膜内蛋白質構造変化の電気光学的研究 : バクテリオロドプシンの電場誘起性構造変化
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Electrooptical methods are very useful for studies on structural and functional changes of membrane-bound proteins, because the membrane potential plays an important role on the structure, and therefore on transport of ions and/or molecules through the membrane. In this review article electrooptical studies of bacteriorhodopsin of halobacterial purple membrane is summarized as an example. Conformational changes of bacteriorhodopsin are induced by electric pulses both in suspensions and in dried films. The absorbance change induced by the electric field of 2-20kV/cm with a duration <100/μs in suspension consists of dichroic and isotropic contributions, which have been suggested to correspond to changes in the orientation of both retinal and tyrosin and/or tryptophane residues and to changes of the microenvironment of aromatic amino acid residues concomitant with pK changes in at least two types of proton binding sites, respectively. The absorbance change due to the external field of-300kV/cm (duration 3-5 minutes) in dried films reveals that the electric field from the intracellular side to the extracellular side of purple species of purple membrane induces an intermediate similar to the cation-depleted blue species. The electric dichroism is also observed in the dried film where the membrane fragments are fixed, suggesting the angular displacement of the retinal transition moment.