Thermodynamic and Structural Characterization of Photosynthetic Reaction Centers by Photoacoustic Detection with a Broad Frequency Band Hydrophone

摘要

Broad frequency band hydrophone was used to carry out photoacoustic measurements in order to study thermodynamic and structural characteristics of photosynthetic reaction centers isolated from Rhodobacter sphaeroides R-26 purple bacteria. Data were extracted on enthalpy and volume changes accompanying the primary steps of photochemistry in the range of 0-500 μs. Considerable molar volume contractions, ΔV_M = -14.4, -7.7 and -4.3 cm~3 for 1, 2 and 0.07 Q/RC were calculated from the measured parameters accompanying the forward electron transport steps after light excitation. By comparison of the enthalpy changes (ΔH) to the Gibbs free energy (ΔG~0) data in literature, a rather large (26 %) entropic contribution to the ΔG~0 is estimated for the P~*Q_AQ_B → P~+Q_A ~-Q_B electron transport. This is in contrast to previous estimations that ΔG~0 is equal to ΔH in these processes. Only a small (4 %) entropic contribution to the ΔG~0 of the P~*Q_AQ_B → P~+Q_AQ_B~- process is estimated.