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

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Abstract
The structure of spinach thylakoid membranes was investigated by sensitive differential scanning calorimetry. Six endotherms are observed between 20 and 85.degree. 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.degree., 54.degree., 65.degree., 72.degree., 79.degree. and 84.degree. C, with the 65.degree. C transition being particularly prominent. At a lower ionic strength, transitions are centered at 44.degree., 61.degree., 66.degree., 70.degree., 78.degree. and 83.degree. C. The 42-44.degree. C endothermic transition (the A transition) can be correlated with the modification of 3 electron-transport components or properties associated with photosystem II: release of Mn from the membrane, the loss of O2 evolution with water as a donor, and 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.degree. 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.

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