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aiw for e 11 structural motifs are observed for MPs: membrane-span-ning a-helix bundles and b-barrels, the former being pre-tions and dynamic studies of MPs might serve as a power-ful tool to understand the biological functions of MPs. The retinal protein bacteriorhodopsin (BR) is a MP found in the purple membrane of Halobacterium salinarium and serves as a light-driven proton pump. It converts the energy of green light (of wavelength 500–650 nm) into an * Corresponding author