Thermal and structural changes of photosynthetic reaction centers characterized by photoacoustic detection with a broad frequency band hydrophone

摘要

Photoacoustic measurements using a broad frequency band hydrophone were carried out in photosynthetic reaction centers (RC) isolated from Rhodobacter sphaeroides R-26 purple bacteria. Data were extracted on enthalpy and volume changes accompanying the primary steps after excitation in the range of 0-500 mus aimed at further characterizing the thermodynamic properties of the RC, Quinone titration showed that the volume contraction accompanying the electron transport is sensitive to the molecular species occupying the secondary quinone site. DeltaV(M) = 14.4, 7.7 and 4.3 cm(3) molar volume contractions were calculated from the measured parameters for 1, 2 and 0.07 quinone/RC after light excitation. Comparing the enthalpy changes (DeltaH) to the Gibbs free energy data in the literature, a rather large (26%) entropic contribution to the free energy changes (DeltaG) is estimated for the P*Q(A)Q(B) --> P(+)Q(A)(-)Q(B) electron transport (where Q(A) and Q(B) represent primary and secondary quinones, respectively). This is in contrast to previous estimations that DeltaG = DeltaH in these processes. On the other hand, only a small (4%) entropic contribution to the DeltaG of the P*Q(A)Q(B) --> P(+)Q(A)Q(B)(-) process is estimated, in agreement with the literature data. Our results are in good agreement with the data obtained earlier. [References: 33]