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New β-sheet architectures. (a) A bifurcated X-shaped β-sheet can fold upon itself on both sides, forming a double barrel. (b) A very large β-sheet can be folded into a β-spiral roll with overlapping edges. This architecture combines features of both β-barrel and β-sandwich. In both parts, for simplicity, the arrows denote β-strands but do not define the strand directionality. The actual β-sheets of these architectures may comprise parallel and antiparallel strands.
