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Phase transitions of the purple membrane and the brown holo-membrane X-ray diffraction,circular dichroism spectrum and absorption spectrum studies
Authors:Kenji Hiraki  Toshiaki Hamanaka  Toshio Mitsui  Yuji Kito
Affiliation:1. Department of Biophysical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560 Japan;2. Department of Biology, Faculty of Science, Osaka University, Toyonaka, Osaka 560 Japan
Abstract:The phase transition of the purple membrane observed by differential scanning calorimetry (Jackson, M.B. and Sturtevant, J.M. (1978) Biochemistry 17, 911–915) has been investigated by X-ray diffraction, circular dichroism and absorption spectrum, in comparison with the phase transition in the brown holo-membrane. The two-dimensional crystal of bacteriorhodopsin transformed into two-dimensional liquid around 74–78°C in the purple membrane and around 50–60°C in the brown holo-membrane. The X-ray diffraction patterns obtained at 78°C for the purple membrane and at 60°C for the brown holo-membrane exhibit several broad peaks. Analysis of the pattern suggests that bacteriorhodopsin molecules aggregate in trimers even above the phase transition temperature. The negative circular dichroism band in the visible region is still present at 80°C in the purple membrane and at 60°C in the brown holo-membrane, but becomes negligibly small at 70°C in the brown holo-membrane. The 560 nm absorption peak due to bacteriorhodopsin changes its position and height drastically around 80°C in the brown holo-membrane as in the purple membrane. X-ray diffraction studies have been made on membranes of total lipids extracted from the purple membrane. No indication of the phase transition has been found between ?81°C and 77°C.
Keywords:Bacteriorhodopsin  Purple membrane  Phase transition  X-ray diffraction  DSC  differential scanning calorimetry
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