The role of retinal in the thermal stability of the purple membrane.

Differential scanning calorimetry demonstrates that the bleached form of the purple membrane does not possess any measurable thermal transition in water, up to 105 degrees C, whereas in 0.1 M phosphate pH 7.5 it shows a transition at about 82 degrees C, with an enthalpy of 110 kJ/mol. In the latter medium, the native membrane shows the main transition at 97 degrees C, with an enthalpy of 390 kJ/mol. The reduced form of the purple membrane shows two small transitions in water, as well as in 0.1 M phosphate, which do not seem to be related to the main thermal transition of the native membrane. Fourier-transform infrared spectra in D2O show that the two modified samples, as well as the native one, undergo similar secondary structural changes upon thermal denaturation. These changes appear to extend through a wide temperature range for both modified forms, particularly for the bleached one. The results suggest that the main thermal transition in the purple membrane is due to a cooperative conformational change involving the disruption of the network of electrostatic and hydrogen-bonding interactions which originate from the protonated Schiff base. In the two modified membranes, these conformational changes appear to proceed smoothly through a rather low or non-cooperative process. The thermal behaviour of the bleached membrane in water resembles that of the molten globule state described for several globular proteins.