Thermodynamics of electron transfer in oxygenic photosynthetic reaction centers: A pulsed photoacoustic study of electron transfer in photosystem I reveals a similarity to bacterial reaction centers in both volume change andentropy

The thermodynamic properties of electron transfer in biological systems are far less known in comparison with that of their kinetics. In this paper th e enthalpy and entropy of electron transfer in the purified photosystem I t rimer complexes from Synechocystis sp. PCC 6803 have been studied, using pu lsed time-resolved photoacoustics on the 1 mus time scale. The volume contr action of reaction centers of photosystem I, which results directly from th e light-induced charge separation forming P-700+F-A/F-B(-) from the excited -state P-700*, is determined to be -26 +/- 2 Angstrom (3). The enthalpy of the above electron-transfer reaction is found to be -0.39 +/- 0.1 eV. Photo acoustic estimation of the quantum yield of photochemistry in the purified photosystem I trimer complex showed it to be close to unity. Taking the fre e energy of the above reaction as the difference of their redox potentials in situ allows us to calculate an apparent entropy change (T DeltaS) of +0. 35 +/- 0.1 eV. These values of DeltaV and T DeltaS are similar to those of bacterial reaction centers. The unexpected sign of entropy of electron tran sfer is tentatively assigned, as in the bacterial case, to the escape of co unterions from the surface of the particles. The apparent entropy change of electron transfer in biological system is significant and cannot be neglec ted.