted with other quinones, the effective Em values of 12 different quinones in situ are estimated. In native reaction centers, or in reaction centers reconstituted with quinones that give a standard free-energy gap of more than about 0.8 eV between P+Q− and P∗, charge recombination from P+Q− to the ground state (PQ) occurs almost exclusively by a temperature-insensitive mechanism, presumably electron tunneling. When reaction centers are reconstituted with quinones that give a free-energy gap between P+Q− and P∗ of less than 0.8 eV, part or all of the decay proceeds through a thermally accessible intermediate. There is a linear relationship between the log of the rate constant for the decay of P+Q− via the intermediate state and the standard free energy of P+Q−. The higher the free energy, the faster the decay. The kinetic and thermodynamic properties of the intermediate appear not to depend strongly on the quinone used for reconstitution, indicating that the intermediate is probably not simply an activated form of P+Q−. It has been suggested previously that the intermediate is PF; however, calculations of the decay kinetics of P+Q− in kinetic models based on this assumption are inconsistent with the thermodynamic parameters determined from fluorescence measurements. In addition, the triplet state (PR) normally formed as a decay product of PF could not be detected. These observations argue that the thermally accessible intermediate is not identical with the PF state that has been characterized previously, though it could be a strongly relaxed form of this radical pair.