The application of a successive annealing procedure to the scanning calorimetric endotherm of the purple membrane from Halobacterium halobium in phosphate buffer, pH 7.5, leads to five thermal transitions beneath the overall endotherm. Circular dichroism and fluorescence experiments have also been carried out with the native membrane heated at the same scan rate as in calorimetric runs (1°C/min) as well as with previously heated membrane samples. These results, together with others from the literature, have been used to suggest a preliminary explanation of the five thermal transitions.
