Bacteriorhodopsin thermal stability: Influence of bound cations and lipidson the intrinsic protein fluorescence

Temperature - induced changes in protein intrinsic fluorescence of native, delipidated and deionized purple membranes are investigated. It is found th at the removal of cations most strongly affects the protein and its thermal stability. The denaturation of dei-BR completes at 70 degrees C, while del ipidated and native BR still maintain their native structure at this temper ature. Both, the quantum yield and the fluorescence maximum suggest correla tion between the Trp-retinal coupling and protein structural stability. The low red shift of the fluorescence maximum caused by increasing of temperat ure indicates limited unfolding of bacteriorhodopsin upon denaturation.