Primary Electron Transfer Processes Research Articles

Electron transfer and spin exchange contributing to the magnetic field dependence of the primary photochemical reaction of bacterial photosynthesis

The yield ϕ T of triplet products “P R” generated in reaction centers of Rhodopseudomonas sphaeroides in which the “primary” acceptor is reduced had been found to depend on external magnetic fields. The magnetic field dependence varies, however, between different reaction center preparations. By means of a theoretical description of the primary electron transfer processes and hyperfine coupling-induced electron spin motion the factors influencing the magnetic field behaviour of the triplet products are studied. The following quantities characteristic of the primary electron transfer in photosynthesis have a strong effect on ϕ T: (1) the rate constants of reversible electron transfer between the initially excited singlet state of the reaction center and an intermediate radical ion pair state; (2) the rate constants of irreversible electron transfer of the radical pair to the ground and excited triplet state of the reaction center; (3) the electron exchange interactions between the radical pair and the “primary” acceptor. From the observed magnetic field dependence of ϕ T estimates for these quantities are obtained. A temperature dependence of the magnetic field behaviour of ϕ T and a magnetic field effect on the fluorescence quantum yield of the reaction center are predicted.

Primary photochemical processes in isolated reaction centers of Rhodopseudomonas viridis

Picosecond and nanosecond spectroscopic techniques have been used to study the primary electron transfer processes in reaction centers isolated from the photosynthetic bacterium Rhodopseudomonas viridis. Following flash excitation, the first excited singlet state (P∗) of the bacteriochlorophyll complex (P) transfers an electron to an intermediate acceptor (I) in less than 20 ps. The radical pair state (P +I −) subsequently transfers an electron to another acceptor (X) in about 230 ps. There is an additional step of unknown significance exhibiting 35 ps kinetics. P + subsequently extracts an electron from a cytochrome, with a time constant of about 270 ns. At low redox potential (X reduced before the flash), the state P +I − (or P F) lives approx. 15 ns. It decays, in part, into a longer lived state (P R), which appears to be a triplet state. State P R decays with an exponential time of approx. 55 μs. After continuous illumination at low redox potential (I and X both reduced), excitation with an 8-ps flash produces absorption changes reflecting the formation of the first excited singlet state, P∗. Most of P∗ then decays with a time constant of 20 ps. The spectra of the absorbance changes associated with the conversion of P to P∗ or P + support the view that P involves two or more interacting bacteriochlorophylls. The absorbance changes associated with the reduction of I to I − suggest that I is a bacteriopheophytin interacting strongly with one or more bacteriochlorophylls in the reaction center.

Electrochemical oxidation of theophylline at the pyrolytic graphite electrode

The electrochemical oxidation of theophylline (1,3-dimethylxanthine) at the pyrolytic graphite electrode has been investigated by voltammetry and controlled potential electrolysis and by isolation, identification and determination of the reaction products. The primary electron transfer process occurs by a 1-electron, 1-proton oxidation of the −N9=C8− double bond to give a free radical species which partially dimerizes to give a previously unreported compound, 1,3-(dimethylxanthyl-8)-1,3-dimethylxanthine. Part of the free radical is oxidized in a further 1-electron, 1-proton reaction to give 1,3-dimethyluric acid which is then immediately further oxidized in a 2e−/2H+ reaction at the −C4=C5− double bond to give 1,3-dimethylric acid-4,5-diol. This latter species is unstable and undergoes secondary electrochemical oxidation to parabanic acid and dimethylurea, and fragmentation to dimethylalloxan and urea and 6,8-dimethylallantoin and carbon dioxide.

Electrochemical oxidation of theophylline at the pyrolytic graphite electrode

Summary The electrochemical oxidation of theophylline (1,3-dimethylxanthine) at the pyrolytic graphite electrode has been investigated by voltammetry and controlled potential electrolysis and by isolation, identification and determination of the reaction products. The primary electron transfer process occurs by a 1-electron, 1-proton oxidation of the −N9=C8− double bond to give a free radical species which partially dimerizes to give a previously unreported compound, 1,3-(dimethylxanthyl-8)-1,3-dimethylxanthine. Part of the free radical is oxidized in a further 1-electron, 1-proton reaction to give 1,3-dimethyluric acid which is then immediately further oxidized in a 2e−/2H+ reaction at the −C4=C5− double bond to give 1,3-dimethylric acid-4,5-diol. This latter species is unstable and undergoes secondary electrochemical oxidation to parabanic acid and dimethylurea, and fragmentation to dimethylalloxan and urea and 6,8-dimethylallantoin and carbon dioxide.

The nature of some primary electron transfer processes in photosynthesis

The nature of some primary electron transfer processes in photosynthesis