Thermodynamics of electron transfer in oxygenic photosynthetic reaction centers: Volume change, enthalpy, and entropy of electron-transfer reactions in the intact cells of the cyanobacterium Synechocystis PCC 6803

The volume and enthalpy changes for charge transfer in the 0.1-10 mus time window in photosynthetic reaction centers of the intact cells of Synechocys tis PCC 6803 were determined using pulsed, time-resolved photoacoustics. Th is required invention of a method to correct for the cell artifact at the t emperature of maximum density of water caused by the heterogeneous system. Cells grown under either white or red light had different PS VPS II molar r atios, similar to3 and similar to1.7, respectively, but invariable action s pectra and effective antenna sizes of the photosystems. In both cultures, t he 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 pho tosystems at these wavelengths, the volume change was determined to be -27 +/- 3 and -2 +/- 3 Angstrom (3) for PS I and PS II, respectively. The energ y storage on the similar to1 mus time scale was estimated to be 80 +/- 15% and 45 +/- 10% per trap in PS I and PS II, respectively. These correspond t o enthalpies of -0.33 +/- 0.2 and -1 +/- 0.2 eV for the assumed formation o f ion radical pairs P-700+F-AB(-) and Y-Z.P(680)Q(A)(-), respectively. Taki ng 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 complex es on the microsecond time scale [Hou et al. (2001) Biochemistry 40, 7109-7 116, 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 ana lysis. Our pulsed PA method can correct the "cell" or heterogeneous artifac t and thus opens a new route for studying the thermodynamics of electron tr ansfer in vivo.