Enthalpy changes during photosynthetic water oxidation tracked by time-resolved calorimetry using a photothermal beam deflection technique

The energetics of the individual reaction steps in the catalytic cycle of photosynthetic water oxidation at the Mn4Ca complex of photosystem I I (PSI 1) are of prime interest. We studied the electron transfer reactions in oxygen-evolving PSII membrane particles from spinach by a photothermal beam deflection technique, allowing for time-resolved calorimetry in the micro- to millisecond domain. For an ideal quantum yield of 100%, the enthalpy change, Delta H, coupled to the formation of the radical pair Y(Z)(+)Q(A)(-) (where Y-Z is Tyr-161 of the D1 subunit of PSII) is estimated as -820 +/- 250 meV. For a lower quantum yield of 70%, the enthalpy change is estimated to be -400 +/- 250 meV. The observed nonthermal signal possibly is due to a contraction of the PSII protein volume (apparent Delta Vof about -13 angstrom(3)). For the first time, the enthalpy change of the O-2-evolving transition of the S-state cycle was monitored directly. Surprisingly, the reaction is only slightly exergonic. A value of Delta H(S-3 double right arrow S-0) of -210 meV is estimated, but also an enthalpy change of zero is within the error range. A prominent nonthermal photothermal beam deflection signal (apparent Delta V of about +42 angstrom(3)) may reflect O-2 and proton release from the manganese complex, but also reorganization of the protein matrix.