Cultural/Ethnology: An Ethnography of Santa Clara Pueblo, New Mexico. W. W. Hill. Edited and Annotated by Charles H. Lange

Charge recombination of the primary radical pair in D1/D2 reaction centers from photosystem 2 has been studied by time-resolved fluorescence and absorption spectroscopy. The kinetics of the primary radical pair are multiexponential and exhibit at least two lifetimes of 20 and 52 ns. In addition, a third lifetime of approximately 500 ps also appears to be present. These multiexponential charge-recombination kinetics reflect either different conformational states of D1/D2 reaction centers, with the different conformers exhibiting different radical pair lifetimes, or relaxations in the free energy of the radical pair state. Whichever model is invoked, the free energies of formation of the different radical pair states exhibit a linear temperature dependence from 100 to 220 K, indicating that they are dominated by entropy with negligible enthalpy contributions. These results are in agreement with previous determinations of the thermodynamics that govern primary charge separation in both D1/D2 reaction centers [Booth, P.J., Crystall, B., Giorgi, L. B., Barber, J., Klug, D.R., & Porter, G. (1990) Biochim. Biophys. Acta 1016, 141-152] and reaction centers of purple bacteria [Woodbury, N.W.T., & Parson, W.W. (1984) Biochim. Biophys. Acta 767, 345-361]. It is possible that these observations reflect structural changes that accompanying primary charge separation and assist in stabilization of the radical pair state thus optimizing the efficiency of primary electron transfer.

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 81%

Observation of multiple radical pair states in photosystem 2 reaction centers

Booth¹,

Crystall²,

Ahmad³

et al. 1991

“…The high signal-to-noise ratio of the present measurements, especially with oriented membranes, showed that this is not exactly true: the fit of the decay of the primary radical pair in the presence of reduced QA by a single exponential showed significant deviations that could be accounted for by assuming that about 10-15% of the radical pair decays more slowly than the dominant part. The reason for this could be a heterogeneity of charge recombination, perhaps due to a relaxation of the radical pair (Sebban & Barbet, 1984; Woodbury & Parson, 1984) or other than exponential kinetics, perhaps connected to a partial formation and decay of the radical pair triplet state. A detailed study of the back-reaction, however, is beyond the scope of this work and will be treated in another publication.…”

Section: Resultsmentioning

confidence: 99%

Kinetic properties of the acceptor quinone complex in Rhodopseudomonas viridis

Leibl

Breton²

1991

The kinetics of electron transfer from the primary (QA) to the secondary (QB) quinone acceptor in whole cells and chromatophores of Rhodopseudomonas viridis was studied as a function of the redox state of QB and of pH by using a photovoltage technique. Under conditions where QB was oxidized, the reoxidation of QA- was found to be essentially monophasic and independent of pH with a half-time of about 20 microseconds. When QB was reduced to the semiquinone form by a preflash, the reoxidation of QA- was slowed down showing a half-time between 40 and 80 microseconds at pH less than or equal to 9. Above pH 9, the rate of the second electron transfer decreased nearly one order of magnitude per pH unit. After a further preflash, the fast and pH-independent kinetics of QA- reoxidation was essentially restored. The concentration of QA still reduced 100 microseconds after its complete reduction by a flash showed distinct binary oscillations as a function of the number of preflashes, confirming the interpretation that the electron-transfer rate depends on the redox state of QB. After addition of o-phenanthroline, the reoxidation of QA- is slowed down to the time range of seconds as expected for a back-reaction with oxidized cytochrome. Under conditions where inhibitors of the electron transfer between the quinones fail to block this reaction in a fraction of the reaction centers due to the presence of the extremely stable and strongly bound semiquinone, QB-, these reaction centers show a slow electron transfer on the first flash and a fast one on the second, i.e., an out-of-phase oscillation.(ABSTRACT TRUNCATED AT 250 WORDS)

“…Perhaps the structural changes accompanying the charge separation process act to inhibit the wasteful direct recombination pathway. The possibility of such a stabilization process has been discussed by Warshel (1980) and by Woodbury & Parson (1984).…”

Section: Discussionmentioning

confidence: 99%

“…Recent studies by Woodbury & Parson (1984) suggest that there is a discrete distribution of states involved on the time scale (t <10 ns) of the early electron-transfer steps in RCs.…”

mentioning

confidence: 99%

Electron-transfer kinetics in photosynthetic reaction centers cooled to cryogenic temperatures in the charge-separated state: evidence for light-induced structural changes

Kleinfeld¹,

Okamura²,

Fehér³

1984

366

378

We have compared the electron-transfer kinetics in reaction centers (RCs) cooled in the dark with those cooled under illumination (i.e., in the charge-separated state). Large differences between the two cases were observed. We interpreted these findings in terms of light-induced structural changes. The kinetics of charge recombination D+QA-----DQA in RCs containing one quinone were modeled in terms of a distribution of donor-acceptor electron-transfer distances. For RCs cooled under illumination the distribution broadened and shifted to larger distances compared to the distribution for RCs cooled in the dark. The model accounts for the nonexponential decay observed at low temperatures [McElroy, J. D., Mauzerall, D. C., & Feher, G. (1974) Biochim. Biophys. Acta 333, 261-277; Morrison, L.E., & Loach, P.A. (1978) Photochem. Photobiol. 27, 751-757]. A possible physiological role of the structural changes is an enhanced charge stabilization. For RCs with two quinones, the recombination kinetics D+QAQB-----DQAQB were found to be strongly temperature dependent. This was interpreted in terms of temperature-dependent transitions between structural states [Agmon, N., & Hopfield, J.J. (1983) J. Chem. Phys. 78, 6947-6959]. This interpretation requires that these transitions occur at cryogenic temperatures on a time scale t greater than or approximately 10(3) s. The electron transfer from QA- to QB was found to not take place in RCs cooled in the dark (tau ABdark greater than 10(-1) s). In RCs cooled under illumination, we found tau ABlight less than 10(-3) s. We suggest the possibility that the drastic decrease in tau AB observed in RCs cooled under illumination is due to the trapping of a proton near QB-.