91Ó°ÊÓ

$\mathrm{β}$-barrels appear in certain transmembrane proteins. Most proteins have 8 to 22 strands in their trans-membrane -barrel. To close the $\mathrm{β}$-sheet in on itself with all strands antiparallel, the number of strands must be even. Porins are trans-membrane proteins that allow molecules or ions to flow through.

Porins are $\mathrm{β}$-barrel structures with a central aqueous channel that is narrowed to produce a $7×11A°$elliptical pore. The channel can handle solutes with a diameter of more than 600D. The material that may move through the central aqueous channel is determined by its sizeand the residues that develop at the trans-membrane.

A solid core can be found in a $\mathrm{β}$ -barrel with eight strands. The diameter of the cross-section of porins grows enough to allow a pore for solute transport when the number of strands increases to 16 or 18 strands.

As a result, thecross-sectional diameter expands to accommodate a pore forsolute transport.