Thermal and Structural Changes of Photosynthetic Reaction Centers Characterized by Photoacoustic Detection with a Broad Frequency Band Hydrophone

• Published in: Photochemistry and photobiology 2001-07, Vol.74 (1), p.81-87
• Main Authors: Nagy, László, Kiss, Vladimir, Brumfeld, Vlad, Malkin, Shmuel
• Format: Article
• Language: English
• Subjects: Electron Transport; Photochemistry; PHOTOSYNTHESIS AND BIO/CHEMILUMINESCENCE; Photosynthetic Reaction Center Complex Proteins - chemistry; Photosynthetic Reaction Center Complex Proteins - radiation effects; Rhodobacter sphaeroides; Rhodobacter sphaeroides - chemistry; Rhodobacter sphaeroides - radiation effects; Thermodynamics
• ISSN: 0031-8655; 1751-1097
• DOI: 10.1562/0031-8655(2001)074<0081:TASCOP>2.0.CO;2

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 μs 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. ΔVM = 14.4, 7.7 and 4.3 cm3 molar volume contractions were calculated from the measured parameters for 1, 2 and 0.07 quinone/RC after light excitation. Comparing the enthalpy changes (ΔH) to the Gibbs free energy data in the literature, a rather large (26%) entropic contribution to the free energy changes (ΔG) is estimated for the P*QAQB → P+QA−QB electron transport (where QA and QB represent primary and secondary quinones, respectively). This is in contrast to previous estimations that ΔG = ΔH in these processes. On the other hand, only a small (4%) entropic contribution to the ΔG of the P*QAQB → P+QAQB− process is estimated, in agreement with the literature data. Our results are in good agreement with the data obtained earlier (Edens et al. [2000] J. Am. Chem. Soc. 122, 1479–1485).