Differential scanning calorimetry of chloroplast membranes: identification of an endothermic transition associated with the water-splitting complex of photosystem II.

The structure of spinach thylakoid membranes has been investigated by sensitive differential scanning calorimetry. Six endotherms are observed between 20 and 85 degrees C, corresponding to order--disorder transitions of different structural domains within the thylakoid membrane. In a medium of relatively high ionic strength, endothermic transitions occur at 42, 54, 65, 72, 79, and 84 degrees C, with the 65 degrees C transition being particularly prominent. At a lower ionic strength, transitions are centered at 44, 61, 66, 70, 78, and 83 degrees C. The 42--44 degrees C endothermic transition (the A transition) can be correlated with the modification of three electron-transport components or properties associated with photosystem II: (i) release of manganese from the membrane, (ii) the loss of O2 evolution with water as a donor, and (iii) a decrease in the redox potential of the hydroquinone-reducible cytochrome b-559. Both the A transition and the ability to evolve O2 are irreversibly lost after heating to 49 degrees C and also after exposure to trypsin, suggesting the involvement of protein in this transition. The interpretation of these observations is that one effect of the A transition involves the thermal disruption of a protein component on the donor side of photosystem II.