The A~-F_x to F_(A/B) Step in Synechocystis 6803 Photosystem I Is Entropy Driven

摘要

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_1F_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 A3),which are assigned to the P_(700)* -> A_1~1F_x step,and (2) a component with approx 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_1F_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 deg 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.