The A-Fx to F(A/B) step in synechocystis 6803 photosystem I is entropy driven.

We have previously reported the enthalpy and volume changes of charge separation in photosystem I from Synechocystis 6803 using pulsed photoacoustics on the microsecond time scale, assigned to the electron-transfer reaction from excited-state P(700) to F(A/B) iron sulfur clusters. In the present work, we focus on the thermodynamics of two steps in photosystem I: (1) P(700) --> A(1)(-)F(X) (<10 ns) and (2) A(1)(-)F(X) --> F(A/B)(-) (20-200 ns). The fit by convolution of photoacoustic waves on the nanosecond and microsecond time scales resolved two kinetic components: (1) a prompt component (<10 ns) with large negative enthalpy (-0.8 +/- 0.1 eV) and large volume change (-23 +/- 2 A(3)), which are assigned to the P(700) --> A(1)(-)F(X) step, and (2) a component with approximately 200 ns lifetime, which has a positive enthalpy (+0.4 +/- 0.2 eV) and a small volume change (-3 +/- 2 A(3)) that are attributed to the A(1)(-)F(X) --> F(A/B)(-) step. For the fast reaction using the redox potentials of A(1)F(X) (-0.67 V) and P(700) (+0.45 V) and the energy of P(700) (1.77 eV), the free energy change for the P(700) --> A(1)(-)F(X) step is -0.63 eV, and thus the entropy change (TDeltaS, T = 25 degrees C) is -0.2 +/- 0.3 eV. For the slow reaction, A(1)(-)F(X) --> F(A/B)(-), taking the free energy of -0.14 eV [Santabara, S.; Heathcote, P; Evans, C. W. Biochim. Biophys. Acta 2005, 1708, 283-310], the entropy change (TDeltaS) is positive, +0.54 +/- 0.3 eV. The positive entropy contribution is larger than the positive enthalpy, which indicates that the A(-)F(X) to F(A/B)(-) step in photosystem I is entropy driven. Other possible contributions to the measured values are discussed.