The enthalpy and entropy of reaction for formation of P(+)Q(A)(-) from excited reaction centers of Rhodobacter sphaeroides

The enthalpy and volume changes for the charge-transfer reaction between ex cited donor and ionized donor and acceptor in bacterial reaction centers we re determined using pulsed photoacoustics. Excitation in the lowest absorpt ion band of the centers at 860 nm minimized the thermal signal caused by de gradation of excess energy. Knowing the free energy of this reaction, -0.86 eV, the determination of the enthalpy, -0.44 eV, fixes the entropy at 25 d egrees C as about one-half (T Delta S = +0.42 eV) of the free energy for th e normal ubiquinone-10 containing centers. This is a larger contribution th an anticipated from previous estimates of the enthalpy. The unexpected sign of the entropy is assigned to the release of counterions from the reaction center surfaces when the charge transfer cancels the dominant opposite cha rges at the interfaces. The enthalpy and entropy of six reaction centers co ntaining exchanged quinones did not correlate with their free energies. The volume contractions ranged from -28 to -42 Angstrom(3) and roughly correla ted with the size of the quinone as expected from electrostriction.