Thermodynamics of Electron Transfer in Oxygenic Photosynthetic Reaction Centers:  Volume Change, Enthalpy, and Entropy of Electron-Transfer Reactions in the Intact Cells of the CyanobacteriumSynechocystisPCC 6803

Abstract

The volume and enthalpy changes for charge transfer in the 0.1-10 micros time window in photosynthetic reaction centers of the intact cells of Synechocystis PCC 6803 were determined using pulsed, time-resolved photoacoustics. This required invention of a method to correct for the cell artifact at the temperature of maximum density of water caused by the heterogeneous system. Cells grown under either white or red light had different PS I/PS II molar ratios, approximately 3 and approximately 1.7, respectively, but invariable action spectra and effective antenna sizes of the photosystems. In both cultures, the photoacoustic measurements revealed that their thermodynamic parameters differed strongly in the spectral regions of predominant excitation of PS I (680 nm) and PS II (625 nm). On correcting for contribution of the two photosystems at these wavelengths, the volume change was determined to be -27 +/- 3 and -2 +/- 3 A3 for PS I and PS II, respectively. The energy storage on the approximately 1 micros time scale was estimated to be 80 +/- 15% and 45 +/- 10% per trap in PS I and PS II, respectively. These correspond to enthalpies of -0.33 +/- 0.2 and -1 +/- 0.2 eV for the assumed formation of ion radical pairs P700+F(AB-) and Y(Z*)P680Q(A-), respectively. Taking the free energy of the above reactions as the differences of their redox potentials in situ, apparent entropy changes were estimated to be +0.4 +/- 0.2 and -0.2 +/- 0.2 eV for PS I and PS II, respectively. These values are similar to that obtained in vitro for the purified reaction center complexes on the microsecond time scale [Hou et al. (2001) Biochemistry 40, 7109-7116, 7117-7125]. The constancy of these thermodynamic values over a 2-fold change of the ratio of PS I/PS II is support for this method of in vivo analysis. Our pulsed PA method can correct the "cell" or heterogeneous artifact and thus opens a new route for studying the thermodynamics of electron transfer in vivo.

Photoelectron Generation by Photosystem II Core Complexes Tethered to Gold Surfaces

“…To calculate the spectrally averaged cross section, we integrated the measured cross section at 470 nm between 365 and 750 nm ( Figure 2). The spectrally integrated average cross section is 25 2 . In PSII CCs immobilized on Au foils, the basic fluorescence kinetics features are preserved.…”

“…In all oxygenic photosynthetic organisms, the reaction centers of photosystems I and II (PSI and PSII RCs) convert photon energy into electrical potentials with extraordinary efficiency (45 AE 10 % and 80 AE 15 %, respectively [2] ) under a wide range of light and temperature conditions. PSII RCs oxidize water to generate oxygen and protons on the lumenal side of the thylakoid membrane, while translocating electrons toward the stromal side.…”

Spectral Signatures of Photosynthesis. I. Review of Earth Organisms

“…This seems to provide a sensible explanation for the uphill exciton transfer from BChl b at 1013 nm to the RC at 980 nm. Mauzerall and coworkers (Hou, et al, 2001a;Hou, et al, 2001b;Boichenko, et al, 2001) quantified, in detail, the entropy changes that occur in PS I, PS II, and cyanobacteria, and found them to be very small. Work on kinetics of electron transfer (Trissl, 1993;Trissl, et al, 1999;Bernhard and Trissl, 2000) implies the optimality of arrangements between the reaction centers and light harvesting antennas to ensure efficient transport and trapping of the excitons.…”

The A-FXto FA/BStep inSynechocystis6803 Photosystem I Is Entropy Driven

“…The thermodynamics of electron transfer in biological system have been of great interests during the last decade and the accumulated evidence supports the view that the entropy changes in electron and proton transfer reactions in chemical and biological system are significant and cannot be neglected. [17][18][19][20][40][41] We summarize the free energy, enthalpy and entropy changes of PSI in Figure 4. The solid arrows are the free energies of reaction obtained from the redox potentials of cofactors in situ, the open arrows are the enthalpy changes observed by means of photoacoustic data, and the dashed arrows are the calculated entropy changes using the Gibbs relation ΔG = ΔH-T ΔS.…”