Changes in Enthalpy of the Joliot-Kok Four Step Cycle to Produce Oxygen in Photosynthesis

Abstract

Changes in Enthalpy of the Joliot-Kok four-step cycle to produce oxygen in photosynthesis.During photosynthesis, oxygen is produced from water in the four step Joliot-Kok cycle. The kinetics of this cycle have been extensively studied but the thermodynamics are still uncertain. An intriguing possibility is that evolution has found a path that allows a constant free energy difference for all the steps. This would strongly contrast with the sequential one-electron steps where the energies vary by over one electron-volt, or 100 kilojoules. Although the free energy or redox potentials of the intermediates are difficult to asses, the methodology of photoacoustics (PA) allows direct access to the enthalpy of the steps. Essentially the heat or enthalpy liberated by a photo-induced reaction causes an expansion of the media. The resulting sound wave is detected with a microphone. In certain leaves the oxygen itself produces a slower sound wave that can be used to verify the Joliot-Kok cycle of oxygen production. The PA measurement requires a weak pulse of light at the trap wavelength, 680 nm, to determine the enthalpy unalloyed with excess energy, interspersed with saturating pulses to advance the S-states of the Joliot-Kok cycle. A half dozen or more pulse pairs are required to obtain values for the four S-states. To enhance the signal to noise ratio, the cycle is then repeated following several minutes in the dark to allow the system to relax to the known state. The saturating flashes must be spaced by about 300 milliseconds to allow completion of the reaction but not allow decay of the S-states. Very preliminary results indicate that the differences in enthalpies of the reaction steps are small. This work was supported by the Rockefeller University.