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

Photoacoustic measurements using a broad frequency band hydrophone were car ried out in photosynthetic reaction centers (RC) isolated from Rhodobacter sphaeroides R-26 purple bacteria. Data were extracted on enthalpy and volum e changes accompanying the primary steps after excitation in the range of 0 -500 mus aimed at further characterizing the thermodynamic properties of th e RC, Quinone titration showed that the volume contraction accompanying the electron transport is sensitive to the molecular species occupying the sec ondary quinone site. DeltaV(M) = 14.4, 7.7 and 4.3 cm(3) molar volume contr actions were calculated from the measured parameters for 1, 2 and 0.07 quin one/RC after light excitation. Comparing the enthalpy changes (DeltaH) to t he 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 prev ious 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 d ata. Our results are in good agreement with the data obtained earlier.