The effects of a gel network structure on purple membrane (PM) spontaneous stacking are discussed, with particular emphasis on the relationship between the PM periodic pre-structures before gelation. Such peculiar intermembrane pre-structures with a periodic distance of ∼20 nm have recently been found to provide an important key to PM stacking, which is critical for the engineering applications of PM. Here, we investigated how the PM stacking differs depending on the gel network of a poly(vinyl alcohol) gel. On the basis of the results, we propose a plausible interplay between the PM periodic pre-structures and the type of gel network for PM stacking. The results indicate that a porous gel network, in addition to the PM periodic pre-structure in solutions, is necessary for PM stacking in immobilized gel samples.

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